Owen Duffy wrote:
Whilst I have reservations about the design pics etc referenced, the
fundamentals of the Yagi itself look good, and with an impoved matching /
balun element, it looks like a practical high performance antenna. They
are certainly becoming more popular in this part of the world where most
home brew designs are DL6WU based (for good reasons, but peformance isn't
the principle goal).

An improved design could persue a more reproduceable balun that had an
adequately high shunt impedance on one side of the DE, or better, a
symmetric balun, and the shunt impedance was tuned out as part of the DE
tuning adjustment. The latter, if done properly, should result in more
symmetric current distribution on the DE, and pattern closer to that
modelled.

On that point, I am often somewhat amused at the out of hand dismissal of
gamma matches because of pattern distortion, and recommendation of
asymmetric baluns in their place. Symmetric baluns are just not very
popular with hams on VHF and UHF.

Owen

Interesting that you are among a group that uses the DL6WU designs.
They were very good for their day, but the K1FOs had better all over
performance, especially regarding wet weather degradation. And they
were around 20 years ago.

The T match combined with what I've always thought of as his "4 element
launcher" is pretty much bulletproof. I think he may have been the
first one to have the first director very tight to the DE, which had
very desirable effects.

All you had to do was splice the first 4 elements from one of his
designs onto a well designed group of directors and you had a great
antenna. OK, not quite that simple, but a good place to start.

And I agree on the gamma. I like the T, but have never been able to see
any real issues from a gamma. As a disclaimer, I only use gammas on 6m.

tom
K0TAR

Apologies for what's going to be a fairly controversial posting and then
not being able to reply, as I'll be away from this newsgroup for a few
days.

Forgive me also for splicing Owen's and Tom's postings together and
replying to both at once.


Owen wrote:

Whilst I have reservations about the design pics etc referenced, the
fundamentals of the Yagi itself look good, and with an impoved
matching / balun element, it looks like a practical high performance
antenna. They are certainly becoming more popular in this part of the
world where most home brew designs are DL6WU based (for good reasons,
but peformance isn't the principle goal).
An improved design could persue a more reproduceable balun that had
an adequately high shunt impedance on one side of the DE, or better,
a symmetric balun, and the shunt impedance was tuned out as part of
the DE tuning adjustment. The latter, if done properly, should result
in more symmetric current distribution on the DE, and pattern closer
to that modelled.
On that point, I am often somewhat amused at the out of hand
dismissal of gamma matches because of pattern distortion, and
recommendation of asymmetric baluns in their place. Symmetric baluns
are just not very popular with hams on VHF and UHF.
Owen


The most that anyone can claim for a balun is that it "helps to promote"
a symmetric current distribution on the DE. Current baluns are quite
successful in enforcing equal and opposite currents at the feedpoint
itself, but that is only one of three requirements for a symmetric
current distribution over the entire DE. The other two are symmetry in
the DE's physical structure and symmetry in its environment, and the
balun can do nothing to 'fix' those.

I'm not quite sure what you mean by "symmetric balun". If you look
closely enough, no balun can be completely symmetric in every detail...
but certainly some baluns and feed arrangements are much more
symmetrical than others. Some help to promote symmetry, while others are
"not part of the solution, but part of the problem".

I'd place the gamma match firmly in that second category. Far from
promoting a symmetrical current distribution in the DE, by its one-sided
physical structure it *creates* electrical asymmetry. An asymmetrical
current distribution at the feedpoint implies that there will be an 'I3'
common-mode current on the feedline, and the gamma match also provides a
hard-wired connection to launch this current onto the outside of the
coax (and usually onto the metal boom as well).

As you may gather, I don't like gamma matches on principle! That has
been confirmed by practical experience with a gamma-matched yagi for
50MHz (which I bought as a kit, foolishly thinking it would save time).
RFI on both transmitting and receiving was severe, but cleared up
remarkably when the DE was rebuilt to use centre feed through a choke
balun. The improvement was simply due to blocking the common-mode
current that the gamma match had been launching onto the feedline, boom,
mast, rotator cable...


Tom wrote, replying to Owen:

Interesting that you are among a group that uses the DL6WU designs.
They were very good for their day, but the K1FOs had better all over
performance, especially regarding wet weather degradation. And they
were around 20 years ago.

The T match combined with what I've always thought of as his "4 element
launcher" is pretty much bulletproof. I think he may have been the
first one to have the first director very tight to the DE, which had
very desirable effects.

All you had to do was splice the first 4 elements from one of his
designs onto a well designed group of directors and you had a great
antenna. OK, not quite that simple, but a good place to start.


DL6WU's yagi development pre-dates K1FO's work, but the latter was
influenced by earlier development work by Tom Kirby, W1EJ. We also have
to remember this was before the modern era of instant communication;
people in different countries often worked for years on very similar
projects with no idea of each other's existence.

Both DL6WU and K1FO designed 'families' of long yagis based on the same
"launcher" structure (the first 4-6 elements) followed by a suitable
array of directors. Also, both of them used a very close-spaced 1st
director to increase the feedpoint impedance of the DE - it had been
done before by cut-and-try, but DL6WU was probably the first to do this
*knowingly* and explain why. (As an aside, G0KSC's new loop feed is
doing essentially the same; the typical width of the loop is very
similar to the typical spacing of a close-spaced 1st director.)


And I agree on the gamma. I like the T, but have never been able to
see any real issues from a gamma. As a disclaimer, I only use gammas
on 6m.

See above!


Back in a few days - have fun, guys.


--

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek