I had a somewhat similar scenario happen on my (somewhat) recently installed
tower. The rotary 80 dipole at the top would have a low SWR until it rained
when it would go up quite high. Within generally a couple days it was back
to normal. I checked everything associated with the antenna to no avail. I
did that because the installer had reassured me all coax connectors had been
properly sealed when he installed everything.
Months later we're installing a 6 meter yagi on the tower and the tower guy,
a different guy, goes to plug its coax in to the relay box ... he says
"they're all wrapped with the exception of one of them which is only 1/2
wrapped and partly exposed" .... yes - it was the 80. He wrapped it with
seal and although it took a couple weeks it eventually got to the point
where the SWR is constant - and low once again. So water was getting in to
the connector - over time it has dried out. If this were anything other than
80, possibly 40 I'd change out the coax - being 80 I don't think there was
any long term damage as it appears to have dried out through use. Of course
when it happened my first thought was connections - but I dismissed that
because of the installer ... turns out I shouldn't have. That is not to pass
blame as we can all make mistakes no matter how experienced we are - it is
however a lesson learned. One that I have 'learned' and relearned many times
in my 50 years as a ham ... never assume.
Gary
K9RX
-----Original Message-----
From: John Lemay
Sent: Sunday, August 27, 2017 8:55 AM
To: 'chuck.gooden' ; kr2q@optimum.net ; towertalk@contesting.com
Subject: Re: [TowerTalk] Heliax acting strangely: Help?
Yes, water ingress.
If you are lucky, it is at one end or other.
Remove both connectors, examine for dampness / corrosion. If necessary allow
to air dry or use gentle heat. Re-check for infinite resistance between
inner and outer.
John G4ZTR
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of
chuck.gooden
Sent: Sunday, August 27, 2017 12:53
To: kr2q@optimum.net; towertalk@contesting.com
Subject: Re: [TowerTalk] Heliax acting strangely: Help?
Improperly installed connetor that allowed water to ingree the cable?
Chuck Gooden - K9LC
-------- Original message --------
From: kr2q@optimum.net
Date: 08/27/2017 6:25 AM (GMT-06:00)
To: towertalk@contesting.com
Subject: [TowerTalk] Heliax acting strangely: Help?
I put out a 160 foot run of 7 eights heliax last year. Worked great. Last
time I was QRV was end of January, 2017.
I recently put some RF out (100 watts...HP for me) and was surprised to see
an SWR in the shack of about 3:1.
Setup:
Outdoor antenna switch box just outside the shack. From there, very short
run of RG8 (under 4 feet) to "start" of Heliax.
160 foot run of Heliax to "back" tower.
Then another "short run" of RG8 to about 80 feet of RG217, which goes "up"
my crankup tower.
What I have done:
Put a dummy load at the rig: shows 1:1 SWR.
Put the dummy load at the "far end" of the heliax by "breaking" the
connection from the Heliax to the short run of RG8. SWR in the shack still
3:1 (all HF bands).
Put the dummy load at the end of the short run of RG8 coming from the
antenna switch box.
SWR in the shack shows 1:1 (dummy working, coax to that point working).
OK, so seems like something is wrong with the Heliax.
I disconnected the Heliax at both ends. If I short inner to outer, it shows
dead short at the "other" end. I know, DC is not RF.
I measure the resistance between center conductor and outer shield of the
Heliax. I expected "infinite," but using my crappy RS ohmeter, it shows
about 500k ohms. I measured other runs I have RG217 (nothing connected at
either end, about same length), and the crappy RS meter shows infinite, as
expected.
So what could go "wrong" with my 7 eights inch Heliax? It is above (on)
ground for the entire length.
I walked it yesterday, picking it up about every 10 feet. Looked "normal"
to me.
Any help/suggestions?
Thanks,
de Doug KR2Q
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