[OKDXA] The weak spot in my dipole

[Perry Jenkins]

Check your coax with a megaphmmeter. Disconnect from the antenna and test. This tests  for breakdown in the dialectic of coax at high voltage. This can find defects that don't show up with continuity checks with a standard VOM. 

Perry N5PJ 

Sent from my iPhone

> On Jun 30, 2016, at 14:22, John Geiger <af5cc2 at gmail.com> wrote:
> 
> spot" in my dipole
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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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