[TowerTalk] "different" trap

[Jim Thomson]

Date: Fri, 22 Nov 2019 12:39:45 -0800
From: Glenn Pritchard <gpritchard7000 at gmail.com>
To: "Joe Subich, W4TV" <lists at subich.com>
Cc: towertalk at contesting.com
Subject: Re: [TowerTalk] "different" trap

<That false narrative about high loss is just not true.
<I?ve been working with traps for over 30 years and have never seen the losses that those two claimed.

<Glenn, VA7UO 

##  The full  sized  Mosely  10m  ele    becomes   aprx  75%  FS on  15M.   The  10M  trap  now  becomes  strictly end  loading on  15M. 
You  require  abnormal sky high  coil  uh  values,  if a loading  coil is  inserted at the  ENDS of a shortened  dipole.  You  can  easily  see    
that  effect on  any  loading coil  software  calculator.   The  software  I use  will  depict the required  loading coil uh in  7  different locations 
simultaneously.....  or  spit  out   required coil  values  for  any location.  Current  through the end  loading coils  is  very  low.

##  say  u  wanted  to  build a 15M  monoband  shortened yagi,  75% of  FS.   The LAST  place you would choose  to  insert  loading coils  is  the  extreme  ends! 
Uh  vales  would  be extreme,  and  the  SRF...self  resonant  freq of the coil  will  get  u  into  trbl  every  time.....  its now  too  low.   Mosely 
uses  low  Q traps  on  10M....  IE:   high L,  low  C...so  they  can  obtain  the  required uh  for  end  loading  on  15M.  

##  In  the  case  of the  Mosely,  we  now have to  follow  up the 15m  end  loading....with a 15m  TRAP.   Traps  are  parallel  resonant,  with  extreme  high
circulating currents through  the  coil, and  extreme  high  peak  Vs  across  the  capacitor.   The  cap is  not  like  how  telrex constructed  traps,  with a NPO TX  doorknob cap 
In parallel  with a coil.   Mosely  derives the capacitance  between the  AL  cover  and  coil  windings....its  distributed  capacitance. 

##  Calculate  or   better yet,  measure the  circulating current and  u will  gag.  The  puny  10  gauge  AL  wire gets  hot  enough, from localized  heat,  to melt  and  deform
the  grooved  polystyrene former.    One  would think the thermal  heat  would  be heat sinked to the adjacent  AL  tubing on either side, but it  does not,  its  localized. 

##  I  can  see the  same  effect  on tapped  tubing coils on a linear amp.  The active  15M  portion of  the  20m  tank coil  gets  hot, but the  adjacent un used  20m turns  are  room  temp. 
##  Swap  out a copper   tank coil for an identical AL tubing coil, and you will now  find out what HOT is all about.  

## 10  gauge  AL  wire is  simply too small  in gauge,  freq  is too high,  and  AL  only conducts  60%  as  good as  copper... at  any freq  from  DC,  160m,  80, 15  etc. 
Run  the  numbers  through brian  Beasleys,   K6STI  coil  3.14  coil  program,  and  u  will  see  the  problem  right  away. 

##  toss  In the slightest  bit of a loss   from lousy coil  connections , at  each end of each  trap, and u just compounded the issues. 

##  There is a reason Mosely rates  their  yagis  at  only 600 watts  average power output...=  3.5A  into  50  ohms.   One partial   work around is the use of  dual  driven eles.   After melting the  polystyrene   formers  on  
several trapped  yagis  and   several trapped verticals, I gave up on traps,  they wont work for my application.   HB  traps  that  will  work  are simply  too large,  and  too  heavy, too  expensive, constructed of  .25  and  .375  OD  silver
plated  tubing and  paralleled   HEC    HT-57/ 59  NPO caps.   

Jim  VE7RF