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Author Topic: Center fed VHF/UHF mobile antenna??  (Read 877 times)
WD6AGJ
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« on: June 06, 2004, 10:26:56 PM »

Hi All,

I am trying to find a good center fed dual VHF/UHF mobile antenna.  I need to mount the antenna on top of a fiber glass camper shell with no ground plane.  I'll be using with an FT-8800 dual band tranciever.  I have run across a few single band antennas that look promising, but nothing dual band.  If there is not a center fed solution, any other suggestions?  73's  JC
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K0BG
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« Reply #1 on: June 07, 2004, 08:01:16 AM »

I don't know of any center fed mobile antennas. Larsen makes a loaded 1/2 wave designed for mounting on fiberglass and similar surfaces. I'm not sure they make it in a dual band however.

Alan, KØBG
www.k0bg.com
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VA3WXM
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« Reply #2 on: June 07, 2004, 11:55:33 AM »

The Larsen NMO2/70 is 1/2 wave on 2 meters and thus shouldn't require a groundplane.

I've had one for several years and it works great!
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WD6AGJ
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« Reply #3 on: June 07, 2004, 08:02:35 PM »

Any idea how I can do a no ground plane solution on both VHF and UHF?
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VA3WXM
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« Reply #4 on: June 08, 2004, 04:31:00 PM »

Sorry, I guess I wasn't entirely clear in my post!

The Larsen NMO 2/70 IS a dual-band vhf/uhf antenna.  The 2 m portion is an end-fed, center-loaded 1/2 wave, the 70 cm is 5/8 over 5/8.  The center loading for 2 acts as the phasing coil for 70.

Theoretically NO groundplane is required with this antenna.  In fact, according to Larsen there are no adjustments you can make to it, i.e. trimming the whip, etc.  Out of the package mine was better than 2:1 SWR on both bands.
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WD6AGJ
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« Reply #5 on: June 08, 2004, 08:46:26 PM »

OK - great.  Thanks for the Larsen tip.  I'll check it out.  Is your mounted on fiberglass or some other relatively non-conductive surface?
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WD6AGJ
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« Reply #6 on: June 10, 2004, 01:08:49 AM »

I contacted Larsen, and they say that none of their antennas are recommended for situations without ground planes - the search continues...
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CROOKIE
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« Reply #7 on: June 10, 2004, 09:20:43 AM »

The problem seems to be the quest for a *dual* band 1/2 wave antenna vs. two single band 1/2 wave antennas. I thought the Larsen 2/70 was ground plane independant *at 2m* even though a 2 by 5/8 (IIRC) at 70cm, therefore needing a GP for the latter, but maybe I read the description copy wrong.

I take it a copper (or aluminum) tape/flashing on the underside of the shell is not an option? The 2m shouldn't care and the 70cm shouldn't need as big a GP as a 1/4 or 5/8 2m would
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W0IPL
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« Reply #8 on: July 04, 2004, 02:38:25 AM »

Look back at QST about three or so years ago. There is an article about a "Sleeve Dipole" that you can make for about $10 in parts.

Pat
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VE3VBO
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« Reply #9 on: July 05, 2004, 09:25:42 AM »

Hi,

I have thought about trying this for a while, but haven't had a need.

Larsen has an open coil LM mount dual band antenna (LMC2/70B). I have also noticed that their Elevated Feed antennas (EF806SNMO) use a stud for the whip that is the same size as the LM mount.

I have taken apart the mast on other elevated feed and, basicly, the mast has the coax peeled back so that the shield is folded (inside the mast) back over the coax that runs to the whip's mounting point. If there isn't enough of a ground inside the mast it should be easy to un-solder the stud and strip back some more shield from the coax.

Hope this helps.

Victor

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