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Author Topic: What happened to my antenna??  (Read 433 times)
WA8FOZ
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Posts: 187




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« on: September 13, 2008, 10:46:23 AM »

This is weird. My (very simple) mobile antenna has suddenly changed resonant frequency - by a lot.

The antenna: a 2-foot mast, a Bugcatcher 480 coil(4" wide - 85 microhenries), and a 6-foot whip. Mounted to the rear quarterpanel with a Breedlove ball mount. Initially, with the full coil in circuit, it resonated at ~3400 kHz. Nice match with an MFJ Unun. I put in taps for 75/60/40/20. Works great.

What happened: Yesterday I set out on a rainy day. I discovered that the antenna would not match. Later in the day, when I stopped along the way (still raining), I discovered that all the resonances had shifted upwards: 400 kHz on 80m, 800 kHz on 40m, 2 MHz on 20m. Oh well, it will return to normal when it dries, I thought. It didn't!

After checking everything, and cleaning and drying the coil and taps, I find that nothing has changed. All resonances remain up. I checked the feed system by attaching my back-up antenna, an MFJ "manual screwdriver." This antenna works - and resonates - as it did before - I have all the resonant spots marked on the MFJ's tube, and they all are the same.

What could have happened? Is the rain a red herring?

I want my resonances back!! Is this possible, or must I just accept the wonders of nature, readjust the taps, add a capacity hat, or just stay at the top of 75m(ugh)?

73,
Bill

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K0BG
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« Reply #1 on: September 13, 2008, 11:06:17 AM »

There are a bunch of things that effect the resonant frequency, and water is one of them. Dirt, high humidity, surface moisture, etc., and it doesn't have to be on the antenna per se.

This said, most of the time when you see a large change in the resonant frequency, and you haven't changed anything but the weather, it means the antenna is mounted too low and/or too close to the body.

The other issue is the MFJ box. They do alright most of the time, but if you experience a vast change in input impedance  (and didn't change the tap), there is a chance you got the included core too hot. That'll zap one as quick as anything. It's easy to check by replacing the antenna with a dummy load.

Alan, KØBG
www.k0bg.com
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WA8FOZ
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Posts: 187




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« Reply #2 on: September 13, 2008, 12:00:14 PM »

Thanks, Alan, I didn't think about the box. The antenna is pretty much in the clear - the coil is higher than the roof of the car. Total height is just a shade under 13' - about as high as you can go here in tree and low bridge country!
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WA8FOZ
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Posts: 187




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« Reply #3 on: September 13, 2008, 06:29:01 PM »

I checked the unun box - working fine. Rechecked the mount - everything is tight and conducting as it should. I took the whip apart from its ferule, cleaned everything, and reattached it. And the other antenna works as before with the same unun, feed, and mount. I'm flummoxed! Something has happened to that coil, I believe, but what? A mystery....
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K0BG
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« Reply #4 on: September 14, 2008, 06:29:40 AM »

Wait until it dries out.

Here's something to consider. As I stated before, the actual resonant point is when X=Ø. If you add some external reactance, capacitive in this case caused by the moisture, the antenna's resonant point will change. Depending on how you determined the correct tap on the UNUN in the first place, will have a direct effect on how much the SWR changes.

Said another way, if the antenna is 25 ohms, and you used say a 35 ohm tap, the change noted will be somewhat higher than it would be is the tap used was 25 ohms.

This points out the need to set matching devices with an antenna analyzer, and not an SWR bridge, especially one built into the radio.

Alan, KØBG
www.k0bg.com
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WA7NCL
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Posts: 625




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« Reply #5 on: September 15, 2008, 09:34:22 AM »

If "resonances" were measured as SWR minimums with an SWR meter somewhere in the line, the minimums were not necessarily "resonances".  Resonance is like Alan said X=0 and it has to be measured at the antenna.

Maybe you made an error in your initial measurements.  If you grabbed an extra chunk of coax and added it to the system with the SWR meter and did your "resonance" (really SWR fiddling).  Then later you added a different length of coax when your "resonances" changed, it could just be the varying length of feed line you added.

If you really want to adjust antennas without the mystery behaviors, then invest in an antenna analyzer.  One that can read R and X.  It will save hours of fiddling and mystery.  You can actually know where your antenna resonates and based on its measured feed point value, figure out a proper way to match it.  

You can even make a guess at efficiency based on what the radiation resistance should be for a given length of antenna vs. what you measure.  Its usally higher, and the extra resistance is mostly loss.  Thats why those ham stick guys are always happy about their great SWR Smiley
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WA8FOZ
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Posts: 187




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« Reply #6 on: September 18, 2008, 08:57:23 AM »

All points are very well taken, most especially my cheapness in not owning an antenna analyzer!

That said, I still find myself on a mission to discover what happened. I have rebuilt, resoldered, remounted, and reconnected everything. I even replaced all cables! Obviously, a whole lot of things could change Xc and/or Xl - just wish I knew what they were. Something (or things) that precipitously  changed a stable match that muchought to be discoverable!

I have heard it said that the difference between a scientist and an engineer is that the scientist likes surprises, and the engineer does not! In my line of work I am a bit of both; I guess I need to be surprised. I also probably need to change the taps on the coil....
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WA8FOZ
Member

Posts: 187




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« Reply #7 on: September 20, 2008, 08:08:37 PM »

Well, you guys shamed me into buying an analyzer (an MFJ-259B), and, sure enough, it fixed everything. After I connected it to my antenna everything returned to its original state! I think maybe a second reconnecting of all grounds and mast connections may have had something to do with it as well, but you never know - maybe the black box scared the gremlins away!

So it's tighten everything regularly. I thought I knew that....

Other observations:
1. As I play with the analyzer, I'm thinking "How could I ever have been without one of these?" But, almost all of my antennas over the past 35 years have been either tuned feeder dipoles, commercial yagis, quads cut to formulae, or random wires. So....until now I didn't feel the need...
2. Interestingly, the SWR readings on my IC-706 were just about dead on...
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