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@g3ueg.co.uk>
To: antennaware <antennaware@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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