After six years of faithful (and effective!) service my Tennadyne T-10
log periodic has recently been showing a different SWR pattern:
2.5:1 or worse across 20 meters
2.3:1 or worse across 15 meters
2.1:1 at 18070 KHz
2.3:1 at 24890 KHz
nearly 1:1 at some parts of 10 meters but bad in others
But there are clear dips to good SWR:
1.2:1 at 15250
1.2:1 at 20750
1.3:1 at 22000
1.1:1 at 25200 .
These readings (and others) were taken at the shack end of the
feedline with a MFJ-249 antenna analyzer.
The feedline is about 100 feet of 9913 and then RG-213 around the
rotator and along the boom, ending in the usual coil at the feedpoint.
Initially I suspected a bad feedline. But, when I connected the
MFJ-249 to the end of the RG-213 (near the top of the tower), I got a
similar pattern of readings.
The feedline, however, may be part of the problem. With a 50-ohm load
in the shack and the MFJ-249 on the tower end, I measured gradually
rising SWR as I swept the frequency from 10 to 25 MHz.
Another possibility is that one side of the RG-213 has come loose at
the feedpoint, so that I'm using only half the antenna. But (measured
from the shack) the DC resistance across the feedline is essentially a
short, which is normal because of the stub connecting the two halves
of the boom at the end away from the feedpoint.
So perhaps the strange behavior comes from some other problem in the
RG-213. To diagnose that further, the antenna will have to come down
(or at least be tipped down). :-(
I'd be grateful for suggestions!
A related question: If the problems are with the coax (both the 9913
and the RG-213), am I seeing normal wear? That is, should I have
simply done preventive maintenance last summer and replaced all the
coax? (Fall is pretty in New England, but we rapidly run out of
decent weather for tower work!)
Many thanks!
Dave, K1HT
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