I made some comparison measurements between various twinlead types after
experiencing some difficulty in obtaining a 1:1 SWR in a Slim Jim 2M J-Pole
using Channel Master 9354 Vinyl / Polyfoam 300-ohm cable I bought at Fry's.
The enhancement/absorption disturbance of the coupled signal (see previous
post, below) by the 1/4-wave
stub measured 6.7 dB with the CM cable, while some old Belden 8225
ribbon-type 300-ohm cable showed an 11.6 dB peak to valley measurement and
450-ohm ladder line using 18 ga solid conductors displayed a huge 35.8 dB
effect. I believe the magnitude of the phenomenon is partly related to the Q
of the resonant stub, and in turn to its ability to reach a high enough
impedance at the open end to match the end impedance of the 1/2-wave
radiator. And of course, surrounding the stub with PVC tubing reduces the
effect a bit, so that the CM cable peak-to-valley measurement goes to 6.0
dB.

I found fair success in wrapping the Belden 8225 stub section with some 1/8"
thick foam sheet one can buy from a packing and shipping store to stabilize
the effect of PVC tubing on the Velocity Factor of this cable. I cut a 1.5"
wide strip from a 24" wide foam sheet and fastened the wrap with Mylar tape
every 2.5" above the feed tap to keep the conductors spaced away from the
PVC
wall. My final design had a 16.53" stub, 38.65" radiator, Feed tap at 1.75"
and gap of 0.25", resulting in a 1.0 SWR at 145.48 MHz with R = 53 and X = 2
on the MFJ-269 Analyzer, and a 2:1 SWR bandwidth of 7.16 MHz (141.80 to
148.96 MHz).

The best SWR I could get with the Channel Master 9354, using a similar foam
wrap inside the PVC was 1.3 at 145.98 MHz. This design had a 16.375" stub,
37.88" radiator, 1.625 feed tap and 0.375 gap. Its 2:1 SWR bandwidth was
5.08 MHz (143.72 to 148.80 MHz). Of course both Slim Jim J-Poles will work
well when
applied at a suitable height, so don't lament too deeply the scarcity of
Belden twinlead - - but if you see any at a swapmeet, it might be worthwhile
to replenish your supply.

73,

Chuck, W6PKP

"Chuck Olson" wrote in message
. ..
I think I've found a way to measure the resonant frequency of a shorted
stub
for the purpose of determining the Velocity Factor of twinlead cable.
Unfortunately it requires the use of an HP3577A Network Analyzer, but I
latched on to a used one a few years ago on Ebay, so I can have all kinds
of
fun with it. But there's a question of how to interpret the results.

I use two small one turn Faraday-shielded loops that I made out of RG58 a
while back to induce or detect signals in various circuitry. For the
purpose
of this measurement, I loosely couple two of them, one from the analyzer
source and the other to its receiver, about 4" apart with the loops
parallel
and axially aligned. Then I introduce the stub midway into the space
between
the loops, oriented for a maximum effect and observe a small peak followed
by a small dip in the received spectrum sweep. My question is, which is
the
true measurement of the resonance of the stub - - the peak or the dip or
the
zero-crossing between them, and why? Up to now, I've felt the dip is the
correct feature to use, since at resonance the stub patently steals a
little
energy from the coupled fields, but that doesn't really say why it steals
energy, or why the peak isn't just as valid an indicator.

Thanks for your help,

Chuck, W6PKP