[FONT=Arial, Helvetica, sans-serif] [FONT=Arial, Helvetica, sans-serif][SIZE=-1]Why does my coax length affect the SWR of my antenna? [/SIZE][/FONT]
[FONT=Arial, Helvetica, sans-serif][SIZE=-1]How many of you change the length of your coax to tune your antenna? One of my good friends said to me, "I think changing the length of the coax is the same as moving the gamma rod adjustment on my Moonraker 4". Sorry to say, this is not true. As most people will find, varying the length of coax to the antenna will vary the SWR that the SWR meter is reporting. Actually, SWR should remain relatively constant no matter how long the coax is or where it is placed on the line (if its 5 feet down the coax from the radio or 50 feet down the coax from the radio). In most cases, the cause of inconsistant SWR meter readings is from poor SWR meter design or component aging / failure. For the SWR meter to read consistant SWR readings on the coax, the meter has to have an impedance itself of exactly 50 Ohms. Any deviation of the SWR meter's self impedance (from 50 Ohms) from poor design or component aging / error / failure will cause slightly inconsistant SWR readings when the SWR meters position on the coax or length of the coax is varied. In practice, generally you will find varying the coax length seemingly effects the SWR reading. Most SWR meters (built into radio and external type meters) and impedance "humps" in coax lines and connectors will cause minor variations in SWR as jumpers and coax length are varied. In reality, the mismatch at the antenna's feedpoint / coax junction is unchanged. Therefore - the actual SWR is unchanged. [/SIZE][/FONT]
[FONT=Arial, Helvetica, sans-serif][SIZE=-1]Another reason SWR could vary is from the situation where the coax is acting as part of the antenna. Not a favorable or normal situation. The signal is traveling back down the outside of the of the coax braid (note power should only be traveling on the inside on the coax braid). Therefore, the coax is part of antenna system and changing the coax length will change the SWR. This situation is more likely to occur in mobile installations. You can try to eliminate this situation (called "Common mode currents") by winding an "RF Choke". Wind about 6ft of RG-213 or RG-8 into a coil (6 to 8 turns). For RG-58 use 4ft with 6 to 8 turns. Wind the coax up, placing each turn right next to one another. Use electrical tape to secure turns together. You should place these as close to the antenna as possible. Right at the antenna coax connection point being optimum. Most times, you can verify that you have common mode currents flowing back down the coax by grabbing hold of the coax while transmitting and moving the coax around. You can watch the SWR waver by moving the coax while transmitting (don't speak into mic!). You have to do this with all the doors closed from inside the vehicle. SWR should waver, if you notice that SWR jumps rapidily between two values, you might have a intermitant (bad) connection in the connectors (PL-259s) on the coax. In most cases of "common mode currents", just grabbing the coax will cause the SWR to change. [/SIZE][/FONT]
[FONT=Arial, Helvetica, sans-serif][SIZE=-1] The "RF choke" described above stops the signal from traveling back down the outside of the coax. The signal inside the coax is * u n a f f e c t e d * by the choke (contrary to what you may have heard about coiling up excess coax). Common mode current kills antenna efficieny. You could have a decent SWR and not realize half your signal is being broadcast into you car (result very poor antenna performance). If your linear amplifier causes serious problems with your car's computer, lights, etc....you may have common mode currents. If moving the coax around the vehicle results in SWR change, this is a good indicator you have common mode currents flowing back down the coax line. [/SIZE][/FONT]
[FONT=Arial, Helvetica, sans-serif][SIZE=-1]This doesn't happen often with base station antennas. Most base antennas have some type of device that will decouple the antenna from the feedline (gamma match, balun, etc.). Make sure you run your feedling (coax) straight down from the antenna, taking care not to run close to antenna to prevent "common mode" currents which could still occur if coax is oriented in a way to pick up strong antenna signal. [/SIZE][/FONT]
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