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Old 04 August 2006, 08:20   #1
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Paired Antennas - for VHF Gurus

Is there any benefit to fitting two antennas exactly the same size to recieve and transmit on i.e. not duplex where one might recieve on one and transmit on the other but not both on both....if that makes any sense.

I'd read, possibly in RIB intl article that two antennas give more signal is it halved? i.e. 12.5 watts per antenna? and does it produce bands of constructive and destructive inteference.
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Old 04 August 2006, 08:32   #2
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Quote:
Originally Posted by Simon B
Is there any benefit to fitting two antennas exactly the same size to recieve and transmit on i.e. not duplex where one might recieve on one and transmit on the other but not both on both....if that makes any sense.

I'd read, possibly in RIB intl article that two antennas give more signal is it halved? i.e. 12.5 watts per antenna? and does it produce bands of constructive and destructive inteference.
In the good old daye when the RX and TX were in diff parts of the radio, some in diff rooms!, this would have been possible and in a lot of cases how it worked.

Most modern radio's have one main PCB containing both the RX and the TX parts of the unit.

Modern radio's will not allow for two antenna working.
You would find it almost impossible to make work with out lots of external bits.

You would also come into issues of mounting as the antenna could and would effect each other.

The only time two comes in to play these days is with a two box DSC system like the M503 and DS100.
You then must have two but the one on the DSC unit is for the RX of DSC calls only..

Having said that two does look good on the A frame, always good to have a spare!!

Best regards
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Old 04 August 2006, 10:26   #3
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Your money is best spent with a better antenna. Look at the specs and the price range. Also get the antenna as high as possible.

There are also antennas that can do a couple of things at once. There are some sweet vhf/cell antennas out there.

You will be shocked at the cell reception that you can achieve with a 3 dp portable antenna but with one of the antennas I noted above I think the boost is more like 9db.
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Old 04 August 2006, 11:12   #4
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Hi
It is possible to Co-Phase two antennas which in theory should give about 3dB of gain.

John is right with
Quote:
Originally Posted by Jon Brooks
You would find it almost impossible to make work with out lots of external bits.
Installing the array is some what convoluted and I wouldn’t recommend it on a RIB.

Link below gives a good insight into what’s involved, but note actual figures relate to 27 MHz.

http://www.signalengineering.com/ult...o_phasing.html

Shaggy
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Old 04 August 2006, 11:19   #5
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would not suggest its a good idea for anything that moves!
Fine for a base install.

Most good marine antenna will have better gain than 3dB.

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Old 04 August 2006, 12:48   #6
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Height has to be more important than anything else - on a RIB you are going to be pretty much limited no matter what you do. Take a hand held VHF up in a plane and you will be amazed the range on it - even though it's got a titchy aerial and 5W.
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Old 07 August 2006, 05:54   #7
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Hi Simon
John is correct, having had to do phase & antenna matching with HF installations for hyperbolic radio ranging possiotioning systems, the short answer is don't - lifes too short, unless you really really need to!
If you want to get more range then as others have said height, good quality cable and decent connectors (not the trashy twist together units) and a decent antenna is the best way forwards.
Procom make some excelent antennas for the marine market in different dB gain levels.
I use the trionic? wire antennas is they are cheapish(£45ish) reasonable quality, thus I don't cry when a £170 fibre glass colinear breaks at sea.
I have two seperate radios on the rib with as much vertical and horizontal seperation as I can get and I have not had any interferance yet.
Hope it helps
Rgds
James
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Old 07 August 2006, 07:17   #8
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Quote:
Originally Posted by Jon Brooks
would not suggest its a good idea for anything that moves!
Devon and Cornwall Police appear to Co-phase their VHF Antennas on the patrol cars, not a good idea in a boat, keep it simple..

Quote:
Originally Posted by Jon Brooks
Most good marine antenna will have better gain than 3dB.
Oops sorry I mean additional 3dB of gain.


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