Thoughts on feeding a yagi...

[Julius Fazekas n2wn]

I'm near the point of installing an old 205BA (5
element 20M yagi) that will be serving a dual purpose.
It will also act as a tophat for a 160/80/40 vertical.
I'd like to ground the driven element. Any thoughts as
to the best way to feed it?
I've been looking at the old standard gamma match,
omega, hairpin and possibly using a broadband
transformer.
Kinda off topic, but might be of interest...
73,
Julius
n2wn
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[Vic K2VCO]

J F wrote:
> I'm near the point of installing an old 205BA (5
> element 20M yagi) that will be serving a dual purpose.
> It will also act as a tophat for a 160/80/40 vertical.
> I'd like to ground the driven element. Any thoughts as
> to the best way to feed it?

The original antenna used a grounded 'beta match'.  See
<http://bama.edebris.com/manuals/hygain/205ba/> for the instruction
manual.  Also, there was an improved model called the 205CA.  Hygain
once had a kit to convert the BA to a CA.  Maybe they still do!

--
73
Vic, K2VCO
Fresno, CA
http://www.qsl.net/k2vco
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[Earl W Cunningham]

Julius, N2WN wrote:

"I'm near the point of installing an old 205BA (5 element 20M yagi) that
will be serving a dual purpose.  It will also act as a tophat for a
160/80/40 vertical.  I'd like to ground the driven element. Any thoughts
as to the best way to feed it?"
----------
The Hi-Gain 205BA driven element is already DC-grounded to the boom
through its hairpin (beta) match system.  Regardless, the driven elements
of any  HF Yagi act like they are grounded on the lower bands if their
coax feedlines are routed all the way down to the ground inside the tower
(or taped to one of the tower legs).
==========

"I've been looking at the old standard gamma match, omega, hairpin and
possibly using a broadband transformer."
----------
On 160m, with that big top-loader, if the tower is 60 feet or more high,
a shunt-feed system using a single (gamma) capacitor at the feedpoint
will work.  A shorter tower may require a two-capacitor (omega) match
system.  No transformer (broad band or otherwise) is necessary.

Ditto on 80m if the tower is 40 feet or more high.

A shunt feed system using a gamma match will certainly always work on
40m, however if the resonant frequency of the structure (tower plus top
loading) is below about 2.5 MHz, the DX results may be disappointing
because of the high-angle radiation from a vertical that size.

Be aware that the structure need not be resonant at any particular
frequency to be an efficient radiator.  Anything will radiate the RF that
is applied to it if it is fed/matched properly.  All that is necessary is
to find the correct height of the shunt wire tap onto the tower where the
resistive part of the impedance at the bottom of the shunt wire is 50
ohms.  Then it's a simple matter to cancel out the reactive part of the
impedance (which is always inductive) with a series (gamma) capacitor to
get a perfect match to your 50-ohm coax feedline.

In any case, a good ground radial system will ensure good radiation
efficiency.

My two 72-foot shunt-fed towers work quite well for DX on both 80m and
160m.  One tower has a TH6DXX at 75 feet and the other has a 402BA at 75
feet.  The towers are spaced 135 feet (which is 1/4-wave on 160m and
1/2-wave on 80m) and are used as a switchable-direction phased array on
those two bands.

Each tower has its own ground radial system of 96 radials, each 125 feet
long.

73, de Earl, K6SE
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