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[TowerTalk] antenna choices for K4XS

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Subject: [TowerTalk] antenna choices for K4XS
From: "Jim Thomson" <>
Date: Wed, 15 Apr 2015 04:02:41 -0700
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Date: Tue, 14 Apr 2015 12:32:06 -0400
From: "Bry Carling AF4K" <>
Subject: Re: [TowerTalk] antenna choices for K4XS

What is your problem with metal enclosed traps?
Are you saying that they cause loss, like the RF going into a shorted turn and 
up the trap case instead of radiating RF energy? How many watts are you 
guessing may be 
turned into heat this way in the average beam trap?
Show us the maths please.  You might be correct.  I am not convinced so far. 

Then there would also be the issue of your RF tank coil in a linear amplifier 
being similarly 
"metal enclosed."  Somehow I am not seeing your point.

####   The capacitance between the al tube and the coil IS the capacitor.   
Another way to build a trap
would be like how Telrex used to  do it.... with a separate HT-50 / 58 style 
NPO hv capacitor , wired
in parallel with the coil. 

##  Enclosing the coil with an aluminum tube drops the loaded Q of the coil 
through the floor.  Remember,
the 10M  trap is a trap only on 10m.  On 15 + 20m, the 10M trap is a coil only. 
  Ditto with the 15m trap,
the 15m trap is a coil when used on 20m.   On 20m there is NO trap.  The 20m 
ele is way too physically short,
and its brought up to resonance by the 4 x coils   from the 4 x traps per 

##If you are going to build a coil, the last thing you do is slide an aluminum 
tube around it.   Whether its from
the dreaded shorted turn effect or not is speculation.   We must have had at 
least a 1/2 doz Mosley  TA-33 jrs
over the years..all  with melted traps in the DE only.  REF + DIR are not the 
problem with power applied.
Mosleys  standard fix for this is to use the larger DE traps from the 33 sr. 

##  when the trap covers were removed from the 33 jr DE traps, the polystyrene 
grooved form inside, that the
small gauge aluminum wire is wound  on..was completely melted !   It appeared 
as though you had run a blow torch 
back and forth across it.   And these had melted during cold wx too.   You 
would think the aluminum tubing sticking out
each end of the trap would heatsink it a bit, but doesn’t.    I noticed a 
similar effect on hot  1/4 inch diam cu tubing coils
on linear amplifiers.   Take a 20m tubing tank coil, and tap it for 15m.   You 
would think the un-used portion of the coil
would act as a heatsink for the used portion of the doesn’t.   With 
lid off, and using a dead cxr and footswitch,
I verified that effect myself.  Portion of coil in use was extremely hot, and 
just 1/2 inch further down the same coil, on the
other side of the tap,  was barely luke warm...go figure. 

##  Sliding an al tube over a coil to make for a cheap and dirty capacitor is a 
novel idea...... but makes for a lousy, low Q coil. 
Telrex didn’t get it right either.  The caps used would fail and blow out.   
Dunno if it was cuz their RF current ratings were exceeded, or
if the peak V across em was too high...or both.   HT-59s  or series- parallel 
HT-57s  could be used, but then each trap  would $100
in caps alone.   12 traps used in a 3 triband yagi. 

##  The other problem with most trapped yagis is they have foolishly stuffed 
BOTH the 10m +15M trap inside the SAME  aluminum 
trap cover tube.   On  15M band, the 10M trap is now a coil.  The 15m trap 
resides 1 inch away  from the 10M trap.   The now way
too physically short 15M ele now consists of the 10M loading coil and the 15m 
trap.    Lousy config imo.   If you want to add a loading coil
to an ele, the optimum placement will be no more than half way out each ele 
half.   The further you place the coils out, the larger the required
inductance.   You can see this effect in any software program that handles 
loading coils for shortened dipoles etc.   Placing the coils at the
extreme ends of the shortened dipole requires a huge amount of inductance. 

##  To do it right would mean separate 10 + 15m traps, with 10M  trap being 
located at  aprx 1/4 wave from the boom. On15m,
the ele is now loaded..via the 10M coils.  The 15m  trap would then be located 
< 1/4 wave from the boom.  Telrex got that part

Jim  VE7RF 



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