Topband: Switching in Extra Antenna Length

[George & Marijke Guerin]

Dear Topbanders.
    The problem with coaxial traps made from RG-58 coaxial cable is one of
losses and power.  If one is running only a transceiver or small amplifier,
500 watts or less, then the coax trap is OK, but it does heat up to about 50
deg C / 122 deg F under keydown conditions in the summer.  At higher power
levels and for lower losses, consider an air or UV resistant PVC or CPVC
form like two inch pipe or conduit which is 2.375 inches outside diameter
(~6 cm OD), wind 9.5 to 11.5 uHy inductor over about 2.8 inches (7 cm) and
tune as an 80 meter trap with 5 KV or better door knob ceramic capacitors
(about 2 cm diam x 2 cm long).  Actual value will be about 215 pfd for a 9.5
uHy inductor and 180 pfd for a 11.5 uHy inductor.  Use at least two
capacitors in parallel to make the desired value, as one capacitor alone
will not take the circulating current in the trap at full power.  For
example 3 x 70 pfd or 2 x 100 pfd, what ever you can find.
    One can also use two pieces of RG-213 in parallel to make the capacitor
and "hide it inside the top of mast" as long as it is about 2 cm inside
diameter.  Then trimming the shield will reduce the capacitance and help
bring the trap resonance to about 3510.  The coax is about 30 pfd per foot.
Connect the shield to the bottom of the inductor, and bring the center
conductor up to the top of the trap inductor.  Make sure the center
conductor is well insulated to prevent arcing at either end.
    good luck    73
George    K8GG