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I know that there have been many similar threads on this, but I still need clarification. At the moment, the only thing I have is a 500' roll of RG-6 and a (very) tight budget, and I should be getting this cheap VHF/UHF 25W mobile soon (mostly going to use it as a base though).

Question: Could I safely use this coax and radio safely? Would I have to do anything "special"? Would this vary with the type of antenna that I use?

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    $\begingroup$ First, do you have access to an instrument that can measure complex impedance and SWE such as a NanoVNA or MFJ Antenna Analyzer? $\endgroup$ – Josh Oct 10 at 20:27
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    $\begingroup$ Do you know how long a cable you'll need? $\endgroup$ – Marcus Müller Oct 11 at 10:22
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    $\begingroup$ Sell the RG6 Coax and buy what you need of something like RG8X. $\endgroup$ – K7PEH Oct 11 at 15:38
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    $\begingroup$ Yeah without a way to test a tuner or custom matching network my answer was going to be: 1) Make sure to cut it to 1/2 wavelength and just try. 2) Don’t do it. Just find some 50 ohm coax. $\endgroup$ – Josh Oct 12 at 15:24
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    $\begingroup$ @K7PEH RG-6 has less loss and greater power handling capability than RG-8X. In fact, RG-6 is about the same as RG-213, except for its impedance. $\endgroup$ – Mike Waters Oct 12 at 17:02
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Well, according to the spec tables in the manual the thing expects a 50 Ω antenna impedance.

Mismatch means a loss of signal energy. How much exactly depends on the length of the 75 Ω transmission line – this makes a system much harder to design predictably.


I wrote all the following (up to the next horizontal line), then stopped to wonder "what might be the antenna connector on this thing?", and then saw what it was. All I wrote doesn't matter to your problem, more in the general case (future readers), jump ahead to the next horizontal line

Three options here:

  1. Matching circuitry to match your 50 Ω transceiver to 75 Ω
  2. Buying 50 Ω cable
  3. Ignore the problem and hope that worst case, the transmitter would withstand its whole output power being reflected back at him if you happen to experience a full reflection due to the length of the cable.

Between 1. and 2., I'd go with:

  • If distance is short, buy cheap 50 Ω cable
  • If distance is long, add matching circuitry

in general, 75 Ω line can be made lower-loss at the same cost. If you know your spool of 75 Ω is "fresh" as in "no water has seeped in from the ends", it's probably wisest to simply match. The matching circuit for a single band (especially if it's as low in frequency as VHF) is easy (you'll find plenty of examples on the web), a series inductor and a shunt capacitor, and cheap.


here is where you'd jump if you just want to read the answer to your specific case

So, you'd technically need to match your transmission line to your radio's impedance, and/or vice versa.

But, looking at the antenna connector on your device:

SO-239 female

😨

That's SO-239, for a PL-259 connector! No way that's matched to any particular impedance. These connectors should have been dead since ca 1948, and really are sub-par. That means that no matter what the impedance of the electronics inside are, the connector ruins them. It's also definitely not designed to go as high as 480 MHz like your radio. By far not.

So, in other words, since these radios don't come with a datasheet that contains any performance / impedance curves to begin with, and just claim 50 Ω, but have a connector that doesn't even allow for that impedance, and in fact doesn't even allow for low-leakage operation at the higher frequencies your radio works at, at all, I wouldn't worry.

You might be losing a bit of power with your 75 Ω line, because the device might actually be maybe 60 Ω (I really doubt you can make this connector be as low as 50 Ω at 500 MHz, it's just too friggin large), but you might also be in luck and your device is closer to 75 Ω than the manual claims.

So, heck, just connect your 75 Ω line to this. It shouldn't work great if the datasheet was right, but should still work with a high probability, but the manual very likely isn't right, so here's to "best effort".

Would this vary with the type of antenna that I use?

Yes. Just as your transceiver, the antenna needs to be matched to your transmission line impedance.

In your case, you need to match your antenna to the 75Ω line, but that's often as easy as soldering in a capacitor and inductor and that's it. And again, you can probably just ignore all this, because we know so little about the radio that all things that we optimize might simply be going in the wrong direction.

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    $\begingroup$ I think you're making the situation sound far worse than it really is. Nearly every ham radio has an SO-239 on it, and it doesn't "ruin" anything. Sure, it's less than ideal, but "ruined" is a strong word. $\endgroup$ – Phil Frost - W8II Oct 12 at 16:32
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    $\begingroup$ Yes a quarter wave at 440 MHz (in polyethylene) is 120 mm, and the whole SO239+PL259 is probably only 40 mm, so even if the impedance is 40 Ohms it'll only make the overall impedance worse by a couple of Ohms. $\endgroup$ – tomnexus Oct 12 at 17:50
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Smith chart shows that 50 Ohm antenna (assumed perfect 50 Ohm real!) connected to 75 Ohm coax cable can result in impedances between 50 and 112.5 Ohm. SWR 1: 1.5. So this so far not mentioned possible solution in this thread may be usefull for your problem: for a single frequency, or for a small frequency band, the use of cable with a multiple of half wavelengths (electrical wavelength! I.E. calculate the physical length from the shortening factor of the cable!) can give or maintain impedance matching. Recommended is measurement with a NanoVNA to assure the impedance at the antenna connector before powering up the transmitter. PA0FSB

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