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

Some comments on the 25-35Ohm range for the impedance of Yagis, which
has VE2PID from my website. Without further details this makes indeed
no sense to say this would be the best for Yagis.

1. You must differ between short Yagis and Longyagis.
2. You must know for what bandwidth the Yagis should be designed.
3. A given result you can reach with a nearly infinite number of
changed parameters.

For short Yagis and the bands 14-1,35MHz, 21-21,45MHz, reduced 10m-
Band from 28-28,8MHz, and for 144-146MHz a low impedance (10-15Ohm)
does not cover the entire band, but gives high gain. A high impedance
with 50Ohm gives away to much gain for a to high bandwidth. For
example a 3-Element-Yagi with 50Ohm direct feed cannot reach more than
5,5dBd. The reasons therefore are pointed out by Roy, W7EL in his
comments about element coupling and element currents.

For the enumerated bands the impedance of 25-35Ohms is indeed the best
for a good balance of gain, pattern and bandwidth. By adding a close
spaced D1 for 50 Ohms you can reach similiar results, but why doing
that? For more weight and additional mechanical problems?
It is better to match the impedance to 50Ohm than to add more elements
for rising the impedance!

For Longyagis the problems are more difficult. You must see the
radiator, D1 and D2 as a unit (radiation center) in a Yagi system. The
very close spaced D1 in a 50Ohm-feeding system acts like an "open-
sleeve-element" and has higher currents than the radiator. The
interaction between these elements can reduce the bandwidth
dramatically, because -j and +j of the impedance increase very fast if
you leave the center frequency.
It is interesting but a fact: You can replace the 50-Ohm-radiator and
a close spaced D1 in several Longyagi systems by one radiating element
with lower impedance and greater bandwidth.
Evolutionary algorithms for optimizing Longyagis find more lower
impedances than 50Ohm for a given gain, pattern and bandwidth. The
matching "by hand" with the close spaced D1 (as Ian, G3SEK writes) in
the finishing construction process is possible but no must.

73 de Martin, DK7ZB