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Author Topic: Adding Coil to AH-4-Tuned System  (Read 7029 times)
KG4NEL
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Posts: 541




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« on: September 14, 2003, 09:54:06 PM »

Hey -

I've got my HF through UHF mobile setup all working now, and of course, I can't stop thinking of ways to improve every bit of it. I guess that's why I'm a ham radio operator...but anyway, I was wondering if there's any way I can improve my HF antenna. Currently, it's a 102" whip tuned by an Icom AH-4. Maximum height above road is around 11.5', which clears local bridges nicely, although it doubles as a drive-by tree pruner on occasion. It tunes pretty well, covering 40-6m, and 60m sometimes, depending on its mood at the time. Never quite satisfied until I run out of money or ideas, there's room for improvement here.

I'd like to get it to tune 75m through 6m. Most of the nighttime HF ARES ops around here are on 75, as well as gaining reliable coverage of 60m. I was thinking of adding a small coil in the middle of the antenna. I have around a 108" height limit for the antenna, so I can still clear 12" bridges. My question is, would I gain 75m coverage if I added a center coil? If so, how much inductance do I need, and what size mast and whip do I need? If I did this, would I lose 6m as well?

Thanks for any help in advance...

Jim, KG4NEL  

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AA4PB
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Posts: 15066




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« Reply #1 on: September 14, 2003, 10:20:28 PM »

Why not use a 75M Hamstick with the tuner for 75M and replace it with the 102-inch whip for the other bands? The Hamstick will present a near 50-ohm load to the AH-4 on 75M and it should easily find a match.

I would not recommend trying to use the 75M Hamstick on other bands as you *may* develop some high voltages and arc-over of the Hamstick loading coil.
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Bob  AA4PB
Garrisonville, VA
K0BG
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« Reply #2 on: September 15, 2003, 09:53:46 AM »

Using any Hamstick (and most other commercial units) on any frequency other than it was designed for is suspect due to the high circulating currents and/or high inter-winding voltages which would be present under such operating conditions.

It is possible to add a coil to the whip so your AH-4 will cover 75 meters, but you have to make sure the coil pitch is as large as possible to avoid the aforementioned problem. And remember, the higher the coil is towards the top, the higher the voltages present.

Look up my call on qrz.com (eham.net call lookup is down as I write this) and you'll see a picture on my coil. Even this one arced on 17 meters until I coated it with high-voltage varnish (glyptol). Incidentally, the antenna is 20 meter resonant and about 13 feet long.

Alan, KØBG
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KG4NEL
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« Reply #3 on: September 18, 2003, 04:05:34 PM »

Using a Hamstick isn't really what I'm looking for, because part of the reason I chose an AH-4 was to be able to switch from band to band without getting out and swapping coils or changing taps.

So at this point, I'm thinking about using one of the smaller Texas Bugcatcher coils and enough extension mast to get it over the roofline. Think this would work as an radiating antenna, or as a mobile Tesla coil?

 
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AA4PB
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« Reply #4 on: September 18, 2003, 06:35:30 PM »

I doubt that you will find a "ready made" coil that will be suitable. Take a look at Alan's picture and notice how few and how widely spaced the turns are on his coil.

By the way, the coil does not need to bring the antenna into 75M resonance on it's own (as in a 75M resonator). All it needs to do is add just enough inductance to bring it to the point that the tuner can match it on 75M. You might find that a 10M resonator, for example, would permit the AH-4 to load it on 75M. You might also consider placing the coil at the base of the whip where it is easier from a mechanical point.
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Bob  AA4PB
Garrisonville, VA
FJGH
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« Reply #5 on: September 28, 2003, 04:56:13 PM »

If you are planning to leave the coil in line all the time, the coil will have too much inductive reactance at higher frequencies.  This will cause more problems than you will ever want.

The concept of an antenna coupler that will tune a wide range like 40m - 6m is exactly what the AH-4 was designed for.  But expecting 80m - 6m is too much using a whip at the length you have.  Even if a typically designed tuner could match it, the efficiency will be even lower than a center loaded mobile antenna without the tuner.
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