[Antennaware] Trap design

David Gould dave at g3ueg.co.uk
Wed May 29 10:18:09 EDT 2019

Yes, I know traps are a compromise and some people don't like them, but 
my feeling is that it is often the design and construction that are the 
main problems.

I plan to build one for 15m (for a 15m/20m elevated vertical) using a 
heavy duty doorknob capacitor and small diameter copper tube for the 
inductor, which should optimise the construction issue.

Question 1)  What should the design frequency be?  I vaguely remember 
reading somewhere that the design freq should **NOT** be the middle of 
the higher freq band (eg 21.225 in this case) but on some other freq 
somewhere between 21MHz and 14MHz.  I have done several Google searches 
and cannot find any references, in particular I would like to know the 
reasoning behind doing this, and  how using such a mid-freq affects the 
length of the two parts of the antenna?

Question 2)  Are there likely to be any issues measuring the resonant 
freq of a trap using a RigExpert AA-54 analyser, or is there a better 
way?    Again I vaguely remember reading that a dip oscillator is the 
best way, but I don't have access to one.


Dave, G3UEG / K1MDX

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