Excellent information, Ian. This is the kind of "education" I was looking
for. :-) I'll check out your website, too, for more info on the clip-on RF
current meter. Sounds like a good thing to have when optimizing an antenna
setup.

Bill / W5WVO


"Ian White, G3SEK" wrote in message
...
Mark Keith wrote:
"John Passaneau" wrote in message
...
But a gamma match is an un-balanced load and a 1:1 balun is not needed.


--
John Passaneau, W3JXP
Penn State University


In a way I agree, and don't bother with chokes on many of my beams.
But I have heard of some that reported some common mode problems with
the gamma match alone. A choke should clean this up, but it will be
pretty small choke for 6m use, and be effective. For 6m, beads might
be a route to consider. ?

The potential problems with a gamma match are that:

1. It is an unbalanced (asymmetrical) connection to the dipole. This
means that the physical centre, where the coax shield is connected, is
not the electrical centre. Therefore there is always an EMF that makes
some current flow on the outer surface of the coax. However, the value
of this current depends on the impedance presented by the shield.
Winding the coax to form a choke at the feedpoint, or adding large
ferrite beads, will increase the shield impedance at the feedpoint and
reduce this stray current.

2. A gamma-matched yagi often has the driven element connected to the
boom at its physical centre... and so too is the shield of the coax.
Therefore as well as launching stray currents onto the shield, the gamma
match also launches stray currents onto the boom of the yagi, which flow
down the mast, the rotator, the tower, the cables etc. Even if you
choke-off the coax, those currents in the metalwork remain.

Note that I did say *potential* problems. Those stray currents are
always going to be there, at some level, but they may or may not be a
problem for you and your station.

On the one hand we have Mark:
Myself, I've never really had any trouble not adding a choke. My 2m FM
3 el has a gamma and no choke. Same for my 3 el 6m yagi. Here, they
are some of my "coldest" antennas as far as rf on the feedline. Maybe
I just lead a charmed life.. MK

On the other hand, I had RFI and severe noise pickup with a very similar
gamma-matched 3 el 6m yagi. A clip-on RF current meter showed
significant currents on the surface of the coax *and* the rotator cable.
When I changed the driven element to an insulated split dipole with a
choke balun, those stray currents all but disappeared - and so did the
RFI and the noise.

Over on the RFI list, we also had a G who put up a new 20-15-10m quad
and was plagued with noise pickup. This went down from S9 to S2 when he
tried a small choke balun at the feedpoint, and with a better balun
(more turns on a larger core) it should go lower yet.

A clip-on RF current meter is an excellent tool for RFI investigations -
at last you can *see* which cables are carrying stray RF, and how much.
You can also *see* which of your solutions is working, and how well. For
homebrew versions see my website, or see the Palomar and MFJ ads.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek