Here are some viewpoints on Coax length, from people that know more about it than I do...
There is a misconception among CB operators that changing the length of your coax can "tune" your antenna system. This is absolutely untrue! This short discussion will detail why.
The coax cable is a means for transferring your RF signal to the radiating portion of the radio system. The cable, in theory, is meant to be a contained, non-radiating link. Because it does not radiate and serves only to transfer RF between two components of the transmitter system, it's performance in terms of efficiency is affected by length, but only in terms of overall resistance. In other words, using a long run of coax will reduce the total amount of signal at the antenna, but only because of loss due to resistance and NOT because of standing waves.
Ideally, you want to check the SWR of your antenna at the antenna feedpoint. In a perfect world, this is the best way. However, we all know that this is ludicrous to expect in a standard base antenna installation. Unless a remote SWR meter head is incorporated, we usually use the standard SWR meter located at the radio. The drawback is that resistance and slight impedance mismatch of the coax affects the overall SWR reading.
Because radio waves are tuned wavelengths of energy, we have to take into account the coax cable length. A typical 11-meter signal has a basic wavelength of 36 feet/wave. "Tuning" the coax for the exact full wavelength tends to throw off the SWR meter by not allowing any standing waves to return to the meter. Excess RF on the coax has been given an ideal medium by which to "hide" electrically from your SWR meter. That is not to say that the excess RF is not returning to the radio, you just can't see it on your meter.
What we want to do is create an environment where any excess RF (standing waves) are rendered as visible as possible to the meter. This is effectively done by using multiples of the 1/2-wavelength of the radiated signal. One half wave for the 11-meter band is 18 feet. However, this is not the length that you will cut your coax. There is another factor that affects the length. This is Velocity Factor. The velocity factor is basically a term for how fast the signal moves through the coax. This factor affects the overall electrical performance of the coax and thus needs to be accounted for when determining the true half wave length
Here are the velocity factors of the various Belden coaxial cables:
RG-59/U (foam) .79
RG-58/U (foam) .79
RG-8/U (foam) .80 9913 .84
Here is how to figure out your true 1/2-wave:
492 x (Velocity Factor) / Frequency (MHz)
For example, I want to figure out the true half wave coax length for RG-59/U (foam) on my home channel (ch. 33 - 27.335):
492 x .79 / 27.335 = 14.219 feet
Now add 14.219 to itself to determine your 1/2 wave multiples. Remember to use every other number. See the example below:
14.219 feet 1/2-wave multiple, 28.438 feet 1-wave multiple, 42.657 feet 1/2-wave multiple, 56.876 feet 1-wave multiple, 71.095 feet 1/2-wave multiple, 85.314 feet 1-wave multiple
and so on . . . .
Use only the lengths that fall on the 1/2-wave multiples and you will be all set.
Now in order to get the true SWR of the system, you have to throw away that 3-foot jumper cable for now. The SWR meter has to fall on a 1/2-wave point on the coax run. Using the example above, you need a 14.219 foot jumper from the radio to the SWR meter, and a 1/2-wave multiple length from the SWR meter to the antenna. If my antenna is 65 feet away from my radio, I need a 14.219 foot jumper from the radio to the SWR meter, and a 71.095 foot length between the SWR meter and the antenna.
In laymans terms, the coax length fools your SWR meter into thinking that your SWR is different to what it actually is !
The ONLY way to know what your SWR actually is, is to use a correct length of coax for the operating frequency
This article was provided to me by Firebreather, Thanks.
Visit his web site
UPDATE 22nd March 2000
I have since discovered this information, which I thought may be of interest to some people who want to know more about the calculations involved.
Since we know the signal makes a half a cycle every 18 feet as it travels through the air on CB frequencies, we can come up with a formula (using simple algebra) to give us the exact length of a 1/2 wavelength for any frequency.
(Channel 33 = 27.335Mhz)
(18 Feet = 1/2 wavelength)
27.335 X 18 = 492 <- There is where 492 comes from.
492 comes from multipling the frequency by the distance of a 1/2 wavelength.
Also here is a second formula for calculating a half wavelenght in inches
5904 / Frequency = 1/2 wavelength in inches.
This infomation was given to me by Scott (not me, another one)
Visit his web site
Your math says a 1/2 wave antenna for CB is 18 feet. That is an error. The CB band in the USA is 26.965 to 27.405 MHz . The center is ch.20 or 27.205 MHz. Cutting an antenna to 18 feet would be a perfect 1/2 wavelength at 26.000 MHz. 1.205 MHz lower than it should be.
To cut a 1/2 wave antenna for 27.205 MHz., take 468 divided by 27.205 = 17.2 feet. or 17 feet and 4 inches.
Your math only tells you what an exact measurement is in wavelength. But you can not cut an antenna that way. It will be too long.
(Also read Dave's comments on the Radio Math page)
This info was ent to me by Steve, thank you.
The SAME SWR ratio or measurement will be measured at ANY point along a feedline, regardless of the pyshical or electrical length of the feedline measured in feet or degrees (at the resonant frequency).
For reference, read Walter Maxwell's book named "Reflections" published by the ARRL a few years back.
It will dispell many myths about antenna systems, SWR, and the like.
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).
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.
Once the RF is removed from flowing down the outside of the line, the real SWR mearsurement can be taken.
Read Maxwell's book!
This was sent to me by Dave, thank you.
Any person who says that, "coax length has nothing to do with SWR's" has no knowledge or understanding of broadcasting. I have been in commercial and private radio broadcasting for 25 years, and it's time to dispel the myths about SWR and coax.
Your coax is a vital part of the antenna system and must be tuned for
Installing an SWR meter at the antenna will measure the SWR's (AT) the
antenna, but not the SWR's (OF) the antenna. Even with the meter connected
at the antenna, any change in coax type or length, will change the SWR
reading on the meter. But it doesn't make sense to check SWR's at the
antenna, because the most important component -THE RADIO- does not see the antenna alone. Your radio is reacting to the entire antenna system, including the coax. Even if you were to set the SWR's at the antenna and make them perfect, the SWR will be different at your radio's end of the coax.
Coax not only has resistance, but it also has capacitance which changes
with different lengths. Resistance (Impedance), determines how the
transmission line and antenna match the radio's requirement to send the
greatest amount of signal to the antenna. But capacitance controls which
frequency the antenna is best tuned at. Changing the capacitance of the
antenna system, will alter the frequency at which your radio sees the best
SWR. Any component you add to the coax line, such as a meter, linear, or
even a coax switch box, changes both the resistance and capacitance. My own radio, a Kenwood TS-570DG, has an SWR meter built in to it.
I also use an
external set of meters to keep tabs on my radio's performance. When I check
the SWR's on the meter in my radio, It shows a perfect SWR at one frequency.
When I check the SWR's with the external meters, they show my best SWR at
frequency almost one full MHz lower. So which meter is correct? They are both correct. My external meter is reading a perfect SWR at Frequency (X). That is because the resistance and capacitance of the coax and antenna is resonant at frequency (X). But
the meter in my radio is seeing the addition of a meter and a 6 foot coax jumper. This changes the resistance and capacitance of the antenna system that my radio is seeing. It also changes the frequency that the antenna system is now resonant at, so the meter in my radio sees a perfect SWR at frequency (Y) 1 MHz higher. Both meters are telling the truth, but the meter in your radio is the most important one to use for SWR. The meter in your radio is reading the SWR closest to your radio's Final output transistors, and that is more important than the SWR at the coax connector on the back of your external meters.
The following is for use ONLY on 11 meters.
For 11 meter operation, use these figures to set up your antenna and coax.
SORRY! I only know the measurements in FEET. Convert them to meters if you need to for better understanding.
For base stations. Cut your RG-8/U coax in multiples of
11 feet. For example.. if you need 50 feet of coax to reach your antenna,
use 55 feet
instead. 11 X 5 = 55. Or 11 feet multiplied by 5 multiples equals 55
total. If you need 37 feet, use 44 feet instead.
11 X 4 = 44. What ever length you need, go to the next highest length that can be divided evenly by 11. Include all jumper cables in that length. Jumper cables should be cut to 6, 9, or 12 feet. Never less than 6 feet, or more than 12 feet. Example. . . If you need 55 feet of coax to reach your antenna, and you want to add a meter in the line. Use 49 feet of coax from the antenna to your meter, and 6 feet from your meter to your radio. 49 + 6 = 55. If you find that 49 feet is too short from the antenna, then add 11 feet to the coax from the antenna to the meters, making your total length, with jumper cable, 66 feet. 11 X 6 = 66. If later on you decide you want to add another device like a linear in the line, do it this way. Do not just add a jumper for the new device. Where your main coax line from the antenna ends inside your home, cut off 6 feet of the coax, and put antenna connectors on each side that was cut, and add your new equipment at that point. The total length of coax stays the same, and the jumper is also the proper length.
For set up on 11 meters in your automobile, use RG-8X or RG MINI coax. Do the same as I instructed for base station installations, except this time cut your RG-8X to 20 feet minimum. Do not use less than 20 feet. Even if you only need 1/2 of that length, still use 20 feet. The extra coax can be coiled up and put in the trunk or under the seat. If it is possible, try to coil the extra coax as close to the antenna as you can get it. If not, the trunk or under the seat is good. It will not cause any problems. A length shorter than 20 feet can cause RF feedback, because of the antenna being so close to the radio. Also, mobile antennas are usually 1/4 wave or less.
This causes more RF to travel down the coax to your radio. A coax length of 20 feet, and coiling the extra coax under the seat helps to stop some of that RF from feeding back into your radio.
I hope I have helped you better understand SWR and antenna systems.
Take care. Dave in U.S.A.
This counter shows the number of hits since the 17th January 1999
Go Back To The Technical Pages
Go Back To The Home Page
Fill Out My Feedback Form
Go Back To The Main Home Page
Copyright © RadioMods 1997-2016