On Mar 17, 2:22 am, Ian White GM3SEK wrote:


Also, it is possible with many designs to increase the feedpoint
impedance towards 50 ohms by adding a director at very close spacing
(about 0.05 wl). That director has relatively little effect on other
performance parameters, so it can be added fairly late in the design
process as a means of matching. (After construction, that close-spaced
director also allows final adjustment of the matching, by bending the
ends towards or away from the driven element.)


The cushcraft A4S yagi I have is designed to have a 50 ohm feed.
It's not half bad for a tribander.. So I think it's possible to have
fairly decent gain and f/b with such a design.
In the manual, they claim 25 db f/b and 8.9 dbd forward gain.
Of course, the gain they claim may be a tad optimistic, but
in using it, it does seem to do ok. We won 10m fone using
at field day about 5 years ago.
I've designed many yagi's using modeling, but can't remember
how many I've done that were designed to have a 50 ohm
feed.. I think I've done a few though.
In most all cases, I design the yagi for what I want in gain and
f/b, and then worry about the matching later. The only
exception might be if I showed a very low Z, which might
add extra matching losses. If you design for max gain, the
feedpoint Z will usually be quite a bit lower than 50 ohms.
I think as long as you are no lower than 10-12 ohms or so,
the losses in matching are fairly low. An NBS yagi shows
appx 12 ohms or so, and I've never noticed any large loss in
feeding one even using a simple gamma match.
The cushcraft A4S uses no matching device at all.
You just roll up some coax for a choke, or add a 1:1 balun.
I use the choke myself..
MK