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I have a homebrew G5RV type dipole that I made around 18 years ago.
Overall it has been a very good antenna for me and I have worked lots of DX
with it. It is around 100 feet of copper wire, joined to a run of 300ohm
Radio Shack twin lead, maybe 25 feet or so. At the twin lead to coax
junction I have a Unidilla 4:1 Voltage Balun, and then 20 feet of RG11
75ohm coax.
A couple of years ago I was using a MFJ 918 1:1 current balun, and on some
of the lower bands, I couldn't keep the antenna "tuned". I am using a MFJ
941D tuner, and I would tune the antenna tuner on 40 or 80 for the lowest
SWR, and once I started transmitting the SWR would jump up to infinity, so
I would transmit a carrier, retune it, and once I transmitted again, same
problem, the SWR would jump up. A few months ago I replace the MFJ balun
with the Unidilla 4:1 Voltage Balun because when I originally built the
antenna, I had a Van Gordon 4:1 voltage balun, and it worked fine.
Changing baluns seemed to fix it for a while, but now I am having the
tuning problem again, but it happening on all bands, not just 40 and 80.
Luckily I have a miniquad for the higher bands, but I tried the dipole on
20 and 17 today with the same losing tune problem.
Before I go climbing around on the roof changing stuff out, does anyone
have an idea of where to start-what might be causing this tuning problem.
Same thing as before, I would load up the dipole to a 1:1 SWR, using a less
than 100 watt carrier, but once I go and start transmitting at full power
the SWR quickly jumps way up. I go retune it,which just requires a slight
adjustment, and the SWR jumps up again, and it just keeps doing this. I
don't think it is the MFJ tuner because it doesn't have this problem with
the miniquad, which also goes through the tuner.
Also, the dipole shows a low SWR (2:1 or less) on 12 and 15 meters, and
when I put the tuner in bypass mode I can transmit on the dipole with no
problems with losing its tuning. On 80m the SWR is also low, and
transmitting on bypass mode to the dipole usually works but once in a while
it will its tuning. On 30m it will show a low SWR initially but once I
start transmitting (in bypass again) the SWR quickly goes to infinity. On
40m when I tune the antenna and then transmit at 10 watts it seems to stay
tuned. Going up to 25 watts detuned it again.
So, any ideas or guesses as to where the problem in my system is?
73 John AF5CC
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