Standing Wave Ratio - SWR
for your Antenna Subsystem
|The antenna subsystem is the most important part of your system. It's tuning can make you or break you. When checking SWR or Standing Wave Ratio, you will be hooking up a simple form of a directional watt meter. The meter measures forward power in the system and then compares it to the reverse or reflected power in the system. Instead of showing you power, the meter is calibrated to show the RATIO of coupling from radio to antenna system. If there is all forward power and no reflected power, the antenna system has a one to one (1:1) ratio with the transmitter. By checking the SWR, you will know if the antenna is too "long" or too "short" for the center of the desired band. You will also be able to evaluate your system performance and troubleshoot problems. For instance a SWR too high (over 3:1) all over the band can be an indication of a bad part or junction, poor ground or poor location. An SWR too low over the whole band (1:1) can indicate a low efficiency antenna or lossy component. Safe operation of your radio should be with an SWR of 2:1 or less. Most people would prefer to be 1.5:1 or less. A good quality antenna and proper installation (location very important) should easily result in an SWR of less than 1.5:1
|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 it 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:
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 = 14.22 feet
Now add 14.22 to itself to determine your 1/2 wave multiples. Remember to use every other number. See the example below:
14.22 feet 1/2-wave multiple
28.44 feet 1-wave multiple
42.66 feet 1/2-wave multiple
56.88 feet 1-wave multiple
71.10 feet 1/2-wave multiple
85.32 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.
Calculate the effective transmission power by subtracting reflection power (Pr) from forward power (Pf).
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