Matching 70 cm Yagi to coax feedline
 

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

look for an old article called "beer can baluns ", in QST, maybe also,in
some of their handbooks. consider, that you need a 1/4 wave length lonf
piece of 25 ohm coax (if you build with no dielectric, will be true 1/4 wave
length, no velocity factor) this will be a "sleeve, or "choke" balun (to
acheive a balanced feed, and will provide you a match under 1.6 to 1!
(assuming your impedence formulai is correct! Math is for your balun Zo
(line required) = Zo (in), Times Zo (out), and then take the SQUARE ROOT
RESULTS OF IT! , (and with the small reactive component, should be
insignificant to the calculation) Good Luck-- Jim NN7K



"Phil" wrote in message
...
Try a T match. The ARRL antenna hand book shows dimensions under the
section
on K1FO Yagis.

Phil, KB2HQ

"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).

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

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

Thanks Sjoerd.

Two questions arise.

I'd thought earlier of using a 1/4 wave transformer, using paralleled
50 ohm coax to give me 25 ohms, which transforms the 12.7ohm antenna
to to 49, say 50 ohms, but isn't the DK7ZB match still an unbalanced
feed to a balanced radiator?

Won't this perturb the directivity?

The same thought came to my mind when I read the DK7ZB pages this
afternoon (it's a neat design idea and is going into my keep-this
notebook!) Seems to me there might be a pretty easy way to deal with
this.

A 1/4-wavelength coax transformer for 2-meter operation, using typical
V=.66C foam-dielectric coax cable is going to be just about a foot
long. I'd think that one could wind the pair of coaxes through a
modest-sized ferrite toroid core a few times and tie-wrap the ends
firmly into place, rather than just running them out along the boom
and back. This should create a reasonably effective choke balun...
something like a Reisert 1:1 style, but acting as an impedance
transformer thanks to the paralleled coaxes. You'd get both the
12.7 : 50 ohm transformation, and the balancing, out of one package.

You'd probably want to wind only part of the impedance transformer
around the ferrite, leaving a few inches to make the connection to the
feedpoint (thus keeping the balun itself from coupling into the
radiating element too badly).

Another possibility: in his book on baluns and ununs, Jerry Sevick
discusses this basic sort of application, and suggests that a 1:1
balun of this sort can be used as an isolation transformer by putting
it 1/2 wavelength back down the coax from the balanced-antenna
feedpoint (a technique which he credits to one of Roy W7EL's
articles). It seems to me that this would work equally well for one
of these 1/4-wave-impedance-transformers-wrapped-into-a-balun. Mount
it back behind the reflector, run 1/2 electrical wavelength of any
convenient coax (or maybe open-wire line?) up to the antenna
feedpoint, and it'd still perform both the balanced-to-unbalanced
conversion, and the proper impedance transformation in that location.

So - does this make sense, or am I being stupid and missing something?

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

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

How would a ferrite core go at 70cm?

Losses might be unacceptably high.

Any ideas?

MikeN


On Fri, 11 Jul 2003 05:35:22 -0000, (Dave Platt)
wrote:

Thanks Sjoerd.

Two questions arise.

I'd thought earlier of using a 1/4 wave transformer, using paralleled
50 ohm coax to give me 25 ohms, which transforms the 12.7ohm antenna
to to 49, say 50 ohms, but isn't the DK7ZB match still an unbalanced
feed to a balanced radiator?

Won't this perturb the directivity?

The same thought came to my mind when I read the DK7ZB pages this
afternoon (it's a neat design idea and is going into my keep-this
notebook!) Seems to me there might be a pretty easy way to deal with
this.

A 1/4-wavelength coax transformer for 2-meter operation, using typical
V=.66C foam-dielectric coax cable is going to be just about a foot
long. I'd think that one could wind the pair of coaxes through a
modest-sized ferrite toroid core a few times and tie-wrap the ends
firmly into place, rather than just running them out along the boom
and back. This should create a reasonably effective choke balun...
something like a Reisert 1:1 style, but acting as an impedance
transformer thanks to the paralleled coaxes. You'd get both the
12.7 : 50 ohm transformation, and the balancing, out of one package.

You'd probably want to wind only part of the impedance transformer
around the ferrite, leaving a few inches to make the connection to the
feedpoint (thus keeping the balun itself from coupling into the
radiating element too badly).

Another possibility: in his book on baluns and ununs, Jerry Sevick
discusses this basic sort of application, and suggests that a 1:1
balun of this sort can be used as an isolation transformer by putting
it 1/2 wavelength back down the coax from the balanced-antenna
feedpoint (a technique which he credits to one of Roy W7EL's
articles). It seems to me that this would work equally well for one
of these 1/4-wave-impedance-transformers-wrapped-into-a-balun. Mount
it back behind the reflector, run 1/2 electrical wavelength of any
convenient coax (or maybe open-wire line?) up to the antenna
feedpoint, and it'd still perform both the balanced-to-unbalanced
conversion, and the proper impedance transformation in that location.

So - does this make sense, or am I being stupid and missing something?