Ohm mismatch

[Ohm]

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

[thewisepranker]

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?

[teckniqs]

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.

[sharky]

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...

[sharky]

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.

[Ohm]

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

[teckniqs]

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