Will 75 ohm coax harm my VHF radio?

What is the design frequency of the antenna? Assuming the antenna can handle the power output of your transmitter and that your antenna is designed for the frequency range in which you will be transmitting, the RG-6 will not, in and of itself, harm your equipment. Your antenna and coax form a "system" with a characteristic impedance. Your rig will most likely has a design output impedance of 50-ohms. If the "antenna system" has a different impedance, you will have mismatch. However, host modern equipment is designed to automatically reduce output power to safe level when transmitting into such mismatched loads. Harm? No. Output loss due to inefficiency? Yes.

This is no big deal really, but here's the story. There are two places to think about this - your radio seeing the 75 ohm coax input, and the 75 ohm coax seeing a 50 ohm antenna. When a source of RF power sees a certain load, it will put a certain amount of power into it. Some funny things might happen with the output filter, but nothing too drastic.

Let's assume the antenna is 75 ohms, so the transmitter sees a nice clean 75 ohms at the coax input. So given that 75 ohms is higher than the 50 ohms the transmitter was design for, the transmitter sees a lighter load, so the power it puts out is lower. Think of the transmitter as a battery with a series resistor of 50 ohms, designed to have you put another 50 resistor on it to ground (the antenna), for a total load of 100 ohms. If the battery is 10 volts, this is E^2/R= 1 Watt. Now you're going to put a 75 ohm resistor where that 2nd 50 ohm one was, now the total battery load is 125 ohms, and with the 10 volt battery you have 100/125= 0.8 Watts. This is 1dB lower, or about 1/6 of an S unit on a receiver (6dB/S). Hardly noticeable.

Now lets look at the antenna end. The coax looks like a source of 0.8 watts coming from 75 ohms (the 50 ohm resistor back inside the radio doesn't matter now), and this RF signal sees an antenna that is 50 ohms. This is a mismatch, a 1.5:1 VSWR, which causes 14 dB of return loss, meaning that some RF power will reflect back, and this power will be 14 dB down from 0.8 Watts, or about .03 Watts, leaving 0.77 Watts left going into the antenna. So you lose 1.13 dB total by going to 75 ohm coax, or about 1/5 of an S meter unit, probalby not even detectable. So there's the first issue, you lose a little bit of power due to two mismatches.

Now the complicated stuff. There are two other issues - can your radio handle the mismatch, and what does the length of the coax do. Depending on the length of the coax, your radio will see around a 1.5:1 VSWR. Look at your radio specs and see if the transmitter can handle this. Pretty sure it will have no problem with it. Most specs are 2:1 without any power drop. But it's possible that something in the radio will detect the reflected power and reduce the output power so the final transistor doesn't blow up. Worse, it could just shut the radio down. This is very unlikely with this minor mismatch. The output transistor is seeing 75 ohms instead of 50, so it's not going to have to work hard, but the voltage swing it sees might be higher with the lighter loading, which at some large mismatch will blow up the transistor. But again, very unlikely with this small mismatch, and a 12V radio. A different story on high power television transmitters where there is 50V on the drain and the FET breaks down at 80V.

OK, now the coax length. I'm going to talk in wavelengths here, you can look up the velocity factor of your coax and convert that to inches if you want. If the coax is any integer multiple of a 1/2 wavelength (1/2, 1, 1.5, 2...), then its impedance doesn't matter, it cancels out of the equation. So, assuming your antenna was really 50 ohms in the first place, your transmitter will see 1:1 VSWR and work at full power. If it is 1/4 wave, or odd integer multiples of that (1/4, 3/4, 5/4,...) then this is worst case, the 50 ohm impedance of the antenna is actually transformed to a worse mismatch, up to 112 ohms, which is an SWR of 2.25:1. Probably the radio won't have any problem with this either, just the power will go down a bit more. Remember, if you have a matched system, the voltage and current along the transmission line is constant, and so the length doesn't matter, but when you have a mismatch, there are standing waves, and the length can cause you to hit a peak or valley in the standing waves, causing the above numbers. Even if you have a nice 1/2 wave length of coax (40" at 2 Meters?), you are now frequency sensitive, you will only get a perfect match at one frequency. And this gets worse the more 1/2 wavelengths you have. If you have 10 feet of coax, thats 3 half waves, and the match gets worse 3 times as fast with frequency change.

Here's a cute trick - at the antenna end, use a 1/4 wave transformer to match the 50 to 75 ohms, then all the mismatch issues go away. This is simply a piece of transmission line 1/4 wave long that is 61.2 ohm impedance, between the end of the 75 ohm and the antenna. You can buy 62 ohm line if you try hard. Here's a funny way to do it, if you want to use cable TV aluminum hard line - hit it with a hammer to flatten it down a little over a 1/4 wave length. This lowers the impedance and makes the 1/4 wave transformer, and if you get lucky, you get to 62 ohms and it all works fine, at that frequency. Probably easier to re-tune the antenna to 75 ohms, but I've done this and it works.

Knowing all of the above, my conclusion is just go ahead and use 75 ohm coax, it will probably work fine. This is probably what some other answer said, but now you know the details. This is the difference between an engineer and a hobbyist - the engineer knows all of the above and comes to a conclusion the long way, the hobbyist says "well, let's try it" and it works great and so comes to the same conclusion. But in some cases, the conclusions are very different.

Never a short answer...

-Dan, KW2T

ive used rg6 with my ge super base and a big stick antenna, the antenna was 23ft. form to bottom at ground, on 11 meter and it worked just fine........... 362.

I won't comment on the cable mismatch, but that antenna is a great 2-meter quarter-wave antenna. It also works well on 70CM; I'm not sure of the pattern on 70, but I get great SWR matches on both bands when fed with 25' of 50-ohm RG58/u.