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Author Topic: AH-4  (Read 657 times)
KD8GIR
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Posts: 6




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« on: April 12, 2008, 05:06:42 AM »

I've got an ICOM AH-4 antenna tuner and I've been told that where I have it located (under the eave of the house) can cause RF problems because of the necessary long ground wire (about 20 ft.).  I put it under the eave because I have limited space.  Is it true you need to mount the thing at ground level? thanks.

KD8GIR
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AA4PB
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Posts: 12696




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« Reply #1 on: April 12, 2008, 06:57:00 AM »

The AH4 is designed to end-feed a wire antenna. If you don't have a good RF ground on the tuner then RF can flow back down the coax and control cables into the shack. Having the tuner on the ground makes it easier to provide a good RF ground. Any time you have a long lead between the ground and the tuner then the ground may be ineffective on some bands.

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WA7NCL
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Posts: 625




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« Reply #2 on: April 14, 2008, 09:33:06 AM »

There is a way around the problem that I have used:

You need to add a balun to the output of the tuner.  You can now feed a balanced antenna of some sort.  There still can be a problem with common mode currents in some situations, but you can try adjusting feed line length to get around it.  I would recommend window line and some sort of balanced dipole antenna if you use the balun.

Another thing you can do to feed a random wire is to add a balun and connect one side of the output to the "long" ground wire and the other side to the random wire.  This forms a sort of offcenter fed wire arrangement.  I have done this from an upstairs ham shack before.  Again some lengths of wire cause problems for the balun, but you can fiddle with the lengths and find something that works.

I have generally found a choke balun made of bifilar wound wire on a ferrite core works well for this sort of thing.  Use wire that can tolerate some voltage, like the center conductor of RG-174 or teflon insulated wire.

The trick with these kludged antenna systems is to experiment a lot until you get something that works for you.  Its kinda hard to analyse them.
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N7UTU
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Posts: 2




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« Reply #3 on: April 14, 2008, 08:30:24 PM »

Good advice from postings above. I might add that improving your RF ground can be beneficial. One ground rod doesn't make much of an RF ground, however three or four spaced apart and connected by copper wire will. It can also be helpful to run some radials. Add a couple at a time, you really can't have too many but there is "diminishing return". Make a couple of long ones and a couple of short ones to begin with. It will help your tuner,...tune whatever antenna you put up.

73, Eric
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KC3EF
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Posts: 69




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« Reply #4 on: April 17, 2008, 09:26:42 AM »

KD8GIR

I have my AH-4 mounted in the attic at the center point of a dipole that is 40ft per leg.  No RFI except for killing the garage door opener if I run more than 50 watts on 30 mtrs.  In my old house I had a loop that ran around the attic with the AH-4 feeding it.  No problem, did not even bother the alarm system.
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OBSERVER11
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Posts: 657




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« Reply #5 on: May 02, 2008, 01:23:52 PM »

I have my AH4 mounted close to the ground, then I go up usig 450ohm ladder to a center insulator, then to a fan dipole... works great!
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