Hi all.

I've got a 70 cm Yagi which is to be matched to a 50ohm feedline,
feeding a 70cm repeater (5MHz split).

The Yagi has an impedance of 12.7+j0.8 (so 4NEC2 tells me).

What are appropriate matching systems to give me the best matching so
that the Tx-Rx duplexer peforms up to its bandpass and notch
specifications.

Thanks

MikeN

MikeN wrote in message . ..
Hi all.

I've got a 70 cm Yagi which is to be matched to a 50ohm feedline,
feeding a 70cm repeater (5MHz split).

The Yagi has an impedance of 12.7+j0.8 (so 4NEC2 tells me).

....

I've simply used a folded-dipole (equal diameters for the two
sections) driven element in similar cases. That should give you very
close to 50 ohms. It's balanced, so use a 1:1 balun to get to 50 ohm
coaxial.

Cheers,
Tom

Thanks Tom

What 1:1 balun construction would you recommend.

MikeN ZL1BNB
On 10 Jul 2003 12:04:58 -0700, (Tom Bruhns) wrote:

MikeN wrote in message . ..
Hi all.

I've got a 70 cm Yagi which is to be matched to a 50ohm feedline,
feeding a 70cm repeater (5MHz split).

The Yagi has an impedance of 12.7+j0.8 (so 4NEC2 tells me).

...

I've simply used a folded-dipole (equal diameters for the two
sections) driven element in similar cases. That should give you very
close to 50 ohms. It's balanced, so use a 1:1 balun to get to 50 ohm
coaxial.

Cheers,
Tom

MikeN wrote in message . ..
Thanks Tom

What 1:1 balun construction would you recommend.

For low-power work with a 75-ohm system, I've hacked a balun by making
a length of twisted-pair that's close to 75 ohms, connecting that to
the driven element on one end, wrapping about three turns of it
through a small ferrite core, and connecting the other end to the
coax. Then I put another couple of ferrite toroids over the outside
of the coax, one where the twisted-pair connects and another about a
quarter wave down from there. It's a bit tough to make a 50-ohm
twisted pair, but it might be reasonable to make a 50-ohm twisted
quad, where the wires opposite each other are connected together. But
what I was doing I wanted to be broadband, and if I was doing it at a
single frequency, I'd look at making a "bazooka" balun, I think, using
half-centimeter diameter coax centered inside a quarter wave length of
copper or aluminum pipe about one and a half or two cm inside
diameter, shorted to the coax outer at the end away from the antenna.
You can use a couple of "donuts" cut out of foam packing material
(styrofoam) to keep the coax more or less centered in the pipe, but
don't use too much insulation because it will change the electrical
length. -- There are several other balun structures that could work,
too. Maybe others will post things that have worked for them.

Cheers,
Tom