T&T: Location of VHF antenna

[C. Marin Faure]

 >do I need to be
concerned with any possible interference from the existing VHF or GPS
antennas?

According to the proprietor of the marine electronics shop where we  
have purchased all our boat's electronics that we have installed  
since acquiring the boat, including a second radio and antenna, VHF  
marine-frequency antennas are not supposed to be separated by any  
multiple of (I  think) 17" (I'm sorry I can't be more definitive, but  
it's been a number of years since we installed our second antenna).    
The reason is that if they are separated by a factor of this distance  
(x1, x2, etc) the antennas can passively interfere with the  
transmission of signals from either one of them.

I have mentioned this in the past and have had several people tell me  
this is not true, so I have gone back to the shop several times and  
asked "are you sure?"  He has hauled out some technical manuals that  
show it to be true.  Perhaps not for all frequencies in the marine  
spectrum and perhaps not in all directions, but if the antennas are  
these distances apart they apparently have the potential to interfere  
with signal transmission.  So we installed our second antenna a  
distance from our first one that was not a multiple of 17" (or  
whatever the correct number is if that's not it) and so far as I know  
have never had a problem.  It's also a good idea, our electronics  
supplier said, to separate VHF antennas as far as is practical.

I have never heard that a GPS antenna can cause any interference with  
a VHF antenna or visa versa.


____________________
C. Marin Faure
GB36-403 "La Perouse"
Bellingham, Washington