VHF Aerials

[Tallulah]

I need to install another VHF Aerial on my boat (2.4m) - does anyone know wheter there are any issues mounting two aerials near to each other - i.e. within 1 metre. If there are issues, how far apart do they need to be? Also, any recommendations on the best 2.4m aerial?

 

[hightech]

Generally you will find most boats with multi antenna installs are quite close to each other, usually either side of the arch.

What are you proposing to attach to each one? VHF on one and AIS on the other??

If they are both transmitting aerial, then in generalyou are unlikely to be transmitting on both at the same time and VHF/AIS units are designed to not have a problem with 25 watts effectively injected directed to the antenna input.

 

[Tallulah]

Thanks for this. Currently I have AIS and VHF split from one aerial. My VHF Tx/Rx range has dropped as a result, hence two aerials. My boat ( a Princess 50) has the usual Princess arrangement of a VHF on one side and FM aerial on the other. I want to double up on the side with the VHF (cable routing makes it hard to do it the other way), so the aerials would be about 1 metre apart. One would be VHF only (Simrad) the other would have a second VHF and an AIS Transponder attached to it. I guess the only time they would be transmitting together if I was using the VHF radio and the AIS was broadcasting positional/course data. Would this be an issue?

 

[Hurricane]

Hi Brian

I seem to remember that you used the same contractors as us.
Like Tallula, our Princess was supplied with two aerials - one each side of the flybridge. One was for the Simrad - the other for the FM radios. I got them to change the FM one for another VHF and connect the Comar AIS transponder to it. The FM reception was than switched to the terestial TV antenna which can also double for FM. There doesnt seem to be any problems when you have AIS on one side of the boat and the Simrad VHF on the other.

Anyway, that was what we did which wasnt really your question. Are you sure you cant route cables so that you dont have to install another aerial?

 

[Richard10002]

[ QUOTE ]
If they are both transmitting aerial, then in generalyou are unlikely to be transmitting on both at the same time and VHF/AIS units are designed to not have a problem with 25 watts effectively injected directed to the antenna input.

[/ QUOTE ]

When I put a second VHF aerial at my mast head, (about 2ft from the original, it was suggested in no uncertain terms that one would fry the equipment at the end of the other, (not by you, I hasten to add /forums/images/graemlins/smile.gif ). I'm pretty sure other sources, confirmed this for me, so I put another one on the pushpit and count the spare at the masthead as a spare, (which I am going to use soon).

Assuming you are right, and I am sure you wouldnt say it if you werent, it gives me, (and others), more options - I dont wish to sound rude, but do have a reference for the above /forums/images/graemlins/blush.gif

Cheers

Richard

 

[oldgit]

Think the things need to be more 1/2 wave length apart.
The further apart the better.
If they are too close together you will get almost the entire Tx output of your VHF going straight into the RX section of something else,something only designed to cope with tiny amounts of energy.

 

[Richard10002]

Hence my question... Geoff is saying:

"VHF/AIS units are designed to not have a problem with 25 watts effectively injected directed to the antenna input. "

and, because of who he is, and what he does, the comment cant be dismissed out of hand - he must have it on good authority, or he wouldnt have said it.