Topband: Switching in Extra Antenna Length

[Richard Karlquist]

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) is to run
a wire down the inside of the tubing to
make a 1/4 wavelength section of air coax.
Short the wire to the tubing at the bottom.

I seriously considered doing this, and
modeled it.  It is excellent OVER A LIMITED
FREQUENCY RANGE.  If you only operate at
1830 and 3795, this would be fine.  However,
the "trap" will allow considerable current
to flow at other frequencies, like 3505,
and 3995, etc.

This leakage current may not be important,
or it may screw up the pattern, depending on
what you are doing.

In the case of the original poster, it would
cause the 80 meter vertical to partially convert
to an inverted L with horizontally polarized
radiation.

A fix for this is to design the coaxial trap
for CW and use a relay to short out the last
5 or so feet for phone.  This relay would
not have to switch high voltage (although the
current would be substantial).

It is also possible to design a trap with
dual resonances for say 3505 and 3795 only,
but not in between.  However, this is tricky.

The trap, once designed and tuned, is bulletproof,
but it greatly complicates the system design process
initially.  In my case, with 4 bands, it really
gets complicated.

The relay, on the other hand, is a no brainer in
terms of system design; the difficulty is in the
component design.

Rick Karlquist    N6RK
richard@karlquist.com
www.n6rk.com
www.karlquist.com