Goodbye trapped antennas?

[K8DO@aol.com]

Hi Bruce...
OK on "The end of Ham Radio."... I agree that discussing antennas is a bunch
more fun...

>On the other hand, the interlaced designs optimize
>element placement, but use only a portion of the boom length for each band.
>I don't think we can automatically reach valid conclusions about relative
>performance of the antennas by applying the boom-length rule-of-thumb.


I have to agree that a trapped, multiband yagi is a bushel of compromises...
but, how much pattern degradation,  in aggregate, results from those
compromises, is the question?    I am of the opinion that it is not as much
as the promoters of a certain antenna design would have us believe... The,
well known, difficulty of accurately modeling trapped antennas forces us to
use other avenues of information...  If you look at the designs of Lawson,
especially his gain optimized PV series, slight position changes (in terms of
% of lambda) in the parasitic elements cause major changes in the pattern...
But, his are unusual designs which are out at the edge of the "area under the
curve" for parasitic arrays, and are flirting with the territory of super
gain antennas, with their extremely low impedences... It is like driving a
Dodge Viper, and using that as a model of the average commuters car... we
need to make a concious effort to keep our attention focused in the 50
percentile area... However, I digress... Back to the topic...

If you take a 20 meter yagi of lambda 0.5 and put  some elements on it and
then use EZNEC, or what ever, to move elements back and forth - always
reoptimizing the tuning for gain after each spacing change - you will find
that you can change the impedence of the driven element considerablly, and
the FB ratio to some degree, but that varying the element spacings results in
minimal differences in the forward gain... then jump from 20 to 10 and retune
the elements, leaving the spacings unchanged, you will find that other than
the need to bring the reflector half way towards the DE, (actually, I simply
add a single 10 meter reflector, which is interlacing an element, I know, I
know) that the other spacings work just fine... I have done this a number of
 times in fooling with antenna designs... What this exercise shows us, is
that for the typical tribander on average lambdas of 0.25 to 0.6, we are not
giving up significant gain by optimizing the spacings for 20 and tossing in
an extra reflector for 10.. In fact, a number of the manufacturers have
noticed this and use exactly that scheme... Given this, one can make the
counter arguement to the independent, interlaced beams on one boom crowd,
that THEIR designs which are not using all of the available boom on each
band, are the compromise, not the traditional W3DZZ design... aaaand, the
interlaced designs with all that tubing will have far more wind drag than the
traditional traps on a fewer number of elements....

Further, if critical spacings on the interlaced designs is what makes them
better than traditional designs, then we need to consider that they use very
thin tubing in order to keep the weight and the the cost from becoming a
government overrun program... This thin tubing wiggles around in the wind
like a plate of spaghetti on a vibrator bed in a cheap motel... So much for
critical spacing in anything other than a dead calm....

How'd ja like them apples?   ...   :)


Denny