The gentleman who has done the bulk of the work on getting our GMRS network up and running is a bit flummoxed by antenna theory. That’s not a surprise, it’s weird and wonderful stuff sometimes and there is a lot of misinformation out there. I answered some questions for him in my usual verbose style, which probably didn’t help matters but I liked the result so thought I’d share it here.
If you’re not an antenna geek, this probably will be boring and indecipherable. If you are an antenna geek, you’ll probably find this infuriatingly inaccurate. But that’s all the apology I’m going to make.
On 9/7/26 18:22, ----- ----- wrote:
Should I put the dummy load up 25 feet in the air where the antenna connection would be? That would seem logical to me to have at the same height as the antenna connection.
That shouldn’t actually matter. But what’s that saying?
“In theory, theory and practice are the same.
In practice, they’re not.”
There are, of course, issues of practicality. But I would endeavor to change as little as possible, and leave as much of the routing of the transmission line untouched as possible and just move the connector a few centimeters from the antenna connection to the dummy load. Split insulation inside coax (“if something can go wrong, it will”) can create a cable where even a gentle bend can give it an impedance bump but it will look just fine otherwise.
How can I tell how much energy is getting transmitted? I thought that was SWR. I assume in this case it’s multiplying the effects of the coax and the antenna. I guess I could test the antenna with a very small piece of coax, too.
The real answer to that question is difficult, but that isn’t the question you really want answered. The real question is how much power is getting transmitted in a useful direction. If you build an antenna that’s perfectly matched (1.00 vSWR) and has excellent transmission line but it radiates everything directly upward, it won’t do you much good unless you’re named Elon and have your own satellite constellation. (I jest; aiming at satellites is non-trivial.)
So one way to answer the question is to get a field-strength meter and take a bunch of measurements on the surface of an imagined sphere at least one wavelength in radius (though ½ wave is probably enough) and then do a whole bunch of complicated arithmetic. There weren’t even spreadsheets when I did it [very early 1970’s], and it was awful. But I’ll bet now there’s an app or website where you just plug your numbers in and it gives you the effective radiated power (ERP) along with a nice 3-D plot of where that ERP is actually going.
But I don’t think that’s really what you’re asking for, even if it is what you really want. I would be satisfied with putting an SWR meter at the feed point of the antenna and believing the power number it gives. For most antennas and low (≤1.5) values of SWR, that will be pretty close to your “real” ERP.
Maybe imagine trying to fill a jar with water from a garden hose. If you’re using a high-pressure nozzle, the water might squirt into the jar really fast but also it will bounce around in the jar and come back out the top. You’ll get wet. So you turn the hose down a bunch, and now there’s no bounce but it take forever to fill the jar. If you get the flow just right, the jar will fill quickly but not squirt back out until the jar gets full and ruins the analogy.
With mismatched impedance, not all the energy traveling down the transmission line line can be absorbed by the antenna. Some of it actually reflects back the line toward the transmitter. When it hits the transmitter, it can be absorbed where it heats up your output stage, or it can be reflected back again toward the antenna until the reflections back-and-forth ultimately serve to heat up the transmission line (which is, after all, never lossless).
(You probably know this, but for -----‘s entertainment, the same thing can actually happen with A/C power distribution lines. It’s just that the wavelengths are so incredibly long (60 Hz, say a velocity factor of 0.7) about 3,500 kilometers that you can usually just ignore reflections. Still, every once in a while someone (the US military, in my experience) will put a hundreds-of-km drop across the desert feeding a big inductive load and the nearside delta-wye transformers will just keep blowing out “for no reason.” Reflections, baby.)
So SWR at the transmitter tells you how much power you’re pumping into the transmission-line-antenna complex. SWR at the antenna tells you how much power is actually being radiated by the antenna (unless the antenna design allows heating and loss, but most don’t at our frequencies and power levels).
I purposefully bought a type N antenna due to line loss of GMRS frequencies but the dang connectors on all GMRS and UHF radios are the SO-239 (can’t remember the numbers of female and male connectors). So I bought a 259 to N type connector and put it on the repeater. I’ve always been curious about the losses there. My feeble research on that aspect seems like it would be very small with just one adaptor in the line.
I’ve only seen significant connector losses with defective connectors (usually wet or dirty insulation) or kilowatt power levels melting the connector components. Not our concern, fortunately.
I do have an infrared camera around here somewhere, but I have no idea if it’s sensitive enough to show heating in a cable or connectors at our modest power levels. Might be fun just to try it out sometime.
—2p