more on traps

Dennis Schaefer
Mon, 17 Feb 1997 16:55:44 -0600

L. B., W4RNL, made a good point in that the poor performance of tribanders
is usually due to shortening of the elements rather than power dissipation
in the traps.  I knew that, but didn't express it very well, because I was
responding to a mentality that  implied that the "inefficiency" of traps
was their downfall.   

A case in point - the original Mosley PRO-57 had an almost full-size
dedicated 20M reflector.  (I don't know why they put the coil in there.
Looks like it would have been cheaper to  make a full-size element).  This
antenna also had some full-sized 10 and 15 meter elements.  It worked
extremely well.  When I did the PRO-57A mod to make it work on 12 and 17
properly, the 20M reflector was replaced by a much shorter multi-band
reflector, and the 10 and 15 elements were replaced by 12 and 18 meter
elements.  The front director became a "multi-director" and I never did
figure out what bands it was supposed to work on.   It may have only had 2
active elements on 20!  No, I don't have a modeling program (yet) to run it
through, but common sense and experience told me that the antenna would
never work the same on 10/15/20.   

Someone else made the point about not talking about gain over a dipole
until you have done a comparison.  That inspired me to go put the 20M
dipole back up and take a critical look.  The dipole is up 40 ft in the
center, (same height as the A3) and the ends are about 35 feet.  It is
broadside to East Coast/EU.   I didn't hear any real DX during the test,
but there were many East coast stations on.   The beam was 2 to 4 S-units
better than the dipole in all cases.  Those are S-units on a Kenwood 440S.
I know, that is a meaningless measurement, but it probably means a little
to someone who is accustomed to using a 440S.   (Many signals were
comfortable on the beam and unreadable on the dipole).

My gut feeling?  The A3  probably has about 4-5 db gain over the dipole on
20 meters.    

I suspect that ANY shortened antenna radiates less efficiently than a full
sized antenna.  As W4RNL pointed out, the exception is antennas with
capacitive hats on the ends, but this is not a freebie - you probably have
as much material and weight involved, so you haven't really gotten much
smaller.   It's hard to cheat the laws of physics.   Bigger is better.  

Traps aren't the only culprits.   I once had a 3 el Telrex on 20 meters,
and its elements were several feet shorter than normal because of a
U-shaped stub in the center of each element.       I'm sure this antenna
was mathematically inferior to one with full size elements, but it sure

Tony tested KLM's traps and found that they have little heating loss.  But
KLM's traps also  shorten the elements, so the KLM probably also suffers
this problem.    They seem to be very effective anyway.    

The A3 is definitely a mediocre little antenna.  It is going to get retired
to field day use soon!  However, it is not a "dummy load".   (If someone
will respond to my previous message about whether or not I should consider
using a Hazer, I will be doing some antenna work soon on the other tower!)

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