I have a very small yard and a 70ft tower with a beam on top. I want to put
up transmitting antennas for 80m and 160m. For the space I have, some form
of vertical seems the best. I do not want to shunt feed the tower as a
vertical because the feed lines run a long way to the tower from the house
along a messenger cable that is 10ft above the ground (no buried coax). The
feed lines would become part of the radiating system and would put a lot of
RF into my shack.

I would like to string a wire quarter wave 80m vertical alongside the tower
but I know it will heavily couple to the tower. I would also like to string
a 160m inverted-L alongside the tower. This would likely couple into the
tower too.

I have seen resonance breaking "traps" described in ON4UN's book. He calls
them "resonant linear traps" shown on page 9-27 section 2.2.11. He forms a
rectangular single turn coil in parallel with a variable capacitor using the
tower as one side of the rectangular coil.

Does this work? Do you have to use the entire length of the tower to form
the "trap"? Could I just use a 4ft segment of the tower half way up with the
electrical effect of breaking the tower up into two shorter pieces?

I have also seen what is called a "resonance breaker" described in the ARRL
Radio Frequency Interference book. It is similar to the one described above
except it is a two turn coil made of insulated wire. The coil is twisted
open to form a figure eight and then has the variable capacitor added. The
advantage of this configuration is that no electrical contact is needed
between the conductor you wish to break "resonance" in. The "trap"
magnetically couples with the conductor to impart the high impedance.

Does anyone have experience with these things? Can I use a GDO to set the
resonance of the "trap"?

Jeff - KA9S