On 2/26/2013 3:57 PM, Dave Platt wrote:
In article ,
Sal salmonella@food poisoning.org wrote:

RELATED: If you know, why do some yagis designs place the first director
quite a bit closer to the DE than the rest of the directors' spacing? In
other yagi designs, the spacing of the directors is relatively consistent,
where DE-to-first-director is about equal to the spacing of the remaining
directors. I haven't seen that difference explained ... yet. Thanks.

You might be looking at what has become known as an "optimized
wide-band" Yagi.

According to one article (http://www.naic.edu/~angel/kp4ao/ham/owa.html)
placing the first-director parasitic element very close to the
driven element, causes the antenna to be behave as if the driven
element's diameter is much larger than it is (approximating the
distance between the DE and the first director).

This has two useful effects: it increases the radiation resistance
(making the Yagi easier to match to the feedline) and increases the
bandwidth. You give up a small amount of forward gain, but this is
said to be modest and to be a worthwhile price to pay for the ease of
feeding (even a direct 50-ohm feed can be used in some cases) and the
increased bandwidth.


Some of us old time EME/weak_signal builders call the first 4 elements
the K1FO launcher.

I did a lot of optimization over the last 25 years, but I didn't change
the first 4 elements of his design much. He got it right and it worked
especially well with a T match.

tom
K0TAR