[Towertalk] 15M on C31XR
Fri, 10 May 2002 05:27:45 -0000
I have a C31XR up at about 70 ft fed with three seperate feeders. Each
antenna is fed with 50m of HD100 coax and each has a choke balun of 12 turns
of coax with a piece of 6 inch dia plastic water pipe as a former so the
turns don't bunch up.
SWR on all bands is at 1.1:1 or better at my chosen centre frequency. These
are 14100, 21200, and 28200. These c/f give me good swr across CW and
sideband segments. To achieve this I removed the tips from the 15m
element,added extra length to the 20m element and used hairpin matches on
all three driven elements. These took some fiddling with to achieve best
The above swr readings were taken with the feeders to the other two yagis
open circuit. Any kind of load placed across the 20m coax whilst using 15m
messes up the SWR on 15m. This has caused me a problem in my SO2R set-up
which I have worked around by putting a 15m parallel tuned circuit in series
with the 20m feed. I used a small L and big C to minimise the effect on my
20m SWR. It's a compromise but it does work.
With good choke baluns on each driven element I have found SWR to be
substantially independent of feeder length. That is with 50m and longer
feeder I haven't tried with feeders shorter than 50 metres.
BTW You did remove the tips from the 15m elements didn't you? The 15m
elements as delivered are too long for the separate feeders option.
I have added a Cushcraft XM240 at about 85 feet (turned 90 degrees to the
C31XR) and I have inverted vee dipoles for 160m and 80m hung just below the
C31XR. None of this seems to have had a materially adverse effect on swr or
performance of the C31XR so far as I have been able to judge.
I hope this helps.