I am using a Unadilla trap as an element of an 80-160m vertical antenna
which is 52 feet high. At that point I connected a top loading wire for
160m. The wire length and orientation is adjusted to achieve 160m resonance
as a quarter wave vertical. Since that length is a half wave on 80 meters,
the feed point on 80 meters has a very high impedance -- several hundred
ohms I found.
I inserted an Unadilla trap about 20 feet out on the top loading wire. That
gave me a vertical antenna impedance that looked like a quarter wave on 80m.
On 160 m it had a resonant point with the resistive component of the
impedance lower than 36 ohms as one expects from a short 160m quarter wave
vertical over perfect ground.
I used the Kilowatt 80m CW version of the Unadilla traps. They are actually
designed for use in a 160-80 dipole. Trap resonance for the traps I received
was about 3605 kHz. The distance for placing the trap out on the top loading
wire proved to be relatively non-critical. Anything from 19 to 23 feet
seemed to perform about the same. The resonant point on 80m will shift
slightly as the trap position changes. The 160m resonant point seems to
depend mostly on the length of the top loading wire, then the angle relative
to ground and finally the position of the trap. On 160 m the SWR is 4.5:1 or
better from 1.8 to 2.0 MHz. It is no worse than that across the entire 80/75
meter band. I use 7/8" CATV for a feed line so the 150 run to the tower has
low enough loss that I can adjust the impedance at the shack to a value that
makes the transmitter feel good without significant line loss.
As I recall this approach is covered in several editions of the ON4UN book.
I was able to model the system with the trap inserted and found that the
model result was within 10% of the measured result [GenRad 1606A bridge].
Tod, K0TO
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