VHF antenna minumum distance apart TX/RX

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Ok so I had some radio equipment installed on a boat and they installed the antennas WAY too close together. One antenna is a standard fiberglass VHF Marine antenna connected to an Icom marine radio. The other is a SS whip (pctel MWV1322s to be excact) connected to a Motorola government radio. Well the government radio is very high wattage (45 or 65) and the antennas are only about 6-8 inches apart. so needless to say when I transmit on the Motorola radio, it overloads the Icom and it now barely pics up any marine traffic.

Since it is just a standard NMO mount I am going to move the antenna for the Motorola radio. Since they are both VHF radios, what is the min. distance apart I need to put them? Obviously more is better but it is on a boat where real estate is limited.

Also, the further I get the motorola antenna away from the Icom, the closer it will be to the vessels radar unit..any issues there with either the radio or radar unit?
 

jonwienke

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To prevent RF interaction just from the presence of the antennas (both radios RX), you want a minimum of 1/4-wavelength. Depending on TX power, you may need a lot more. With 50W or more TX, you may need several feet to prevent actual damage to the RX radio.
 

prcguy

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There is not that much difference in RF pickup between two VHF antennas 6in apart and 16in apart or 1/4 wavelength at VHF and there will be a lot of transmit energy going into the receivers in both cases. Another problem with antennas that close is the radiation pattern will probably look like a 2 element beam and far from omni directional.

A good rule is to keep antennas at least 1/2 wavelength apart or at least 3ft at VHF and even at that spacing the radiation pattern may look like a clover leaf with good signal going out in four directions and lousy performance in between the clover leaf pattern.

I would point this out to whoever installed the antennas and have them put them much farther apart. and give them some vertical spacing if possible.
prcguy
 
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I could get a little bit of vertical difference at the expense of as good of a ground plane. but even then it would only elevate it 2 foot or so and it is a 4 foot antenna so.........But that would also give me an extra 2 foot or so distance from the other antenna while raising it above the radar dome.
 
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