TT:
Last time I reported on my small success in loading my Trylon 64-foot
tower on 160M. At that time I said I thought the three parallel shunt wires
were tapped too high up on the tower (they were about three feet below the
tower top at 61 feet.) My SWR curve showed a low point (1.2:1) around 1.79
or so MHz.
Since then several of you have suggested that I lower the tap. Because
I had to climb the tower today to replace a bolt I found on the ground near
the base of the tower - it was from an improperly tightened boom-to-mast
saddle clamp on the 20M DE of the Skyhawk - mea culpa - I unbolted the
shunt wires and brought them down to 51 feet (five feet below the rotator
shelf.)
I remeasured the SWR of the new feed arrangement. Now it's resonant on
1.840 MHz (!), up from 1.79 MHz. Min SWR is STILL at mid-range on the
variable series cap (I measured 625 pF max cap), same as before the move.
This tells me that the TAP POINT determines the resonant freq of this type
of shunt feed; the cap value is determined by the length, and thus
inductive reactance, of the shunt wires. Remember: I didn't shorten the
wires, just moved the tap point from ABOVE the top PVC support to BELOW the
top PVC support.
MFJ 259/731 values are:
1.79 MHz R= 32 X=4 SWR= 1.5:1
1.80 35 5 1.4
1.81 39 6 1.3
1.82 42 7 1.3
1.83 45 4 1.1
1.84 47 3 1.0
1.85 49 5 1.1
1.875 49 9 1.2
1.90 50 10 1.2
1.95 56 16 1.3
FWIW. Next step is loading on 80 with a similar wire set on another
tower face.
73 de
Gene Smar AD3F
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