Topband: Switching in Extra Antenna Length

[Tom Rauch]

> If you insist on using a trap at the top,
> I think the best way to make a trap (if
> the vertical is made of tubing) 

If you insist on using a trap, avoid a coaxial cable trap. They are 
easy to build, but are also very lossy on the frequency where they 
are resonant compared to almost any other trap style. That is because 
they are at a voltage point, and do not have a high EPR (equivalent 
parallel resistance). 

They are moderately lossy on the pass frequency, but that isn't too 
important because they are at a low current point when "not 
trapping".

There is no compelling reason to make a trap resonant in the band. If 
you do use a coaxial trap (or any other low-Q trap) at least tune it 
OUTSIDE the band you are trapping by parking it below the band a few  
percent or more. Then you fudge the lengths of the section below the 
trap and the length of antenna after it, to move the system 
resonances into the desired range. 

What this does is limit the voltage across the trap to more modest 
values, so the trap does not cause excessive loss.

  





 73, Tom W8JI
W8JI@contesting.com