[Antennaware] Trap design

[Don Webster]

David,

Yardley Beers, W0JF wrote an article in the August 1987 issue of Ham Radio about a "new approach to designing trap antennas". It dealt with the design frequency of the traps being about  midway between the two bands.

I built a 3 element yagi for 12 and 17 meters using his approach and it works very well. You have to adjust the length of the elements to compensate for effects of the traps.

Don 
K9MUF

-----Original Message-----
From: David Gould <dave at g3ueg.co.uk>
To: antennaware <antennaware at contesting.com>
Sent: Wed, 29 May 2019 9:35
Subject: [Antennaware] Trap design

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.

73,

Dave, G3UEG / K1MDX



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