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Author Topic: Dual antennas with dual bands?  (Read 370 times)
KG0WX
Member

Posts: 8




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« on: November 07, 2008, 09:26:11 AM »

I have an interesting idea - what if you wanted to
use TWO dual band high performance 2m/440 antennas in
phase? I'd be mounting them on the roof of my F-150
4x4 with 2 NMO mounts on either side of my Sidekick
hf screwdriver and feeding them in phase, kinda like
the way some semi's use two antennas.

Obviously I get 3db gain over a single antenna BUT I'm
worried about UHF performance. I know you need 1/4 wave
 of a specific impedance coax but on UHF it would be 3/4
wave. What about losses in the junction between cables
and the losses in the cables at UHF freqs?

Can it be done? Is it worth it on UHF?

P.S. I'm thinking about using the SG-7500NMO's from
Diamond based on reviews I've read here....

Ken KG0WX
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K0BG
Member

Posts: 9833


WWW

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« Reply #1 on: November 07, 2008, 11:22:08 AM »

Almost anything can be done, but somethings shouldn't be, and this is one of them. What's more, it's playing the Gain Game, and since you're talking FM here, you don't magically double your distance covered by doubling your gain. Even 10 dB won't significantly increase the distance.

Alan, KØBG
www.k0bg.com
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N8EKT
Member

Posts: 371




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« Reply #2 on: November 12, 2008, 07:34:45 AM »

First of all why would anyone want a directional antenna on a moving vehicle?

Second, a Larsen NMO-270BK in the middle of the roof will outperform nearly anything else you will ever try.

Large amounts of gain in a mobile is counter productive.

It will simply create more flutter and multipath as well as dead spots.

Buy the Larsen, the you'll have the best.
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K9BDG
Member

Posts: 3




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« Reply #3 on: November 12, 2008, 10:45:25 PM »

The radio's power will be split among the 2 antennas, so you won't be gaining anything per say. It will just make the overall setup more directional for front and back comms.

You also need to consider the impedance change on the total setup, thus you might have to change the coax impedance, etc, etc to reduce the SWR.

The reason why big rigs use an antenna on each side is because it makes the overall setup more directional for front and back communications. Obviously this is at the expense of comms to the sides. For big rigs it works as they typically want to comm with another station ahead or behind them on the same highway.

This might be what you want to achieve or not.

Cheers
Andries
K9BDG
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