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Author Topic: Icom AH-4 Tuner to whip antenna - what wire?!  (Read 1662 times)
JCRD
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« on: June 15, 2006, 07:33:48 AM »

Hi

About to put together a mobile set up, I've read that there should be a direct connection from the AH-4 tuner's terminal to the whip and NOT to use coax.

Question is, what is the best thing to use exactly? Just insulated copper wire? What gauge? Or perhaps something else?

Also, do I ground both the whip antenna mount to the vehicle and the AH-4 to the vehicle? At the same point or doesn't it matter? Or do I just ground the AH-4?

Sorry, this is probably old hat to many of you, but I just want to get it right.

Thanks in advance,

James
MW0ZAP
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K5LXP
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« Reply #1 on: June 15, 2006, 11:35:50 AM »

Ideally, the tuner will be inches away from the whip feedpoint.  Because this point can be either a current or voltage node depending on whip length and frequency, you want to make sure the wire is both beefy enough to handle current, and high voltage too.  Using the center conductor off of a piece of RG-8 coax should work pretty well.

The mount really doesn't have a ground per se, it's just a mechanical assembly to isolate the whip from the chassis.  The tuner should be grounded to the chassis as close as possible to the whip feedpoint, and since most cars are made of metal, the antenna mount will be connected to the same metal as the tuner by default.

Mark K5LXP
Albuquerque, NM
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K0BG
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« Reply #2 on: June 15, 2006, 12:20:23 PM »

Mark's dead on, but there is one more thing to consider, and that is high RF voltages.

An ordinary ballmount, or insulator will not stand up to the RF voltages present (>10KV). Geo Tools sells a Delrin insulator for the exact purpose (http://www.geotool.com/antmount.htm).

If you want more information, go to my web site and look under Antennas.

Alan, KØBG
www.k0bg.com
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AA4PB
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« Reply #3 on: June 15, 2006, 06:08:47 PM »

I've also found that black heat shrink tubing over the center conductor from RG-8 protects the RG-8 insulation from deterioration.
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K8YS
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« Reply #4 on: June 17, 2006, 10:06:03 AM »

I have used spark plug wire, the wire type, not the graphite type. I bought a small spool at AutoZone.
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AC2RC
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Posts: 112




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« Reply #5 on: June 19, 2006, 08:32:10 AM »

I use West Marine's  GTO 15 High Voltage Wire . It's a 15,000 volt , #14 wire.
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WA4PTZ
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Posts: 528




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« Reply #6 on: June 20, 2006, 09:00:13 AM »

I used the center conductor of a piece of RG-58 and
grounded the shield . It worked fine with my 100
watt mobile for years.
73,
Tim
WA4PTZ
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AA4PB
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« Reply #7 on: June 21, 2006, 05:46:15 AM »

Coax (shield grounded) will work if the antenna is near self resonance (like 10M for a 102-inch whip) but on lower bands the capacitance of the coax (center conductor to shield) will bypass most of the signal around the antenna. The shorter the whip and the lower the frequency, the worst this will become. This is because a short whip (in terms of wavelength) presents a very high impedance and it doesn't take much parallel capacitive reactance to provide an alternate path to ground, bypassing the antenna. Think of it in terms of two parallel resistors. A 1000 ohm resistor (the antenna) in parallel with a 10 ohm resistor (the shunt capacitive reactance). Where does most of the current flow? Through the 10 ohm resistor. The 1000 ohm resistor (the antenna) receives only a small portion of the available power.

If you are using a short whip with an AH-4 on all bands then you should NEVER, NEVER use coax (with a grounded shield) between the tuner and the base of the whip. You want to minimize the shunt capacitance. That means single conductor feed, as short as possible, spaced away from the vehicle and other grounded conductors as much as possible, and a good quality insulator.

And yes, it is all related to efficiency of the antenna system. It is quite possible to make contacts with a system mis-using coax in this manner because it is quite possible to make contacts with very low effective radiated power. With an HF mobile system you are already at a disadvantage because of the short antenna and often poor location. In my book you need to get as much of the available power as possible into the antenna and not waste it in the mis-application of coax feed.
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JCRD
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« Reply #8 on: July 05, 2006, 07:40:42 AM »

Thank you all for your help and advice, looking forward to getting it all up and running.

Regards,

James
MW0ZAP
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WA4MJF
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« Reply #9 on: July 05, 2006, 04:13:11 PM »

Alan, I was thinking about HF Mobile
and I use ASP balls and springs.  
They worked well for 110 watts on
10 and 6 Meter FM rigs.

Would I need something beefier for
the 102" whip and tuner as discussed
above.  I would probably use #12 or
# 10 copper strand or solid?

Tnx es 73,

Ronnie
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WA4MJF
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« Reply #10 on: July 05, 2006, 04:15:33 PM »

AA4PB, why would there be deteriation
of the coax?   I never had that problem
with VHF FM rigs.

73 de Ronnie
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