Hi John,
My “guess” was based on recent experience with a Cushcraft 3 element yagi this
past summer. I wanted to put it on the tower, but it had been sitting in the
back yard for a couple of years, so I wanted to check it before hauling it up
there. I used a new toy (one of those $60 1-60 MHz HF antenna analyzers). With
the antenna pointing up and the reflector a few feet above ground, the SWR was
way-way off.
The antenna was low enough that I could reach the driven element while standing
on the ground and I set the toy to read the antenna impedance. In starting to
re-adjust the gamma rod, I found that it was quite wet inside. I took it all
apart, dried what I could, and then let the sun dry the rest. After drying for
a few hours, I got back to it and my new toy made for the easiest adjustment of
a gamma match ever. The analyzer let me see where to set the shorting bar for a
50 ohms resistive component, and the complex impedance could be easily brought
close to zero by moving the inner rod. Not surprisingly these adjustments wound
up being right where they had been before water had messed up the match.
When all done, I wrapped the ends of the rod with good electrical tape (like
Scotch 33) and then slathered over the tape with some aquarium grade RTV. So
far so good, it still shows its low SWR after lots of rain here since going
onto the tower.
As K4SQC pointed out, we need to keep water out of connectors too. I use the
tape and RTV process with them but also add dielectric grease inside them
before wrapping with the tape.
GL with it and 73,
Chet, N8RA
From: nosigma@aol.com <nosigma@aol.com>
Sent: Wednesday, December 26, 2018 4:52 PM
To: chetsubaccount@snet.net; vhfcontesting@contesting.com
Subject: Re: [VHFcontesting] Polishing the Yagi
Chet,
I expect you are correct, the sudden change from the week before the contest
where it would take a tune anywhere to about halfway into the contest where it
barely worked at 51MHz was probably due to water in the gamma match. That felt
like way to sudden a change to be due to slow oxidation. I will see what I can
do to prevent water intrusion. Thank you for suggesting this. I knew I needed
to address water in the gamma match it but have been afraid to touch it since
took a full day of work with a spectrum analyzer to get set close to 50ohms and
flat across SSB and FM. I will dive in and seal them up. If I get it wrong I
know my buddy Jacek and his magic blue tooth remote head spectrum analyzer will
get it dialed in next weekend.
Jay,
Thanks, I got very lucky with conditions and a great turn out from Hams on FM.
You asked.......but who does that? Well I do, sort of. I keep Anti-Sieze on
all hardware thats R&R'd, I also keep a die grinder and spare hardware on hand.
I did have my 6m telescoping elements seize up January of 2017 with ice so I
now coat all my antenna's with silicon spray (including inside the telescoping
tubes) before the January contest. The silicon spray is used to keep ice from
forming inside the tubes locking the telescoping tubes together (I pull outers
for transport). It also lets all antenna's shuck ice as it forms during the
contests. 0300 UTC, 25 degrees and dropping, soaking wet, numb fingers, all
alone on a mountain top trying to pack up with seized telescoping elements, ah
the joyous memories we make contesting.
Thanks guys. I appreciate the advice. Always learning.
73
John
KM4KMU
-----Original Message-----
From: Chet S <mailto:chetsubaccount@snet.net>
To: vhfcontesting <mailto:vhfcontesting@contesting.com>
Sent: Tue, Dec 25, 2018 9:01 pm
Subject: Re: [VHFcontesting] Polishing the Yagi
Hi John,
If this antenna has a gamma match, my guess is that water got into the gamma
rod, thus changing the effective series capacitance, and de-tuning the match.
73 and Merry Christmas,
Chet, N8RA
-----Original Message-----
From: VHFcontesting <mailto:vhfcontesting-bounces@contesting.com> On Behalf Of
John Young via VHFcontesting
Sent: Tuesday, December 25, 2018 7:08 PM
To: mailto:vhfcontesting@contesting.com
Subject: [VHFcontesting] Polishing the Yagi
In sept of 2017 a buddy came over with a spectrum analyzer and we dialed in the
vswr for my four low band Yagis to under 1.5:1 for FM and under 2:1 for SSB.
Everything was great through the weekend before september 18, though I did need
use the radios built in tuner for 6m going into sept 18.
Midway into the sept 18 contest I got rained on quite a bit and 6m vswr started
rising to the point where the tuner couldnt get the 6m vswr under 3:1 on 52.525
(which cut power way back) but I could just barely get a tune around 51MHz so I
QSY'd FM to there. No problems working SSB in the 50's using the tuner.
The antenna is a cushcraft 5el beam with telescoping elements. The smaller
tubes have wraps of tape on them for stops so I can disassemble for transport
and quickly assemble it.
Last week I disassemble the elements and found light oxidation on all the
mating surfaces. Using 220grit paper in a dowel rod as a flapper I polished
inside the tubes and then polished th outside of the small tube by hand with
220 grit.
Today I got around to testing. Without the tuner I got 1.5:1 @ 52.525 and was
able to 1.2:1 with the tuner from 50 to 53MHz. Hope to get to 1.1:1 on 52.525
after it gets hooked up to the analyzer.
Heard that polishing is an XYL's tale but I am a believer. Anyone had a
similar experience?
73
John
KM4KMU
Sent from AOL Mobile Mail
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