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Antennas for Aviation Radio and VSWR

A VSWR meter is a device which allows you to measure the "transfer ratio" between your radio receiver and the antenna. It is the measure of how efficiently the antenna is able to transmit energy along the coaxial cable and into the receiver. A rating of 1 is perfect - you get all of the energy to the receiver, none is lost to problems, like reflections. The higher the rating, the greater the loss. A rating of 3 means that 25% of the "power" received at the antenna is lost as it goes down to the receiver. Basically, the rating of 1 is perfect, while bigger numbers imply opportunities for using a better antenna design.


(If you are very technically inclined, and want to learn more about VSWR analysis, I recommend this site: VSWR (antenna-theory.com)


I have a special type of antenna, which is called a 'J-Pole' mounted at roof height. I mounted it a long time ago and I remember that it was tuned for the higher end of the aviation band. Today, I put it on my VSWR meter and took a look at the results. I then compared that result to three other antennas down in my lab. It's all interesting and informative.


First, the roof mount antenna looks like this. I don't think it even reaches above the trees, which makes it even more interesting when you consider that aviation radio frequencies are limited by 'line-of-sight', and I've captured aircraft 218 miles away using SCARP and this antenna.



The coaxial cable is routed from the antenna, down to my basement workshop. I'm using RG-400, low loss cable, with a run of about 30'. When I attach this antenna and cable to my VSWR meter, it looks like this. There's a lot going on in this pic, but I'll point you to the only thing we're concerned about in a moment.


We're concerned about the VSWR number, and it is represented by the blue line which starts on the left side and ends up on the lower right side, with a "1" blue medallion. That line represents what happens to the VSWR as we sweep frequency from the left, 118.0 mhz, to the right, ending at 138 mhz. The blue line is at 1.34 at the very right side of the screen, (you can see it noted at the upper right side of the screen in digits). At the left side of the screen, the number is 8.00 (which you can't see called out on the screen, please trust me.)


Summary: the VSWR for my roof mounted J-Pole antenna varies from a high value of 8 @ 118 mhz to low value of 1.34 @ 138 mhz. The absolute low point is around 135 mhz, which is how I tuned the antenna, and very close to 1.

In the most practical terms, 1.34 is near perfection, while at 8, 60% of the received antenna power is lost. My antenna is great for 128 to 138 mhz, but non-optimal from 118 to 128 mhz.


For the next antenna to analyze, I wanted to look at a car roof mounted antenna which had been also tuned for aviation. My friend Neal loaned it to me.



And here is its VSWR plot.


This shows a VSWR ranging from 2 @ 118 mhz, then ending at 4 @ 138 mhz. As the user of this antenna wanted to communicate around 123 mhz, the design of this antenna is excellent. It would work very well with SCARP across the entire spectrum. Approximately 1/3 of the received power is lost at the higher frequencies.


**** to be continued ****



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