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Old 25 February 2005, 12:46   #1
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Aerial positioning

A really techy question for tonight!

I`m looking at replacing my old Silva VHF (hey it came with the boat!) and putting a Icom M421 in its place. This is gonna be a straight swop but I thinking about keeping the old one on board in a different location on the back of a seat pod.

So I`m gonna need more electrics and another aerial to do this.

My question is, if I put another aerial on the a frame its likely to be at exactly the same level and inclination as the existing one so will this cause problems, particularly if for whatever reason I`m transmitting at the same time on both radios on different channels?

Chris
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Old 25 February 2005, 13:31   #2
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Yeh, it might.
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Old 25 February 2005, 14:12   #3
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Very likely!
be much better off using one antenna and a splitter between the two radio`s - so only one set at a time will be in use - but in an emergency you could switch to the back up radio if required!
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Old 25 February 2005, 15:08   #4
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So if I want to transmit on both at the same time what do I have to do position the other aerial?
Chris
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Old 25 February 2005, 17:15   #5
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Quote:
Originally Posted by stevetheboat
Very likely!
be much better off using one antenna and a splitter between the two radio`s - so only one set at a time will be in use - but in an emergency you could switch to the back up radio if required!
Stevetheboat,
I would have to dissagree with that. There would be more loss in the tx/rx by using a splitter than if you had two antennaes in proximity. You would get impedance missmatches causing high standing wave reflections (SWR). (gosh thats sounds really techie )

Remember the phrase KISS : keep it simple stupid
.
Two antennaes on the A frame will interact, but this will be pretty insignificant, and give you a backup if one should give problems.

Dont struggle to make things easier!
Tim'mers.
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Old 28 February 2005, 04:02   #6
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Please don't go down the splitter route, this is sort of okay if one unit it recieve only - but having spent large amounts on the correct kit it was deemed a waste.
If you are going to have two sysems onboard ( like I'm installing ) remember how I got the NSSA systems set up. You want at least 1.5m horizontal seperation and at least one antenna's worth of vertical seperation.
YOU MUST ALSO MAKE SURE THAT YOU DON'T HAVE BOTH SET TO THE SAME FREQUENCIES AND TX ON ONE UNIT as you will probbably toast the input on the other unit.
Give me a call if you want to go over it
Jelly
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Old 28 February 2005, 09:05   #7
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So if I want to transmit on both at the same time what do I have to do position the other aerial?
Chris
Follow Jelly's advise. But even then I'd be surprised if you didn't get interference between the units if transmitting at 25 watts with the antennae so close to each other. The antenna will transmit least from its end, so vertically above/below with the maximun separation is likely to be your best bet.
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Old 28 February 2005, 12:21   #8
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<Alarm Bells>

I have and ICOM 401 and an ICOM DS-100 with two identical aerials mounted as far apart as possible on the A frame(probably only about 1m apart), but with no verticle separation.

I think as I transmit a DSC call, the 401 switches to channel 70. Is this likely to cause a problem?

Anyone else using a DS-100, how have you rigged your aerials?

TIA

David
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Old 01 March 2005, 05:26   #9
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Two antennas and two radio's should be ok.
You need to get the two antenna's as far away from each other as you can.

Due to the short distance you have on the A frame you will get some of what we call "de-sense".
This will mean that as you TX on one you will have the potential stop the second set RX as well.

This effect gets less, a great deal less, once you get over 1m apart

I agree a co-ax switch will cause you loss and will get salt water damamge very quickley.
Never seen one made for out door use.

Just as a guide you get on average 1db loss for every connector and (based on average quality RG58 co-ax cable) 1db for every 10m of co-ax.
If you have 3db loss you have halved you power output.

As far as D100 goes.
The antenna on this is for RX of ch70 only.
The TX of DSC and all voice RX/TX is done via the antenna on the M401 or M503.
You will get "de-sense" if the antenna are very close but again this effect gets less over 1m apart.

This then will only effect an incoming DSC message if you are TXing on the radio.
I have not heard of anyone suffering and problems with this to date.

I hope this all helps

Regards
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Old 01 March 2005, 11:30   #10
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<Bells Cancelled>

Ta Jon. Nice clear explanation.

David
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