[VHFcontesting] Polishing the Yagi

[Chet S]

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 at aol.com <nosigma at aol.com> 
Sent: Wednesday, December 26, 2018 4:52 PM
To: chetsubaccount at snet.net; vhfcontesting at 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 at snet.net>
To: vhfcontesting <mailto:vhfcontesting at 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 at contesting.com> On Behalf Of John Young via VHFcontesting
Sent: Tuesday, December 25, 2018 7:08 PM
To: mailto:vhfcontesting at 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



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