Coax Feedline Lengths To Avoid

5 of coax used as the counterpoise will work. Antenna manufacturers often tell their customers to cut their coax feed lines to a specific length in order to get a good match. A common thought of many is since this is coaxial cable, we need velocity factor adjustments. Lower-loss, double-shielded coax, such as LMR-400UF is best for the rest of the feedline (typically 25-50 feet). Absolutely avoid using RG-58, even for jumpers! Very short runs of good quality RG-8X may be acceptable up to 1. All this does is mask the problem, by moving the SWR node to a different position along the feed line. If you need to add feedline to this antenna, feedline lengths near 65 feet, 130 feet, and so on, should be avoided for ease of tuning on the 80 meter band. 1 - Power, voltage, and current along RG-58/U coax under matched and mismatched conditions. The term used to describe the general idea of keeping the feedline and antenna separate is 'decoupling. I allowed about 3 feet for "feedline" (to the left of the choke). Although those losses may be low compared. (Avoid 130, 260 ft). Coax is OK, as long as you understand impact of loss on the bands to be used on… • Try to make antenna length at least ½ wavelength of lowest frequency. Avoid the use of adapters as much as possible and try to use the shortest amount of coax as possible. It is usually specified in dB/foot and is a linear relationship. 287 MHz, or a dipole length of about 32. 8MHz to 30Mhz coverage. The feedline was terminated in a BNC plug, which you can see loosely coiled for storage at the left. Of course, if you decide to use 50 ohm coax to avoid this problem, then you get a similar mismatch at the antenna, 50:72. The truth is, if the antenna. Imperveon 50-ohm cable from Times Fiber Communications, 358 Hall Ave. Some transceivers, amps and/or tuners do not like a balun installed. The solution isn NOT to use magic coax lengths, you need to fix your antenna so that it isn't inducing common-mode currents on your feedline. For longer runs, use an ultra- low loss cable, such as Belden 9913, to reach the antenna site, then use RG-8X. Always cut feedline to the minimum usable length. To confirm more than 3 rooms, please call 1-888-444-OMNI (6664) and an Omni Hotels representative will gladly assist you. For Series 6 cable, typical loss values are as follows:. 357 Mc to be exact. The dipole coaxial line has the jacket removed from a central portion thereof to expose the outer conductor which is there severed and spread apart to form a gap exposing the dielectric layer, the lengths of coaxial line on each side of the gap comprising dipole. In my opinion, 1/2" Heliax is the minimum acceptable feedline--although I would try to keep the length of the feedline as short as possible. The "choke", which is the second coil of wire from the left (with 4 pieces of white tape), consisted of 8 turns of the feedline coax, with a diameter of roughly 3-1/2 inches. Signal resistance increases with feedline length. unbalanced feedline certainly will. You need to avoid getting it too close to metal or laying it on the ground. acknowledged, but that doesn't necessari- ly mean that your Stray will be appearing in QST. 25in of RG-174 the coax end is reached. Basically the 75 ohm side of this transformer (BNC connector) is connected to the 50 ohm output of the radio and the 50 ohm side (F-Connector) of the transformer is connected to the 75 ohm feed line or load. Note these approximate quarter wavelengths, in feet:. Simulations suggest what feedline lengths to avoid and offer a confirmation why portable users with short feeds never seem to have trouble. To avoid problems matching or feeding any dipole antenna with high impedance lines, keep the lines around these lengths. Side by side, it is easy to see the difference between RG174, RG58 and other RG 213 coax cables. This is derived for the 2M VHF of amateur allocation, 144 MHz – 148 MHz. In case you wish to use a reflector, cover pieces of cardboard with tin foil and attach to the plank. Cut the dipole wire to be a little longer than the computed length – it’s easier to cut wire than to extend it!. Then the antenna must have an electrical length of 66 feet, and suitable feedlines would have electrical lengths of 8, 14, 16, 24, 25, 41, 46, 77, feet. 5dB Measurement At transmitter: SWR is 2:1 (11%loss) not great, ok At antenna –using the graph here SWR at the antenna is 20:1 ! Not good at all Cost another 7dB in mismatch loss Result 1/10 of your power is being radiated. The required physical lengths are shorter as determined by the velocity factor of the feedline. The truth is, if the antenna. A short length of very low loss coaxial cable connects your transmatch to the RemoteBalun™. thin coax like RG174, high loss versions of coax or cheap coax knockoffs, with just a few. When calculating optimum lengths, we must use the velocity factor of the feedline. The braid of the coax will become part of the antenna. What I can tell you, per your question, is to change the SWR by changing the length of the coax, you're not changing SWR at the feedpoint where the antenna meets the coax - what appears we're doing is now an impedance transformation of using the coax as a feedline and hence the transferring of Resonance to more of an Impedance Transformation. matching of an antenna feedline that has not been matched previously. These losses are due to the resistance of the wire, dielectric losses, etc. • All shields are to be connected on same side 73, Hartmut, DG7YBN. In order to deliver the most power to the antenna, the impedance of the antenna has to be the same as the line and the radio (matched). Feedline is 450 Ohm "window" ladder line down through an aluminum framed window into a MFJ 949E tuner into my IC-730 and RigBlaster Plus/desktop PC. The plots were calculated using the "high accuracy" (NEC Sommerfeld) ground model with a. 046 in radius AWG 16 Velocity Factor. In some versions, this coax acts as a broadband feedline balun instead of the usual transformer type. Understanding that the dipole and feedline will radiate, what are optimum feedline & dipole lengths to avoid TVI to the surrounding neighborhood?. The balun [NV2K] built is a bifilar 1:1 design, with two parallel. Normally, in the case of normal (balanced) dipoles, we try to avoid this from happening. , 3184 Quebec st. Feedline radiation can be reduced by placing toroids on the feedline at the antenna. Since 5/8 wave length antenna is no longer matching the transmission line impedence, a feeding coil must be added between the antenna and the feedline so that the antenna can match the impedence of the tranmission feedline. There is only one rule for coax length: Use the shortest practical length possible to go from radio to antenna. , but there is enough space to fit some pieces of the rigid feedline. 1 ratio on the ol SWR meter you will need to use 70 ohm coax to feed the antenna. Coax is so much easier in this regard- simply lay it across the ground, or perhaps bury it just slightly. It allows you to compute the effects of loss through different types of coax with any frequency, line length, or power level. Suppose that we connect a typical coaxial cable to the feedpoint and use 100' of the line to reach the shack. Use fingers only to tighten. Now, if for some reason your SWR changes with differing lengths in your coaxial feedline, then you've got common mode RF flowing on the OUTSIDE of the coax (braid). The cable cut-off length is 80 1/4". 36 feet as a starting point. It's not required for coaxial cable, and not all coax has it (semi-rigid springs to mind) - but most does, so I guess it deserves a mention. Horizontal antennas less than a quarter wavelength long and verticals less than an eighth wavelength high are inefficient, narrow banded, or both. (Not shown. Use a Balun or Feedline Choke. Certain lengths of coax can be used as a conjugate match to match two unequal impedances and is required in SPECIAL cases but never in normal everday one-antenna systems. The longer the length of a piece of coaxial cable, the more signal is lost. Connect a 50-ohm antenna feedline coax to the ANT 1 jack on the back of the AT-100Plus, and optionally connect a second antenna feedline to the ANT 2 jack. The SMA connector is the coax connection. "But, coax is coax, isn't it?" "I use LMR-400 for all my stations at home. Some transceivers, amps and/or tuners do not like a balun installed. Ca= lculated the attenuation based on the loss and then used the SWR121 to te= st the same length of coax at the same freq. To reduce or get rid of this unwanted RF, use a "balun" transformer or RF choke which would consist of several turns on a 8 in diameter form. My lengths for 146 MHz target frequency was 3. The common configuration for the G5RV is ladder or window line for a certain length and then coax to the shack. size to the coaxial cable feedline, it is often made from a scrap piece of the same cable. >Sorry Eddie, Cable length does matter! >The avoid line radiation (nothing to do with swr) you should back the >complete system an even length. A “balun”, a device which blocks common mode currents from the feedline, is often employed. Another common misconception is that changing a feedline's length will change the SWR. • Try to use wire antennas and open/ladder line feedline. I got a set of plans for a Dual-Band J-Pole that works AWESOME!! I use the antenna listed in the plans and Get EXCELLENT SWR and TX/RX. 82 of some foam insulated coax on 144. This is true no matter how many series traps, isolating devices or systems, or common mode suppression devices, installed at the feedpoint. 1 ratio on the ol SWR meter you will need to use 70 ohm coax to feed the antenna. The second drawback is that you need to be careful of how you route balanced lines. In our example that would be a trim for about 14. I use a power divider consisting of a half wavelength of 35 ohm impedance feedline to match a pair of 50 ohm yagis phased on 2m to feed a long length of 50 ohm feedline to the shack. People seem to spend a lot of time messing with the length of antenna or feedline trying to get an acceptable SWR on multiple bands. You will also need to peel off the digit display protector with a slot screw driver. Certain lengths of coax can be used as a conjugate match to match two unequal impedances and is required in SPECIAL cases but never in normal everday one-antenna systems. I've also seen l14-inch hardline soldered to the driven element. The random wire antenna is probably one of the least expensive, easiest and cheapest HF antennas to use if you have a tuner and you want to get the "most" out of a length of "random" wire without having to pull out that calculator, doing the math, getting the center insulator built or bought, running the feedline, and all the rest that goes. Less lossy than coax. The type of feedline you choose can make a big difference on the the performance and range of your antenna system. 5Mhz and 30Mhz. From there it goes directly to a four output distribution amp (at least on paper). If you are building a “resonant feedline antenna,” where you are defining the active antenna element with an inductor on the feedline, then Q matters. Coax is so much easier in this regard- simply lay it across the ground, or perhaps bury it just slightly. Feedline Coils – Traps and Inductors. It is the signal lost during to and fro transmission between the radio and antenna. The required physical lengths are shorter as determined by the velocity factor of the feedline. 120’ of RG-8X Coax feedline @ 144mHz loss = 4. Inside the coax, the common-mode current is zero. 3, but they do have a bearing on the applications shown in Fig. But then, you no longer have a G5RV. Well, I wouldn’t say that it is absolutely critical, but if you want to avoid tuning difficul-ties on some bands, you shouldn’t stray too far from this length. 50 or 1 wave length of the center frequency of the antenna you are connecting to. SWR Feedline 450 Ohms. Inside the coax, the common-mode current is zero. The reason for 5/8 antenna. For hams, baluns are particularly useful to connect a dipole antenna, which is naturally balanced, to an unbalanced coax feedline. If line loss is low it is perfectly acceptable to make SWR measurements at the transmitter end of the feedline. Use a jumper wire if you have to. Herman, you may have common-mode current on your feedline (RF coming back to your radio on the coax braid). 4 feet) coaxial cable to connect it to the rpi. LMR-600 UltraFlex coax has a good deal weight to it. Coaxial cable commonly used for Amateur Radio has an impedance of 50 ohms while ladder line impedances can vary from 300 to 600 ohms. Reduce the gain control on the transmitter to a minimum, and place the transmitter in t tstandbyt. An apparatus has an improved antenna pattern for a cross dipole antenna. The no-feedline EFHW In the previous article featuring a standalone EFHW with autotransformer we confirmed the end fed half-wave antenna can function with practically no feedline. The reference impedance and the cable impedance show a 16:1 ratio. A feedline impedance of 50Ω and a load impedance of 25Ω produces an SWR of 2:1. 1 MHz at +10 dBm. In our example that would be a trim for about 14. Except that when the coax is used to make the line long enough to reach from transmitter to antenna the performance is obviously worse. Following are the benefits or advantages of Coaxial Cable: * Due to skin effect, coaxial cable is used in high frequency applications (> 50 MHz) using copper clad materials for center conductor. Coax length is not critical. There's 12 feet laying on the ground. To confirm more than 3 rooms, please call 1-888-444-OMNI (6664) and an Omni Hotels representative will gladly assist you. The common configuration for the G5RV is ladder or window line for a certain length and then coax to the shack. 5MHz; an 88′ dipole is 31% of a wavelength at 3. It is the quality of coax which makes the difference. 34 to 35 meters - 111 to 115 ft. They can be made to work but usually not by a HF beginner. To determine the proper length of coax, use the following formula:. However, it would be very difficult to route open wire feedline between my operating location and the attic, so I wanted to use a coaxial cable feedline. At one time, RG-58, RG-8X and RG-8U were military standards but now these terms are used rather loosely and refer primarily to the size of the cable. Amateur Radio transceivers are designed to work with an impedance of 50 ohms, so you must use 50 ohm coax, or find a way to convert the 300 to 600 ohm impedance of ladder line to 50 ohms. Caution must be exercised when connecting the RCS-4 to prevent damage to station equipment. Prepare split lead on coax in accordance with figures 2 and 3. Results were less than a te= nth of a dB difference. With the view to establish a quick and easy multi-band antenna deployment for portable and camping operations a simple long wire antenna with an earth or earth plus counterpoise arrangement with a 9:1 voltage unun including a tuner or simply with a tuner is one possible solution. Place the RT 100 near the feedpoint and the virtually eliminate all feedline loss due A dipole antenna designed for one frequency can function over a broader frequency range with a manually or automatically adjusted antenna tuner. The coax shield is grounded at the house entrance. Inside the coax, current I1 flows on the outside surface of the centre conductor, and current I2 flows on the inside surface of the shield. The calculator now shows that for the length of coax the loss is 0. The plots were calculated using the "high accuracy" (NEC Sommerfeld) ground model with a. For hams, baluns are particularly useful to connect a dipole antenna, which is naturally balanced, to an unbalanced coax feedline. What kinds of antennas should the newcomer avoid? First off, avoid the shortened all-band antennas. However it will take whatever your feedline can take and you can throw into it. Space constrictions prevented the 80m extensions from being longer than 3m. For maximum power transmission, 50 Ohm coax cables are used to match the RF PA's output impedence. The great Ed Humphries, N5RCK, has some good plans for a fold-up Jpole antenna using nothing more than that flat, twin-lead, 300-ohm "TV antenna" wire. For VHF 2M, the center frequency is about 146MHz. (See decoupling information, page 12). Side view of the almost terminated antenna - it's only missing the top cover. 50 or 1 wave length of the center frequency of the antenna you are connecting to. Disadvantage of this last, it is harder to bend. Proper grounding is essential for lightning protection. For coax cable as transmission line the the loss [dB] in dB per meter between antenna is multiplied by the cable length, plus adda few tenth of a dB for each connector used. A common thought of many is since this is coaxial cable, we need velocity factor adjustments. HF-Radio - authorSTREAM Presentation. Examples are RG-8 (thick), RG-213, 9913. Whatever antenna you chose, if it is fed with coaxial cable you should use a choke balun. I decided on 37 feet of 300 Ohm ladder line as an acceptable non-resonant length for the feedline. R&L Electronics: Text Questions to 513-868-6399: 800-221-7735: Virus Update We are open for Curbside Service and Shipped Orders. Buy Coax Cable Connector - Coaxial Cable Connector - Antenna Cable - RG8X Coax Connector - Coaxial Cable 50 Ft - RG8X Coaxial Cable - UHF Antenna Cable - Male To Male Cable - 15. impedance to more closely match 50 ohms. The cable cut-off length is 80 1/4". Well, I wouldn’t say that it is absolutely critical, but if you want to avoid tuning difficul-ties on some bands, you shouldn’t stray too far from this length. Herman, you may have common-mode current on your feedline (RF coming back to your radio on the coax braid). In case you wish to use a reflector, cover pieces of cardboard with tin foil and attach to the plank. These may include wires, towers, down spouts, screened porches, feedline from other amateur antennas or TV antennas, metal pipes on your property or adjoining properties. It is usually specified in dB/foot and is a linear relationship. In the absence of common-mode currents, then the optimum feedline length is 0, because a longer feedline only increases your feedline losses. After an additional 7ft 8. LMR-240UF coax is higher-loss but well shielded and lightweight, making it useful for the vertical run up lightweight masts. I am using LMR-600 UltraFlex coax for the rotator loop and feedline connection to this antenna. The gain on a dipole is reference or ZERO. Side by side, it is easy to see the difference between RG174, RG58 and other RG 213 coax cables. “Knead” (squeeze and press) the Coax Seal™ again to assure a perfect seal. Coax length is not critical. Ca= lculated the attenuation based on the loss and then used the SWR121 to te= st the same length of coax at the same freq. I expect to use three dipoles (one for each band), all coming together in the middle and fed with a single feedline through a 1:1 W2AU-type balun. Feedline is 450 Ohm "window" ladder line down through an aluminum framed window into a MFJ 949E tuner into my IC-730 and RigBlaster Plus/desktop PC. The only cure is a very good common mode choke at the feedpoint. ⚫ install common mode chokes on the coaxial feedline and preamp power cable ⚫ bury cables about 12 inches deep for optimum null depth ⚫ Avoid re-radiated signals from nearby antennas and power lines ⚫ locate the antenna as far as possible from other antennas and power lines The “Magnetic” Loop is a specialized antenna. I just did a test myself where I wrapped a length of hookup wire spirally around a 6 foot length of RG-174 and then connected the hookup wire to the input of a spectrum analyzer. The two terms generally mean the same thing. A "balun", a device which blocks common mode currents from the feedline, is often employed. When mounting an antenna on the roof or in the attic be sure to use quality feedline (coax). At lowest swr/ reactance the frequency displays the 90 degree frequency under test, Simply shorten coax carefully in small increments until you are at the degree/ frequency you need. Another common misconception is that changing a feedline's length will change the SWR. For VHF 2M, the center frequency is about 146MHz. 75ohm coaxial cable (cable TV) for the descent to the issuer. This can throw off the antenna's radiation pattern, affect tuning, etc. In our example that would be a trim for about 14. In my opinion, 1/2" Heliax is the minimum acceptable feedline--although I would try to keep the length of the feedline as short as possible. current will flow along its length. After the war, amateur radio operators began using the readily available surplus coaxial cable for their antenna feedline systems. For 2 meters. So I built one for the new 60 meter band. Deliberately use an oddball length such as 11 feet, 13 feet, 20 feet. 706 m shorter than the coaxes # 1, 4 feeding the outer dipoles. Reduce the gain control on the transmitter to a minimum, and place the transmitter in t tstandbyt. How much energy will we lose if we use this cable as a feedline? We can arrive at some answers by using a program like TLW. Avoid routing the line parallel and close to other conductors or structures for any significant distance. Detail of feedline connection to the 900-MHz Cheap Yagi. Don't let it become pinched and avoid sharp bends. At this point the center conductor is extended with a 7ft 8. You may attach an alligator clip to the plastic on the top of the antenna in order to easily hang it. But, in this case we want the feedline to radiate! Extra Radiation. Coaxial cable consists of an inner conductor with an insulated covering (dielectric), which is then covered with a braided wire or foil sheathing (shield). The coax is not just feedline: it’s an intimate part of the antenna! Furthermore, as we’ve seen, it’s connected back through your electrical wiring to your neighbor’s dimmer switch. You can figure that you will lose some of your signal in the coax going up to the antenna, and you will lose some of your received signal strength coming back down when the antenna is used for receiving. The cable is connected to two wires coming from the antenna and carrying currents I4 and I5. A feedline impedance of 50Ω and a load impedance of 25Ω produces an SWR of 2:1. coax lengths to reach them. For a 100' length of 600 ohm, homebrew feedline, you'll spend about $36-$40 for spreaders (IF you can still purchase them) and roughly $65-$80 for a 500' roll of 14ga stranded wire. EXE program approximates the optimum feedline lengths given the length of a horizontal dipole and the velocity factor of the. The total length is given by 468/f where f is the frequency in MHz. Examples are RG-8 (thick), RG-213, 9913. Choose feed line that is designed for little signal loss when setting up for high frequency emissions (such as RG-213 coax). RG-8/U coax is recommended. 5MHz; and; a 40m half wave is 24% of a wavelength at 3. If you are building a “resonant feedline antenna,” where you are defining the active antenna element with an inductor on the feedline, then Q matters. To build a good one that will handle high power. of 66%, so we do this. For the best efficiency, the long coaxial cable run should be from Point A to Point B, keeping the influence of the antenna impedance mismatch from affecting the coaxial cable feeder. You will also need to peel off the digit display protector with a slot screw driver. Fine-tune the settings and enjoy the show. These losses are not significant in the short line length used in Fig. Compendium #6) that losses soar (much higher than coax) when they're wet. 5 MHz, the traps are far from their resonance frequency and artificially increases the length of the antenna, which still resonates half wave. This is the basis for a vertical or Marconi antenna--a quarter wavelength antenna that relies on a ground. Maximum efficiency and absolute minimum feedline radiation will occur when the antenna is tuned as in the steps above. 7dB loss in the entire length! Since hardline is so rare in the amateur radio and land mobile radio world, in this article we are going to focus on coax and corrugated copper feedline. Use fingers only to tighten. As stated above, it is especially a problem at the point where the balanced feedline meets the unbalanced 50 Ohm feedline used with our transmitters. Find the inductance of a 1 m length. Avoid the use of adapters as much as possible and try to use the shortest amount of coax as possible. Deliberately use an oddball length such as 11 feet, 13 feet, 20 feet. For instance, to work 40m be sure the antenna is at least 10m or 33' long. Well, I wouldn't say that it is absolutely critical, but if you want to avoid tuning difficul-ties on some bands, you shouldn't stray too far from this length. picture on the first page, with the feedline sloping down at a 45 degree angle. Never center feed a full-wave antenna with any feedline close to a multiple of a halfwave long. You may attach an alligator clip to the plastic on the top of the antenna in order to easily hang it. With this knowledge in hand, we can avoid feedline lengths that increase feedline currents. I use a power divider consisting of a half wavelength of 35 ohm impedance feedline to match a pair of 50 ohm yagis phased on 2m to feed a long length of 50 ohm feedline to the shack. Arrange the Coax Feedline so that it exits the antenna at a right angle. Shield attaches to the short section feed point. For fixed stations, coaxial hardline is probably the best choice. It is best to use coax in multiples of. This is true no matter how many series traps, isolating devices or systems, or common mode suppression devices, installed at the feedpoint. Some examples: a G5RV is 35% of a wavelength at 3. 080 m) makes 360 degr. the G5RV behaves no differently from any other random-length, random-feedline antenna. Lightning arrestors are small devices that are attached inline of your coax feedline. Use fingers only to tighten. In the absence of common-mode currents, then the optimum feedline length is 0, because a longer feedline only increases your feedline losses. Many companies often call the same item different names to confuse you or make it difficult to compare the same item between companies. • All shields are to be connected on same side 73, Hartmut, DG7YBN. RG-8/U coax is recommended. What kinds of antennas should the newcomer avoid? First off, avoid the shortened all-band antennas. RG-8X is popular these days because it is light and easy to handle. This cable is readily available, easy to install, and will perform well for rooftop runs of up to 60 or 70 feet at 50 MHz. 66 MHz, or with 0. 35" for the 50 ohm coax (VF: 66%). the supplied coax jumper cable, or a similar 50 ohm coax cable rated 125 watts or higher. There are many designs to chose from. Typically, the impedance of the radio and the coax feedline are the same and constant (e. * The higher the frequency, the more signal loss. At least once a week we receive a request a for high ratio balun (6:1, 9:1, 12:1) to manage the transition from high impedance ladder line / open wire feedline to coax. Connect a 50-ohm antenna feedline coax to the ANT 1 jack on the back of the AT-600Pro, and optionally connect a second antenna feedline to the ANT 2 jack. The terminals are connected with 50 ohm coax pieces from the bottom terminals to the top where they are connected to the feedline from the transceiver. 2:1 or better. acknowledged, but that doesn't necessari- ly mean that your Stray will be appearing in QST. 5MHz; and; a 40m half wave is 24% of a wavelength at 3. These may include wires, towers, down spouts, screened porches, feedline from other amateur antennas or TV antennas, metal pipes on your property or adjoining properties. EXAMPLE A coaxial cable has a capacitance of 90 pF/m and a characteristic impedance of 50. Do not substitute an analyzer for this step. The left and center pictures show an L-coupler of length lC of the type used for the multiple resonator devices and single resonator devices, respectively. >So, let say you are using RG-8 >It has a vol. Results were less than a te= nth of a dB difference. • All bands are connected together via the coax links and are all fed by a single feedline. Within a couple of months after installation, the antenna system was already exposed to several storms with 100 km/h gusts - no damage to any of the parts! I am not using coax feedline, but 450 ohm "window" line. Suppose a plot is made of the antenna length desirability for one side of a half wave dipole for 80 metres. matching of an antenna feedline that has not been matched previously. Page 7 Figure 6 Balun and Feedline Connections Recheck all mechanical connections for tightness NOTE: For operation at power levels above 500 and seal with Coax-Seal® or some similar watts, construct a balun using RG-213/u coaxial substance. Buy Coax Cable Connector - Coaxial Cable Connector - Antenna Cable - RG8X Coax Connector - Coaxial Cable 50 Ft - RG8X Coaxial Cable - UHF Antenna Cable - Male To Male Cable - 15. These losses are not significant in the short line length used in Fig. Since the lightning will also follow the coax into your radios, you need to also isolate the coax from the antenna. This is derived for the 2M VHF of amateur allocation, 144 MHz - 148 MHz. For VHF 2M, the center frequency is about 146MHz. Feedline Lengths: The feedline length is also an important part of a "friendly" antenna design. Additionally special care was taken when the feedline was routed through windows to get it into the house. Non-resonant antennas can produce a complex radiation pattern with sharp peaks and nulls, e. For Series 6 cable, typical loss values are as follows:. Disadvantage of this last, it is harder to bend. This should couple quite effectively to a nearby cell phone on the ground, whilst the Yagi up a tree would improve on both transmission and reception. The common configuration for the G5RV is ladder or window line for a certain length and then coax to the shack. Having 30 feet of coax coiled up in a corner will reduce the effectiveness of the antenna dramatically. The way I understood it to tune dual Wilson antennas you take a single coax of equal length and check SWR (and make adjustments if necessary) on each antenna seperately, then hook the dual coax up and away you go. EXE program approximates the optimum feedline lengths given the length of a horizontal dipole and the velocity factor of the. Excess coax between your radio and antenna mount should never be wound into a circular coil of less than 12" in diameter. So, why is the selection of feedline so important? Fir. of the antenna employ 50-Ohm coaxial cable as the feedline. Coaxial cable commonly used for Amateur Radio has an impedance of 50 ohms while ladder line impedances can vary from 300 to 600 ohms. This cable is readily available, easy to install, and will perform well for rooftop runs of up to 60 or 70 feet at 50 MHz. lower on all bands, due to the additional loss of the coax feedline, which is 25m of RG213. How much energy will we lose if we use this cable as a feedline? We can arrive at some answers by using a program like TLW. There can be some RF though the balun and a not mentioned common mode choke will knock it out. For the best efficiency, the long coaxial cable run should be from Point A to Point B, keeping the influence of the antenna impedance mismatch from affecting the coaxial cable feeder. By letting it do so, the outer shield of the 22 feet long vertical coax (RG-8X) radiates to fill in the gaps in the signal pattern radiated by the top portion of the antenna. The great Ed Humphries, N5RCK, has some good plans for a fold-up Jpole antenna using nothing more than that flat, twin-lead, 300-ohm "TV antenna" wire. • If you are installing a choke/isolation balun in your coax feedline at the equipment end of your feedline, it is good practice to use a short coax jumper of 3 to 5 feet in length between the balun and any piece of equipment. Coax Feedline Cable Installation Procedure: The Antenna Coax Feedline is a 20’ length of RG59 Coaxial Cable. While studying this question enroute to writing an article on "A Slightly Off-Center-Fed Diople" in QST for September 2019, I learned that connecting the feedline to the antenna at a current node will essentially prevent the antenna from taking power at that frequency. a 50 ohm coaxial cable to your transmitter. Connect a 50-ohm antenna feedline coax to the ANT 1 jack on the back of the AT-100Plus, and optionally connect a second antenna feedline to the ANT 2 jack. 67' inverted V dipole with the apex at 30'. 35" for the 50 ohm coax (VF: 66%). The Antenna Coax Feedline is a 20' length of RG59 Coaxial Cable. Coaxial Length Calculator Version 5. At one time, RG-58, RG-8X and RG-8U were military standards but now these terms are used rather loosely and refer primarily to the size of the cable. I decided on 37 feet of 300 Ohm ladder line as an acceptable non-resonant length for the feedline. It allows you to compute the effects of loss through different types of coax with any frequency, line length, or power level. At the center, a 5/8" hole allows you to mount a through-chassis coax UHF connector. The feedline connects to the Antenna Connector on the assembly. There is only one rule for coax length: Use the shortest practical length possible to go from radio to antenna. My biggest concern was with feedline length efficiency issues due to impedance mismatches and RF losses. HF-Radio - authorSTREAM Presentation. (260m) and about 1000 ft. You should have insulators at the end and the center to prevent arcing at higher power and interaction with the objects at the end. For a 100' length of 600 ohm, homebrew feedline, you'll spend about $36-$40 for spreaders (IF you can still purchase them) and roughly $65-$80 for a 500' roll of 14ga stranded wire. The higher the frequency of the signal passing through the coaxial cable, the higher the loss over a given length. 12: Close-ups of the installation with the feedline spacers. 5dB Measurement At transmitter: SWR is 2:1 (11%loss) not great, ok At antenna –using the graph here SWR at the antenna is 20:1 ! Not good at all Cost another 7dB in mismatch loss Result 1/10 of your power is being radiated. Use a "liquid" (plastic) electrical tape product to seal the coax connector rear end--and the coax junction once you install the feedline. One end is at low impedance, so. Inside the coax, the common-mode current is zero. It is caused by the same situations that cause common mode current in coax transmission lines. RG-8X or Super 240 coax is recommended. Amateur Radio transceivers are designed to work with an impedance of 50 ohms, so you must use 50 ohm coax, or find a way to convert the 300 to 600 ohm impedance of ladder line to 50 ohms. This line should not be attached directly to the mast. Additionally special care was taken when the feedline was routed through windows to get it into the house. Disadvantage of this last, it is harder to bend. Three 8 Oz fishing weights for launching into trees. For longer runs, use an ultra- low loss cable, such as Belden 9913, to reach the antenna site, then use RG-8X. The plots were calculated using the "high accuracy" (NEC Sommerfeld) ground model with a. 64 for each leg). Feedline loss. - On 144MHz you can get away with just using equal PHYSICAL lengths of phasing lines (assuming you are using the same velocity factor of cable for all cables). Its challenger is the famous Belden 9913 , more expensive but very well protected against RFI and well adapted to very high frequencies. The following DOS IMAXGRAF. NOTE: Weather seal the coax fitting by covering it with neoprene or resistant compound. install common mode chokes on the coaxial feedline and preamp power cable bury cables about 12 inches deep for optimum null depth Avoid re-radiated signals from nearby antennas and power lines locate the antenna as far as possible from other antennas and power lines The “Magnetic” Loop is a specialized antenna. If A Longer Cable Length Is Required, The User May Add Standard Rg-58 A/U Cable Between The Radio And System Cable, Preferably In Lengths That Are Multiples Of Nine Feet. The weight saving is massive!. To reduce or get rid of this unwanted RF, use a "balun" transformer or RF choke which would consist of several turns on a 8 in diameter form. Conventional cross dipole antennas exhibit nulls in their antenna patterns, which can cause antennas to deviate from a standard or specification. I allowed about 3 feet for "feedline" (to the left of the choke). Following are the benefits or advantages of Coaxial Cable: * Due to skin effect, coaxial cable is used in high frequency applications (> 50 MHz) using copper clad materials for center conductor. See the article by K3BC on the Bibliography page for an example of a balanced feed arrangement. Improper termination of cable shields within equipment (the Pin 1 problem) injects. The easiest way to obtain more gain is to take your basic dipole and extend its length to about 1. So, why is the selection of feedline so important? Fir. With openwire feedline we may also incur common mode current. When mounting an antenna on the roof or in the attic be sure to use quality feedline (coax). The coaxial feedline is attached as follows: center conductor to the hook of the "J" pole and shield to the opposite side of the "J" as shown in above. • Try to use wire antennas and open/ladder line feedline. Here is a list of favored Dipole lengths:* 40 to 41 meters - 132 to 135 ft. It allows you to compute the effects of loss through different types of coax with any frequency, line length, or power level. d) Treat the cable gently when routing it through the vehicle. 91 450 Ohm Ladder Feedline Note: Avoid close proximity between the ladder line and any metal object (at least 3-6 inches) and do not lay the excess ladder line on the ground!). At the bottom of the mast I made extra room for the feedline to avoid stress and potential damage. This may not seem much of a loss but look at the Power Compensatoragain and you will see that 100w from your transmitter would have to be increased to 108. Reduce the gain control on the transmitter to a minimum, and place the transmitter in t tstandbyt.   This is a common misconception and when using a loop, doublet or double extended Zep (and several others) for multiband operation will result in "operational frustration". Suppose that we connect a typical coaxial cable to the feedpoint and use 100' of the line to reach the shack. So I built one for the new 60 meter band. The dipole coaxial line has the jacket removed from a central portion thereof to expose the outer conductor which is there severed and spread apart to form a gap exposing the dielectric layer, the lengths of coaxial line on each side of the gap comprising dipole. A “balun”, a device which blocks common mode currents from the feedline, is often employed. Again, simply avoid those resonant lengths. The coax used is RG-174. Insertion loss is also an important parameter. If you connect a coax feedline straight up to a dipole antenna, the shield of the coax absolutely will radiate. The SMA connector is the coax connection. Avoid routing the line parallel and close to other conductors or structures for any significant distance. This line should not be attached directly to the mast. The SWR will remain 2:1 even if the feedline length is changed. If you run into matching problems on some bands, try another balun ratio (e. Connect a 50-ohm antenna feedline coax to the ANT 1 jack on the back of the AT-100Plus, and optionally connect a second antenna feedline to the ANT 2 jack. Page 7 Figure 6 Balun and Feedline Connections Recheck all mechanical connections for tightness NOTE: For operation at power levels above 500 and seal with Coax-Seal® or some similar watts, construct a balun using RG-213/u coaxial substance. Recommended feedline for the 64DX antenna is. At lowest swr/ reactance the frequency displays the 90 degree frequency under test, Simply shorten coax carefully in small increments until you are at the degree/ frequency you need. The trick now is to find a length which is close to an odd multiple of ¼ wavelength on all bands of interest. If you use coax, check a coax loss calculator to understand impact of coax loss on your system. Search Results for Cable, Coax, Wire. * The higher the frequency, the more signal loss. The truth is, if the antenna. Leads to the element are short and direct. using the supplied coax jumper cable, or a similar 50 ohm coax cable rated 600 watts or higher. com The random wire antenna is probably one of the least expensive, easiest and cheapest HF antennas to use if you have a tuner and you want to get the "most" out of a length of "random" wire without having to pull out that calculator, doing the math, getting the center insulator built or bought, running the feedline, and all. Suppose that we connect a typical coaxial cable to the feedpoint and use 100' of the line to reach the shack. If you need to add feedline to this antenna, feedline lengths near 65 feet, 130 feet, and so on, should be avoided for ease of tuning on the 80 meter band. So overall length should be 468 / 5. Total feedline (assuming LMR-400 coaxial cable), right around 300 feet. Don’t forget On most of the J Pole designs out there, a choke should be used as close to the feedpoint of the antenna as possible to help prevent rf on the feedline and creating difficulty with SWR readings. 50 or 1 wave length of the center frequency of the antenna you are connecting to. They can be made to work but usually not by a HF beginner. The easiest way to obtain more gain is to take your basic dipole and extend its length to about 1. What is the correct length of coax to use between your transmitter and aerial at VHF, UHF and Microwave bands, or should there be a correct length?. Installation of the RF suppressor - Remove the existing ground wire and replace it with a length of RG-8 coax transmission line, enough to reach the ground rod and into the shack to connect to the ground bus. Feedline radiation can be reduced by placing toroids on the feedline at the antenna. This can often be corrected by using 18 feet lengths of high quality coax. Each end of the outer shield of the stub is connected to the feed system. Less lossy than coax. If I slope the feedline to my house entry point, the 450 ohm feedline length will need to be about 100-110’ which should keep it non-resonant for any HF bands. At high frequency, as gets larger, the values of R and G becomes less significant in comparison with L and C. com FREE DELIVERY possible on eligible purchases. I was also concerned that most folks do not have the experience to correctly install SMA connectors onto the coax (which requires expensive specialty tools). I have tried this approach and found that it usually works better on some bands than others. For the best efficiency, the long coaxial cable run should be from Point A to Point B, keeping the influence of the antenna impedance mismatch from affecting the coaxial cable feeder. This practice is often applied when, in fact, it might be more efficient, or even more convenient, to use balanced open wire lines. The cable cut-off length is 80 1/4". The height of this dipole should be at least 1/8 wavelength to 1/3 wavelength directly above the reflector wire. * The higher the frequency, the more signal loss. 698 71/90= 7. It allows you to compute the effects of loss through different types of coax with any frequency, line length, or power level. You can figure that you will lose some of your signal in the coax going up to the antenna, and you will lose some of your received signal strength coming back down when the antenna is used for receiving. You need to avoid getting it too close to metal or laying it on the ground. I am on the third floor of a building (a total of six floors), the antenna is placed on the southern balcony. The AT-100Pro is designed to interface directly with many popular ICOM and Yaesu transceivers,. One 40 foot RG-58C/U coax feedline with PL-259 connectors Two 20 foot RG-58C/U transmission lines, one with BNC and the other with PL-259 connectors 3 each nylon heaving lines / halyards. Lower-loss, double-shielded coax, such as LMR-400UF is best for the rest of the feedline (typically 25-50 feet). Coaxial Length Calculator Version 5. How much energy will we lose if we use this cable as a feedline? We can arrive at some answers by using a program like TLW. The terminals are connected with 50 ohm coax pieces from the bottom terminals to the top where they are connected to the feedline from the transceiver. We recommend an 8' x 3/a" copper ground rod. Cable Length. Feedline Coils – Traps and Inductors. “Knead” (squeeze and press) the Coax Seal™ again to assure a perfect seal. for 915 MHz and Table 2 for 9021903 MHz. At least once a week we receive a request a for high ratio balun (6:1, 9:1, 12:1) to manage the transition from high impedance ladder line / open wire feedline to coax. I spent a bit of time trying to devise a way to minimize coax related obstacles. Antennas radiate and so they all expose you to some RF,though this antenna ,being NOT a half wave on any current band tends to avoid the exposure within the operating area. However it will take whatever your feedline can take and you can throw into it. ' Usually, something that inhibits RF currents is placed at a strategic place on the antenna base or feedline. (20 meters), the wire length exceeds sixty feet for 40 and 80 meters. At one time, RG-58, RG-8X and RG-8U were military standards but now these terms are used rather loosely and refer primarily to the size of the cable. Coaxial cable commonly used for Amateur Radio has an impedance of 50 ohms while ladder line impedances can vary from 300 to 600 ohms. Such antennas desirably have an omnidirectional antenna pattern. Likewise v = 0. There are many rules-of-thumbs for lengths of dipole to avoid and lengths of parallel feedline to avoid. located near the antenna's feedpoint to prevent coax feedline re-radiation, disruption of the antenna's radiation pattern, sub-optimal vertical performance, and RFI in the shack. Here is a list of favored Dipole lengths:* 40 to 41 meters - 132 to 135 ft. In free air space, the wavelength (in feet), is 492 / Freq (in Mhz). For a 100' length of 600 ohm, homebrew feedline, you'll spend about $36-$40 for spreaders (IF you can still purchase them) and roughly $65-$80 for a 500' roll of 14ga stranded wire. The antenna will still perform very well, even with the feedline at this angle. By Todd, KY4TS A common overlooked part of putting together a repeater system is the selection of proper feedline. Additionally special care was taken when the feedline was routed through windows to get it into the house. In my search I've read suggestions of using 18 to 21 Feet of coax cable for a 1. Space constrictions prevented the 80m extensions from being longer than 3m. It's expensive, but long-lasting and has very low loss compared to conventional coax. Buy Coax Cable Connector - Coaxial Cable Connector - Antenna Cable - RG8X Coax Connector - Coaxial Cable 50 Ft - RG8X Coaxial Cable - UHF Antenna Cable - Male To Male Cable - 15. 5 and 7 MHz can be fed off the same feedline, allowing efficient operation on both bands. The coax shield is grounded at the house entrance. A "balun", a device which blocks common mode currents from the feedline, is often employed. Naturally, if these metal objects are short you can get closer than 30'. However, this conclusion would be warranted only for cases in which the coax end was either free or was isolated by some attenuation device to the degree that it acted like a free end. I didn't find there was much, if any interaction among them. Open-Wire Feedline Lloyd Butler - VK5BR. Avoid running the balanced Use coax for feedline runs inside your house. To in theory get a 1. In free air space, the wavelength (in feet), is 492 / Freq (in Mhz). That's because the feedline acts as an impedance transformer, and converts what might be an unmanageable im-. The limitation is ladder line lengths, not cut in multiples of half wave lengths of the bands used. To avoid problems matching or feeding any dipole antenna with high impedance open wire lines, keep the lines around these lengths. At the bottom of the mast I made extra room for the feedline to avoid stress and potential damage. Compendium #6) that losses soar (much higher than coax) when they're wet. thin coax like RG174, high loss versions of coax or cheap coax knockoffs, with just a few. Formula for feedline length based on freq. 9’ of 450-Ohm feedline was determined by equation, rather than by experimentation. Unless you model your own scenario in some detail, it is probably a good idea to strive for a dipole length of 35% of a wavelength or longer. The W8NX antenna, without the traps, is about the right length for an EDZ on 20 meters, but the high reactance requires parallel feedline to avoid losses. Currently I use a 5m (16. Understanding that the dipole and feedline will radiate, what are optimum feedline & dipole lengths to avoid TVI to the surrounding neighborhood?. Feedline - The length of 63. You can figure that you will lose some of your signal in the coax going up to the antenna, and you will lose some of your received signal strength coming back down when the antenna is used for receiving. Inside the coax, the common-mode current is zero. If you are interested in a copy, send me an e-mail!. Previous Work This antenna system is based largely on previous work done by amateurs for decades, using feedline segments of carefully-selected length for matching two feedlines. (See decoupling information, page 12). The "Best" Random Wire Antenna Lengths - Blogger. I often find the just changing the feedline length will correct that problem, along with any anamolies in the SWR curve that depart from theoretically predicted results. People seem to spend a lot of time messing with the length of antenna or feedline trying to get an acceptable SWR on multiple bands. 300W carrier it gets plenty HOT, but I will be listening on 160 and 80 and operating on 20 and 40, and I thought that the 4:1 balun would be easier and ladder line be a lighter feedline than coax. Transmit into the dummy load and check for 1:1 SWR on all bands. 698 71/90= 7. 66 VF a physical length of 37. If I slope the feedline to my house entry point, the 450 ohm feedline length will need to be about 100-110' which should keep it non-resonant for any HF bands. should try to buy quality feed line, avoid the small-diameter coaxial cables for runs greater than 50 feet up to 30 MHz, and seal the points where moisture can enter the line. The worst possible line lengths are shown in brackets: 160 meter dipole; 35-60, 170-195 or 210-235 feet. The longer the length of a piece of coaxial cable, the more signal is lost. Find the inductance of a 1 m length. A microwave antenna assembly is disclosed. The SWR will remain 2:1 even if the feedline length is changed. >102" whip and 102" coax x the volocity factor of the coax. matching Of an antenna feedline that has not been matched previously. After some arbitrary length a snap on choke is used to create the common mode choke. The limitation is ladder line lengths, not cut in multiples of half wave lengths of the bands used. Feedline Coils – Traps and Inductors. The fewer bands, the fewer high impedance regions to avoid. \lia%~h Tka-5 90om. The use of ladder line to feed half wave dipoles for multi-band use is preferred due to low transmission line losses due to high SWR's, much better than coax. matching of an antenna feedline that has not been matched previously. We recommend an 8' x 3/a" copper ground rod. Side by side, it is easy to see the difference between RG174, RG58 and other RG 213 coax cables. Since the lightning will also follow the coax into your radios, you need to also isolate the coax from the antenna. The no-feedline EFHW In the previous article featuring a standalone EFHW with autotransformer we confirmed the end fed half-wave antenna can function with practically no feedline. 2:1) then try moving the band switch up and down one position and repeating steps 2) - 5). Always cut feedline to the minimum usable length. * The higher the frequency, the more signal loss. To avoid problems matching or feeding any dipole antenna with high impedance open wire lines, keep the lines around these lengths. Maximum efficiency and absolute minimum feedline radiation will occur when the antenna is tuned as in the steps above. - On 144MHz you can get away with just using equal PHYSICAL lengths of phasing lines (assuming you are using the same velocity factor of cable for all cables). That should be good enough for a repeater, right?" Well, not necessarily. The output option determines whether it is an UNUN or BALUN. circuit occurs in the feedline. com The random wire antenna is probably one of the least expensive, easiest and cheapest HF antennas to use if you have a tuner and you want to get the "most" out of a length of "random" wire without having to pull out that calculator, doing the math, getting the center insulator built or bought, running the feedline, and all. For maximum power transmission, 50 Ohm coax cables are used to match the RF PA's output impedence. I was having a real bad time with the elevated radials of my 160m antenna=. To prevent the boom from sagging due to the weight of the coax, M2 Systems provided us with a custom boom truss setup which is shown installed on the test mast. I allowed about 3 feet for "feedline" (to the left of the choke). Coaxial cable loss is normally specified in dB loss per 100 feet of cable. Amateur Radio transceivers are designed to work with an impedance of 50 ohms, so you must use 50 ohm coax, or find a way to convert the 300 to 600 ohm impedance of ladder line to 50 ohms. Performance Figure 3 shows the far-field elevation plots for frequencies of 7, 14, 21 and 28 MHz. The way I understood it to tune dual Wilson antennas you take a single coax of equal length and check SWR (and make adjustments if necessary) on each antenna seperately, then hook the dual coax up and away you go. Choose feed line that is designed for little signal loss when setting up for high frequency emissions (such as RG-213 coax). ' Usually, something that inhibits RF currents is placed at a strategic place on the antenna base or feedline. A feedline should be chosen based on the length of the cable run and the frequency of the signal. The SMA connector is the coax connection. 25in of RG-174 the coax end is reached. It is best to use coax in multiples of. Previous Work This antenna system is based largely on previous work done by amateurs for decades, using feedline segments of carefully-selected length for matching two feedlines. The only difference is that the shield of the coax is grounded, and the ladder line is not, so it acts in common-mode to bring in and radiate induced RF. If you have to, the W2DU is a choke balun (beads around the feed coax) that mounts at the feed point; it works well, with minimal losses, since it is not a voltage balun. com FREE DELIVERY possible on eligible purchases. Excess coax between your radio and antenna mount should never be wound into a circular coil of less than 12" in diameter. It allows you to compute the effects of loss through different types of coax with any frequency, line length, or power level. Connect a 50-ohm antenna feedline coax to the ANT 1 jack on the back of the AT-100Plus, and optionally connect a second antenna feedline to the ANT 2 jack. At one time, RG-58, RG-8X and RG-8U were military standards but now these terms are used rather loosely and refer primarily to the size of the cable. 12: Close-ups of the installation with the feedline spacers. A short length of very low loss coaxial cable connects your transmatch to the RemoteBalun™. That's because the feedline acts as an impedance transformer, and converts what might be an unmanageable im-. This practice is often applied when, in fact, it might be more efficient, or even more convenient, to use balanced open wire lines. 8MHz to 30Mhz coverage. Less lossy than coax. Its challenger is the famous Belden 9913 , more expensive but very well protected against RFI and well adapted to very high frequencies. * The higher the frequency, the more signal loss. Wire Lengths for Various Band Combinations. com I wish more people would go out into the wild with a 9:1 balun and recommended length(s) of wire and also a resonant end fed half wave (EFHW) using a 64:1 type transformer and correct wire lengths for either a 40-10m or 80-10m version. 5Mhz and 30Mhz. ; T9B2 50 ohms is the impedance of the most commonly used coaxial cable in typical amateur radio installations. R&L Electronics: Text Questions to 513-868-6399: 800-221-7735: Virus Update We are open for Curbside Service and Shipped Orders. When calculating optimum lengths, we must use the velocity factor of the feedline. picture on the first page, with the feedline sloping down at a 45 degree angle. You'll lose a good 2 dB just from the ohmic losses, and even more if your antenna isn't very nearly a PERFECT impedance match to that of the coax - and that's figured at 14 MHz. TYPICAL INSTALLATION: Between one balanced-line antenna tuner and several balanced-line antennas. 2:1 or better. Avoid running the balanced Use coax for feedline runs inside your house. Side view of the almost terminated antenna - it's only missing the top cover. One rule-of-thumb suggests avoiding combinations of feedline electrical length plus one leg of the radiator that are odd multiples of an eighth-wavelength. As we discussed on the first page of this article, a length of wire or coax that is 1/4 wavelength long is an impedance inverter. Coax feedline chokes (and impedance transformers) all have an unbalanced input and are made with several output options dependent on whether the output is balanced or unbalanced. Avoid the use of adapters as much as possible and try to use the shortest amount of coax as possible. The ground wire is about 11 feet long. Side by side, it is easy to see the difference between RG174, RG58 and other RG 213 coax cables. Even a very short length of coax is sufficient to enforce this to a very high degree of accuracy. I was also concerned that most folks do not have the experience to correctly install SMA connectors onto the coax (which requires expensive specialty tools). However it will take whatever your feedline can take and you can throw into it. This will prevent the feedline from becoming part of the antenna which can cause all sorts of problems.
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