coax myths???

[K7MEM]

None of your questions have simple yes/no answers. There are too many conditions that apply. That is why I linked to the article by M. Walter Maxwell, W2DU/W8HKK, in my original post. Especially good reading is on page 11, which lists 27 true statements about antenna matching and SWR.

So basically for a single mobile ant, all that is needed is whatever length it takes to get from the radio to the antenna? Correct?

Also is it better to check SWR at the antenna or the back of the radio?

In general, yes, the length of the feedline should not matter. If changing the line length changes the SWR, then there may be something wrong with the installation. This could indicate poor connections, antenna currents on the outside of the feedline, insufficient ground, etc.

Assuming all of the connections are good, it shouldn't matter where you check the SWR. But there are several conditions where, measurement from the radio end can lead to false readings.

For example, long lengths of high loss cable will attenuate the power getting to the antenna. But it will also attenuate any reflected power caused by a mismatch. Because SWR is based on forward and reflected power, the reduced reflected power may make your SWR look better than it really is.

Martin - K7MEM

 

[JayMojave]

Wa1nic said:

"It is best to check the SWR at the antenna base, and not with an SWR meter but instead with an impedance analyzer."

No not really. For a mobile installation you would need to cut the coax near the antenna, install PL-259 connectors, and a double female SO-239 connector when finished. That's what I meant by a pain, it reply #18.

And the measured SWR at the antenna as compared to the radio end of the coax will be pretty much the same. So there is no advantage in measuring the SWR at the Antenna for most antenna installations.

I have measured SWR at the base of Base Station Antennas, and then at the radio end of the coax. The differences are the loss/attenuation in the coax as mentioned. Ive done this on new antennas designs or antennas that I have built to see what the loss's are ect.

But again the SWR measurement at the radio end of the coax is a valid and correct SWR that the radios see's when transmitting. And is a good test to on any antenna installation. It is not the total all antenna measurement but is good start in the antennas performance.

The impedance analyzers are great to use, I have a few of them. During a late rainy night I just had to have a Yagi Beam Antenna up and running for some rare contacts out of Alaska. My batteries died in my MFJ analyzer and I couldn't find the darn AC wall wart supply. I grabbed my old CB SWR Meter and after some major adjustments and cussing at myself for assembling the antenna way wrong, as my diagram said to assembly the antenna correctly ( I can out smart myself at times) the SWR dropped to just a needle widths of SWR. Cranking the tower all the way up and pointing the Yagi Beam Antenna North to Alaska I finally made a contact. So hang on to your SWR Meters. And keep extra batteries and the wall wart supply handy.

Jay in the Great Mojave Desert