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Old 04-12-2011, 15:19   #16
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Re: VHF Coax

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13 years with no problem leads me to believe that having them 12" apart isn't hurting anything
Well, you haven't had the AIS receiver in the system that long as class B was introduced just 4 years ago but even so, if it was going to damage it, it would likely have done so by now. Just for the OP's sake though, it's not the prudent thing to do. You obviously have an excellent installation there, your fine with the antenna's in very close proximity and you've had no issues with it. I'd be a little too concerned about transmitting 25 watts on the primary VHF that close to an AIS receiving antenna. The splitter has some loss so that may be helping the AIS receiver. Who knows? At least the 2 meter rig is removed in frequency by around 10Mhz.

Eric
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Old 04-12-2011, 15:28   #17
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Re: VHF Coax

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Originally Posted by fairbank56 View Post
Well, you haven't had the AIS receiver in the system that long as class B was introduced just 4 years ago but even so, if it was going to damage it, it would likely have done so by now. Just for the OP's sake though, it's not the prudent thing to do. You obviously have an excellent installation there, your fine with the antenna's in very close proximity and you've had no issues with it. I'd be a little too concerned about transmitting 25 watts on the primary VHF that close to an AIS receiving antenna. The splitter has some loss so that may be helping the AIS receiver. Who knows? At least the 2 meter rig is removed in frequency by around 10Mhz.

Eric
Yes, sorry about that - the AIS is 3 yrs old. The 2 meter and VHF are 13yrs. The splitter is not an active splitter, but a mechanical A-B type, so probably not a big loss.

But as far as proximity, I see lots of boats with two antennas on the masthead. I also see lots of boats - particularly powerboats - with antenna farms on arches. These antenna are as close together as the ones on my masthead. I also see antenna groupings on radar poles - with multiple vhf/ais antenna in closer clusters than my masthead. Something must be working OK because none of them are complaining. Perhaps the electronics in newer equipment have better and more robust reject/attenuation circuitry?

Mark
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Old 04-12-2011, 15:48   #18
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Re: VHF Coax

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Originally Posted by colemj View Post
Something must be working OK because none of them are complaining. Perhaps the electronics in newer equipment have better and more robust reject/attenuation circuitry?

Mark
It's just that 12" seems mighty close to me. I'm a service tech and Iv'e had clients bring in their VHF radio for me to just "check out" before the season and many times Iv'e found weak receiver sensitivity due to bad RF amps and they are suprised to hear that because they thought their radio was working fine. Their just not hearing the weaker signals that they normally would have. Not saying it was because of antenna placement, could also be from bad connectors, nearby lightning strikes or other issues. But like you say, there's only so much room and there's always a compromise when you have multiple antenna's.

Eric
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Old 04-12-2011, 16:17   #19
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Re: VHF Coax

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I'm a service tech But like you say, there's only so much room and there's always a compromise when you have multiple antenna's.

Eric
Eric im installing my AIS 2mt above my VHF off set about 1mt would this be too close?
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Old 05-12-2011, 15:09   #20
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Re: VHF Coax

I contacted Shakespere and they suggested mounting a seperate antenna along the mast trunk below the mast head , they have a standoff bracket 4716 which mounts to the mast and extends the antenna 8" from the mast on a 45 degree angle. looks clean and simple, out of the way except for going up the mast, I think this is way to go.

Thanks to all

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Old 05-12-2011, 15:12   #21
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Re: VHF Coax

They suggested mounting it on a 45 degree angle? Maybe I didn't understand that bit. Will mounting against the mast (8" off) give the antenna a blind spot or effect its radiation pattern?

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Old 05-12-2011, 15:37   #22
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Re: VHF Coax

Poorly written, mia a culpa, the bracket is at a 45 degree angle to obtain a standoff of 8", The antenna is plumb to the mast, (one side vertical and the other horizontal). Their tech guy stated that this was what the bracket was designed for and I will get good performance. Eight inches out from the mast, I am no RF guy, seems it would shadow, the mast is about 7.87" wide, I am inclined to give it a try. The only place I see a complete shadow is at 90 degree abeam ship and even then the shadow is not a 100 percent.

What say you

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Old 06-12-2011, 11:58   #23
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Re: VHF Coax

You can use the masttop for the radio antenna (the thick cable) and place the AIS antena lower (e.g. on the arch) and use a normal (fine) cable.

I think the only justification for the AIS antena high up would be if you have a class A transceiver.

b.
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