Coax Length Issues Simplified
Question: What is the "correct" length of coax?
Answer: The shortest length that makes it from the radio to the antenna.
Question: Are there any exceptions to the above rule?
Answer: 75 Ohm harnesses for Co-phasing is the only exception.
Question: Why do most mobile antenna makers recommend 18 feet of coax?
Answer: You got me, they claim you should use 1/2 wavelength multiples of coax. 18 feet isn't even close to being a 1/2 wavelength in any 50 Ohm coax you will find. Check some commonly used coax using the above formulas. RG-58, the most commonly used mobile antenna coax length would have to be 12 feet to be a 1/2 wavelength. RG-8X would need to be 14 feet.
Question: Ok, seriously nerd, when I trim my coax it changes my SWR. You can't tell me it's not good to lower my SWR from 1.5 to 1.2 by taking off a few extra feet of coax.
Answer: Hey, I'm not a nerd! Go ahead, change your coax length. If you change coax length and it affects your SWR in minute amounts, everything is working fine. If your SWR was 2.5:1 and putting in a 4 foot jumper brought it down to 1.3:1, this large change indicates you have real problems...i.e. common mode currents (see above). Really, you should be changing the antennas length to alter SWR. NO special length of 50 ohm coax is going to fix or lower your SWR signficantly and/or boost performance. Period.
Question: I notice when I change coax length, my "modulation" needle jumps more / harder / faster when I talk. I get more watts out of the radio (verified by a watt meter) with certain lengths of coax. Is there a certain length that will allow my radio to put out the most power?
Answer: No, there isn't a magic certain length that will do this. Certain lengths of coax will allow your radio to "see" a load that it can couple with better which results in more power out of the radio / amp. Unfortunately, there isn't a good way to determine what length you need to allow the radio to put out the max wattage its capable of. I am only refering to newer solid state radios with transistors. Old tube radios usaually have devices built into them to tune the radios finals impedance to match that of the input end of the coax. If you want to accomplish the same effect with your solid state radio, pick up a device known as a "antenna tuner". The term antenna tuner is misleading because it doesn't actually tune the antenna - or take the place of tuning the antenna - it simply lets the radio couple to the antenna system with better efficiency. Other more appropriate names for the antenna tuner are the transmatch or feedline flattener. If your SWR is low (below 2:1), don't expect to notice a (performance) difference from using an antenna tuner.
Question: Why do so many people recommend using 1/2 wave mutiples of coax? Will it really hurt me if I take the time to measure a 1/2 wavelengh multiple of coax?
Answer: The idea of using 1/2 wavelength multiples of coax comes from the fact that the antennas feedpoint impedance is "mirrored" at the input of the coax when using the said length. Many operator make / made the assumption that was a good thing because it was just like having the antenna hooked right to the radio / SWR meter. If anything other than a 1/2 wavelength mutiple is used, the impedance the radio / SWR meter sees is the antennas feedpoint impedance transformed to some other value of impedance. So, if your antenna has a feedpoint impedance of 25 Ohms and you use a 1/2 wavelength wave length of coax, the radio will "see" 25 Ohms in the input end of the coax. If you were to use some other length, say a 1/4 wavelength of 50 Ohm coax, the radio would "see" an impedance of 100 Ohm. What consequences does this have? None. Whether the impedance is 25 Ohms or 100 Ohms, the SWR is STILL 2:1. No matter what the 50 ohm line length, the resultant SWR is still 2:1...at the antennas feedpoint, at the input end of the coax and at any and every point along the coax line.
Many operators take half the truth of transmission line theory and make up their own rules. If you have been reading my page since its inception, you know I used to be "uneducated" when it came to transmission line theory. Sorry to admit I thought coax length was important. It was drilled into my head by somebody I respect(ed).
This isn't the easiest part of CB to wade through. Hopefully I've covered this with enough detail to set everyone straight. Many beginner amateur radio operators and students have misceptions and make false extrapolations in tranmission line theory. There are many conditions that must be stated when simplifying things. I have made one assumption here. I have been assumming the coax loss is negligible. At CB frequencies this is a pretty safe assumption to make.
Question: Ok then, why is the length of 75 Ohm coax line important? If coax length doesn't matter, why is 75 Ohm coax different?
Answer: I've heard this a question more than once. The fact your radio has a 50 Ohm jack on it is the reason you use 50 Ohm coax. The antenna is designed to have a feedpoint impedance of 50 Ohms. When all the impedances match, maximum power is transfered from the radio, through the coax, out the antenna. Using coax with a (characteristic) impedance of 75 Ohms can potenially transform the antennas feedpoint into another value..another value such that the resultant SWR will vary with line length. This function is handy for matching antennas to the feedline that do not have feedpoint impedances of 50 Ohms. This is beyond the scope of this section, but antenna makers can specify certain lengths (1/4 wave) of 75 Ohm coax to achieve the proper match between radio and antenna. Again, this form of matching is not possible with 50 Ohm coax.