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Author Topic: Why does my random wire try to arc on 80m?  (Read 1743 times)
KC2VDM
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Posts: 145




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« on: March 12, 2012, 07:35:04 PM »

Something here that hopefully some antenna/RF guru could solve.  I have a random wire hooked up to my tuner in my room. The wire goes out the window, but a foot or so still runs inside until it actually gets to the window ledge. Tuning up on 80m, I had about 50 or so watts CW going out whie tuning the antenna. Then once I kicked it up a bit more, I heard that dreaded buzzing sound. The wire (insulated) was trying to arc to another wire running by. I stopped trying to tune up on 80m.

On 40 and 20 everything is fine, even at 150w out. I don't get it. SWR on the two bands is less than 1.7.

Thanks in advance for any help.
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W8JX
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« Reply #1 on: March 12, 2012, 08:06:31 PM »

It is likely that the feed point impedance of your wire is very high on 80m resulting in a very high voltage being developed even at a lower power level than on 40 and 20. Increasing its length will likely help.
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KG4NEL
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Posts: 373




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« Reply #2 on: March 12, 2012, 08:14:18 PM »

Something here that hopefully some antenna/RF guru could solve.  I have a random wire hooked up to my tuner in my room. The wire goes out the window, but a foot or so still runs inside until it actually gets to the window ledge. Tuning up on 80m, I had about 50 or so watts CW going out whie tuning the antenna. Then once I kicked it up a bit more, I heard that dreaded buzzing sound. The wire (insulated) was trying to arc to another wire running by. I stopped trying to tune up on 80m.

On 40 and 20 everything is fine, even at 150w out. I don't get it. SWR on the two bands is less than 1.7.

Thanks in advance for any help.

Do you have any kind of radials/counterpoise, or is it just a single wire?

SWR by itself isn't usually a good indicator of antenna performance, as lots of dummy loads can attest to Smiley
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WB6BYU
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Posts: 12979




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« Reply #3 on: March 12, 2012, 09:11:48 PM »

How long is the wire?

If it is relatively short (such as 20' or so) then the voltage will be higher at the base on
80m than on the other bands because of the high reactance.   Otherwise if the wire
is close to a half wavelength it will have a high impedance at the feedpoint - usually
a half wave on 80m would also be a multiple of a half wave on 40m, but it wouldn't
be if you were tuning up in the 75m phone band.  It is also possible that the OTHER
wire it is trying to arc to is resonant on 75m but not on 40m.

You might pick up a length of the insulating sleeve material used for electric fences and
run the wire through that to provide more insulation around it.  (This is often used for
running a "hot" wire along the ground past a gate, for example.)


Just because you can adjust a tuner to get a low SWR tells you little about the antenna
itself.  In many cases you can disconnect the antenna a the end of a feedline and the
tuner will still be able to match it.
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K0ZN
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Posts: 1525




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« Reply #4 on: March 12, 2012, 09:53:57 PM »


Most Respectfully: The cold reality is that this is a technical hobby, consequently, knowledge is critical to success.

Pick up a copy of the ARRL Antenna Book and put in a couple of hours studying antenna basics. It will be the best investment
you can possibly make in your station at this point. Antennas are not complex, but they also have some specific parameters that must be met.
A few 'quick answers' on here is nowhere near enough to cover the basics of antenna theory.

Your antenna is what connects your station to the rest of the world; there is no substitute for some knowledge on the "how and why".

The ARRL Antenna Book gives graphic information that is CORRECT and you avoid a lot of myth and misinformation that is floating around out there.
You can pick up an older copy cheaply off of Ebay, etc. and the info is still as good as gold. The booktime you put in will pay itself back many times
over in more fun, less frustration and more QSO's.

73,  K0ZN
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KG4NEL
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Posts: 373




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« Reply #5 on: March 12, 2012, 10:02:16 PM »

I'll be honest, I learned a lot more about end-fed antenna configurations by reading Tom, W8JI's excellent material than what's in the ARRL Handbook/Antenna Books. They're still must-have references, but in some cases I can understand how one may get left with more questions than answers after reading.
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W5DXP
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« Reply #6 on: March 13, 2012, 08:11:11 AM »

I heard that dreaded buzzing sound. The wire (insulated) was trying to arc to another wire running by. I stopped trying to tune up on 80m.

Consider that the feedpoint impedance for a 20 foot vertical wire used on 80m is 2.7-j913 ohms. At 50 watts, the feedpoint voltage is 4000 volts.

You didn't say how long your "random wire" is but 1/2 the G5RV length of 102 feet, i.e. 51 feet, might be a good start in the right direction.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
KC2VDM
Member

Posts: 145




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« Reply #7 on: March 13, 2012, 02:12:21 PM »

Thanks for the answer's guys.  The antenna is roughly (just my guess) 40-50ft long. I originally just put it up for a Quick N' Dirty RX antenna, but figured it couldn't hurt to try transmitting on it. There are no radials, Just hooked to the random wire terminal of my MFJ-941E tuner.

I've been meaning to pick up one of the ARRL's antenna books, but never got around to it. It's sounding more and more like a useful investment. I'll Have to give Tom's book a look also.

Soon enough I'm just going to get another Dipole up. Until then, This antenna seems to do okay on 40 and 20. The first QSO on this antenna was last Sunday with a ham in Texas. Not too bad in my opinion.
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G0GQK
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Posts: 634




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« Reply #8 on: March 13, 2012, 02:38:51 PM »

It would really help you a lot if you spent some time reading articles about antenna's and other things involved in the hobby. This is where being a short wave listener years ago gained you lots of knowledge listening to the Old Boys, what they said, discussed and argued about.

Sadly it seems most interest with amateur radio is based on contest activity, having thousands of contacts which last 30 seconds from which you gain nothing, no name , QTH and not even a genuine signal report !

mel G0GQK
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W0FM
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Posts: 2052




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« Reply #9 on: March 13, 2012, 02:52:24 PM »

If it were me, I'd add a 1/4 wave piece of wire (cut for your 80M operating freq) to the ground terminal on your tuner.  Lay it along the floorboard in the room.  It doesn't have to be cut perfect, but should certainly help. 

You are working with half a non-resonant dipole and all kinds of wierd things will happen.  The counterpoise wire should help things.

73,

Terry, WØFM
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KC2VDM
Member

Posts: 145




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« Reply #10 on: March 13, 2012, 08:05:42 PM »

Thanks for the tip about the counterpoise, if I can find the wire i'll give it a try.

And yes Mel, I feel the same way. Contesting has (to me at least) always seemed kinda dull and pointless. I only reason I ever answer a contest station is to see how well I'm getting into that part of the country.  Grin  Besides that, They waste the spectrum.
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