TRIBANDER STACKING - MORE FUN THAN "QRL ?" !
K8DO at aol.com
K8DO at aol.com
Wed Aug 2 14:47:27 EDT 1995
The antenna engineers estimate 0.5 dB for trap loss in the average tribander,
which puts coil heating well below the melt down zone... I suspect their
estimate to be in the ball park....
Trapped antennas are not inherently worse than linearly loaded antennas...
Both introduce losses... A well done, trapped beam, with optimum LD ratio on
the coils, properly spaced turns, heavy gauge wire, and efficient condensers
(love that word) will compete with any other shortened beam... But, efficient
traps are more difficult, and much more expensive, to build than linear
loading... (Obviously, a poorly done trap will be have more loss than well
done linear loading...)
The key here is efficiency.... The shorter the elements become the lower the
radiation resistance... Low radiation resistance means higher currents, which
means increased (I^2 * R) losses... As a general rule, a heavily, shortened
beam will have more TOTAL losses (not just trap loss) then a mildly
shortened beam - which will, itself, have more losses than a non shortened
Considering all these items, it is understandable that an untrapped antenna
with high radiation resistance might outgun a (theoretically) higher gain,
more elements, trapped antenna, with low radiation resistance... If, in
addition, we are comparing stacks, then the higher radiation resistance
antennas are going to have less induced stacking loss (mutual coupling causes
increased I^2*R currents) than lower radiation resistance antennas will...
And finally, factor in that the new stack of C3's /M2's (or whatever) with
unweathered coax/baluns/relays etc. and shiny, new solder joints, is being
compared to a 5/10/15 year old stack of TH6/7's (or whatever) and you may
have the recipe for for a complete blowout... :>)
Cheers ... Denny K8DO at AOL.COM
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