Can you use a 75ohm antenna like the triax fm5 as a broadcast antenna. I understand the mismatch to 50ohms is not great, will it result in a swr of 1/1.5? Is there anyway of easily converting an antenna from 75ohm to 50ohm

If your dipole is tuned EXACTLY to frequency and has an input impedance of exactly 75 Ω, hence the imaginary part will be 0 Ω, the VSWR will be 1.5. However, this will never happen and you will end up with something much higher.

How much power are you expecting to put into it?

Ohm wrote:Can you use a 75ohm antenna like the triax fm5 as a broadcast antenna. I understand the mismatch to 50ohms is not great, will it result in a swr of 1/1.5? Is there anyway of easily converting an antenna from 75ohm to 50ohm

I know someone that's done this but they had to make a balun to be able to do it, it simply looked like a ferrite choke with a few turns or so contained inside the centre cap and gave a perfect match. I have no idea if it was 50 or 75 ohms though.

You can series match using two pieces of cut length cable..

Use this Site and download the calculator, i've used it for over 10 years with excellent results.

http://www.seed-solutions.com/gregordy/Software/SMC.htm

You should ALWAYS be using a balun anyway; but lets not open that can of worms again...

Also the pre built balun on the triax will only allow a certain amount of power <5w through it.
if you look at the centre cap you'll see its a printed circuit board, you need to rip this out or you'll never get any signal out of it, also remember the triax is a 4.5 dB gain directional antenna most of your power will travel forward.

I user a similar 6dB Log and I had nothing but drama trying to get that to match a TX above 15w, not saying it's impossible - but a lot of time and patience to get it right.

Probably start with 2w for testing then move up, will look into series matching, think it's covered in VHF/UHF manual. Will be using balun. I understand the issue with their being no perfect ground, without matching is the best probable result a 1.5swr? How does the extra number of elements that a yagi has effect the ohms compared to your basic dipole? Guessing it makes for more experimenting

Good point, the dude I know said it was only good for up to 10w.

...Best thing to do would be to just build a purpose built beam antenna for your frequency.

You can use a caluculator like this one, although it's only for 3 elements types as far as I can remember you just space your other forward director elements the same distance that the first one is away from the radiating elements.

http://www.csgnetwork.com/antennae3ycalc.html