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Author Topic: Vertical all of a sudden out of tune?  (Read 2903 times)
VA2FSQ
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Posts: 511




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« on: February 23, 2013, 01:39:09 PM »

Does anyone have any hints for me?  I have a Hy-Gain AV620 antenna which has been operating very well until recently.  We have had a couple of wind storms hitting 100 kph and it seems that since then the antenna now gives high swr.
I took a look and can't see anything wrong, but can't investigate until the snow is gone (it's on the roof).
Hooking up a antenna analyzer shows that the resonant frequency has shifted down by about 2MHz i.e. 12.2MHz gives 1.2 and 14.15 where it used to be resonant is about 3.2.  17m is similar whereas the other bands now never get below about 2.1 .
It's as if the antenna has all of a sudden gotten 2 feet longer! 
Any ideas?  Anyway to use the analyzer to get a better insight?

Thanks
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VA2FSQ
K0ZN
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Posts: 1553




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« Reply #1 on: February 23, 2013, 05:37:43 PM »

Hi.  I don't have that antenna, but from the looks of it, there are quite a few "mechanical" parts/connections. If you had very high winds, obviously, the odds favor
something mechanical failing or coming loose and de-tuning it. Could be water or ice; loose coax connector, ground connection, etc. Without a close visual inspection, it is pretty hard to just guess what the issue could be. It could also be in the matching components, etc. It is kind of rough when an antenna fails in the winter and it is hard to get at it to check it out; you have my sympathy!

73,  K0ZN
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VE3FMC
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« Reply #2 on: February 24, 2013, 03:20:01 AM »

With these 100 mph winds (you are lucky the antenna still stands) did you also get snow? If there was no snow on the roof and now there is that could be effecting the SWR. I have seen this happen with my 80 and 40 meter dipoles which have the ends close to the ground. One winter we had a higher than normal snow accumulation and the SWR on both dipoles changed after a big snow fall. Nothing else changed on those dipoles other than having the snow buildup get closer to the dipole ends.

Once the snow melted the SWR changed on the two dipoles.
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K1CJS
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« Reply #3 on: February 24, 2013, 07:21:36 AM »

Could be that the antenna sustained a direct hit from something the wind blew into it, possibly breaking a part--or even loosening the connections so that the elements slipped together or apart.  That is one reason that a yearly antenna check is a good idea--to correct deficiencies before they cause a problem.

No, I don't know how long the antenna has been up or if you do do that check, I'm just mentioning a possible cause.  In any event, you're going to have to get up there and check it.  Doing anything else--even with an antenna analyzer--is just guessing.

One thing you may be able to do is to use a pair of field glasses to visually inspect the antenna from the ground.  If there is something that shows up that way, you may be able to start your repair planning before you actually visit the roof--and that is about all you can do.
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KC4MOP
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« Reply #4 on: February 24, 2013, 11:26:28 AM »

If the feed point for the antenna is under the snow, maybe de-tuning the antenna.
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VA2FSQ
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« Reply #5 on: February 24, 2013, 11:44:15 AM »

Hi,

Thanks for all the comments.  The antenna seems pretty independent of the snow under it as well as all weather that had been thrown at it..  But, I do like the idea of the binoculars!
I'll give it a shot.
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VA2FSQ
VA2FSQ
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« Reply #6 on: February 24, 2013, 11:59:46 AM »

Well the binoculars did the trick!  Given the wind situation and storms (even Sandy hit up here a bit) , the antenna has slipped down the mast about 6 inches and now one of the counterpoises are sitting right on top and touching the mast effectively putting the counterpoise at ground.
On my way up...
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VA2FSQ
VA2FSQ
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« Reply #7 on: February 24, 2013, 12:06:44 PM »

Looks like that's not going to happen...took one step on the slanted roof, holding on to a window, and there is no grip what so ever....gotta wait for the snow to go... Cry
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VA2FSQ
K0ZN
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« Reply #8 on: February 24, 2013, 06:27:05 PM »


Well.... the good news is that you know what the problem is and it is a cheap/simple fix.  Good luck..... remember this is a HOBBY....not
worth getting hurt over.  Maybe you can make up some kind of pole/push fixture you can work from the ground??.....

73,  K0ZN
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VA2FSQ
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« Reply #9 on: February 24, 2013, 06:35:20 PM »

The base of the vertical is about 20 feet up, the push rod, probably is too long.  But I can reach with a ladder.  I'll have to visit my neighbour with the long ladder tomorrow....
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VA2FSQ
K1CJS
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« Reply #10 on: February 25, 2013, 04:26:51 AM »

Glad to hear that you've found the problem!  Isn't it funny how some thing just ...happen?  Well, as K0ZN said, please don't put yourself in danger trying to get it fixed, and that includes just trying to push it back up.  If the clamps loosened somewhat, all you're going to do if you try that is to possibly loosen them some more--and without tightening them again, the antenna may just slip back down--and possibly further!  

Best you wait till you can get up there yourself and do the repair properly.  We should be having some good days soon--after all, the groundhog saw his shadow!   Grin

BTW, just a suggestion, but when you can make it up there, and if you have a battery powered drill, maybe you could drill a 3/8 or 1/4 inch hole through both the mast and the antenna mount section and secure it with a bolt, lockwasher and nut.  It would keep the antenna from slipping down again.  73!
« Last Edit: February 25, 2013, 04:33:44 AM by K1CJS » Logged
VA2FSQ
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Posts: 511




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« Reply #11 on: February 25, 2013, 04:51:09 AM »

Very good point. The mounting of this antenna is quite poor compared to the cushcrAft which slide on over the mast. This one is attached via u bolts that make it difficult to attach and hence tighten properly.
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VA2FSQ
VA2FSQ
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« Reply #12 on: February 26, 2013, 08:06:37 AM »

Fixed!!
I'll do a more permanent fix when the snow is gone.
Thanks
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VA2FSQ
K0ZN
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« Reply #13 on: February 26, 2013, 03:47:50 PM »

Good work!  Suffering with some cold fingers for a few minutes is a small price to pay to get going again!  True Ham spirit......  chuckle....

Oh.... one of ham radio's oldest laws.... if you have to put up an antenna when it is cold and miserable or cut or bang your fingers while making it, it will work better !!
Just some of the sacrifice that the RF God's require to allow good radiation !

73,  K0ZN
« Last Edit: February 26, 2013, 03:50:34 PM by K0ZN » Logged
VE3FMC
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« Reply #14 on: February 26, 2013, 04:43:54 PM »

Good work!  Suffering with some cold fingers for a few minutes is a small price to pay to get going again!  True Ham spirit......  chuckle....

Oh.... one of ham radio's oldest laws.... if you have to put up an antenna when it is cold and miserable or cut or bang your fingers while making it, it will work better !!
Just some of the sacrifice that the RF God's require to allow good radiation !

73,  K0ZN

Nah he would not have gotten his fingers cold. In Canada we don't bother with winter jackets until it is -20 C  Grin
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