Bill VanAlstyne wrote:
Well, went back to the books and restudied the section on common-mode
currents on feed lines, and it appears that such currents are only caused
when a balanced driven element is fed with an unbalanced feeder. Since (if I
understand it correctly) the gamma match excites the driven element only on
one side, there is no way for the antenna to couple common-mode currents
into the feed line.

If you bring the coax feedline off parallel to one of the driven elements,
it will certainly couple common-mode currents into the feedline.
--
73, Cecil http://www.qsl.net/w5dxp



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"Bill VanAlstyne" wrote in message ...
Well, went back to the books and restudied the section on common-mode
currents on feed lines, and it appears that such currents are only caused
when a balanced driven element is fed with an unbalanced feeder. Since (if I
understand it correctly) the gamma match excites the driven element only on
one side, there is no way for the antenna to couple common-mode currents
into the feed line.

That said, I'm not sure I really understand how the gamma match works and
whether it represents a significant compromise. The Antenna Book doesn't go
into a great deal of detail about the gamma match, treating it basically as
a "subset" of the T-match. This, coupled with the fact that many amateurs
I've talked with unequivocally trash the gamma as a poor matching system
that should not be used where there is any choice, makes me want a better
understanding of this whole field.

So, the questions:

1) Is the gamma match inherently inferior to a balanced matching system?

Probably so to a degree, but I don't think it's enough to worry about
unless you have a specific problem with common mode currents. You have
to remember, the balanced T match has to be converted to the
unbalanced coax also. If not done properly, it could have problems
also. Most use coax baluns on those. To me, the higher the freq, the
more I would probably want to use a T over a gamma. On HF, where it's
really not that critical, I find gamma matches to be just fine here.
My 2 and 6 meter beams both use gamma matches, and I've never really
had any problems. But if I did, a choke at the feedpoint should clean
it up. As far as the gamma being a poor device that should not be
used, thats hogwash. 1000's have used those things with good results.
But this is not to say there aren't better methods to use. As far as
radiation from the element, as long as you have no common mode current
problems with either device, the end result is the same. MK

"Bill VanAlstyne" wrote in message

Comments, please...

Bill / W5WVO

I might add...To me, the importance of using a better matching device
would depend on how long and how many elements, and also the freq.
Naturally, if I'm on VHF or UHF with a 15 el yagi, the pattern is
tight, and suppression of the common mode currents is fairly critical
to accurate steering and a good clean pattern. So I'd want to use the
T most likely.
But for smaller 3-4-5 element yagi's, I hardly see it worth the
bother. The patterns are fairly broad, and it's not a big deal to have
a small amount of current on the line. In practice, I hardly seem to
notice any problems at all here, but some do. Adding a choke of some
type will clean that up if needed.
HF? Again, hardly worth the bother. To me anyway...Add a choke if you
want to sleep better at night. Actually, my present tri-bander has a
50 ohm feedpoint, and only needs a choke or 1:1 balun. I just wind a
coax choke. MK

Mark Keith" wrote in message
om...
I might add...To me, the importance of using a better matching device
would depend on how long and how many elements, and also the freq.

This is an excellent point. I hadn't considered that the REAL-WORLD impact
of common-mode currents would depend on frequency and desired antenna
pattern.

My yagi is kind of on the borderline of the examples you mention, being a
fairly long-boom (18') 5-el yagi for 50 MHz. But front-to-rear is important
to me, as well as keeping the side lobes under control, so I've opted to
install a Radio Works line isolator (characterized to include 6M) at the
feedpoint of the gamma match, in preference to a simple coax air choke. This
should go up this afternoon, and we'll see how much difference it makes (if
any).

Bill / W5WVO

"Mark Keith" wrote in message
om...

...As far as the gamma being a poor device that should not be
used, thats hogwash. 1000's have used those things with good results.

Here's a couple questions about gamma match construction:

1) Commercial gamma matches typically employ a gamma rod and integral
coaxial capacitor, constructed of two coaxially mating pieces of tubing/rod
separated by a piece of dielectric tubing. Do you know of any reference that
gives a formula for computing the capacitance of such a coaxial capacitor?

2) Antenna modeling programs like YO by K6STI require a specification for
the diameter of the gamma rod. But due to the coaxial capacitor
construction, I'm not sure what diameter is actually being called for. Do
modeling programs typically assume a coaxial capacitor integral to the gamma
rod, or do they assume a molded capacitor between the feedpoint and a
monolithic gamma rod?

Bill / W5WVO

Bill VanAlstyne wrote:
"Mark Keith" wrote in message
om...


...As far as the gamma being a poor device that should not be
used, thats hogwash. 1000's have used those things with good results.


Here's a couple questions about gamma match construction:

1) Commercial gamma matches typically employ a gamma rod and integral
coaxial capacitor, constructed of two coaxially mating pieces of tubing/rod
separated by a piece of dielectric tubing. Do you know of any reference that
gives a formula for computing the capacitance of such a coaxial capacitor?

Reference = "Reference Data for RAdio Engineers", Fourth Edition, ITT,
1956, page 134.

[SNIP] C = (7.36 * Er)/(log(b/a))
whe C = pf/ft
Er = Relative permittivity
b = major radius
a = minor radius


2) Antenna modeling programs like YO by K6STI require a specification for
the diameter of the gamma rod. But due to the coaxial capacitor
construction, I'm not sure what diameter is actually being called for. Do
modeling programs typically assume a coaxial capacitor integral to the gamma
rod, or do they assume a molded capacitor between the feedpoint and a
monolithic gamma rod?

Bill / W5WVO