I've been working with some fishing boats and these guys want to have
redundent VHF setups - two radios, two antennas. Only one would be used
and turned on. My concern is that the unused antenna could couple as a
parasitic element and effect SWR of the operating antenna. Do I really
need to worry about it? If so haw far apart do the two antennas have to be?
--
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SPAM is just too much to take

Ted wrote:

I've been working with some fishing boats and these guys want to have
redundent VHF setups - two radios, two antennas. Only one would be used
and turned on. My concern is that the unused antenna could couple as a
parasitic element and effect SWR of the operating antenna. Do I really
need to worry about it? If so haw far apart do the two antennas have to
be?


Ted,

One way is to place the aerials at a "non 1/4 or 1/8" wavelength apart from
each other but VHF is really not that bothered if you are more than a 1/4
away.

If you really want to be sure, just tilt the spare aerial 90 degrees in the
horizontal plane. That will limit the interaction even further.

I imagine the spare receiver have a 50 Ohms input impedance but only while
powered up! This means that there are no good way to tell whats at the end
of the spare aerial...

But, forget about the details and just install the aerials at a more than
but at a non-1/4 of a wavelength distance apart and all will be good
enough!


Cheers


Dan / M0DFI

In article ,
Ted wrote:

I've been working with some fishing boats and these guys want to have
redundent VHF setups - two radios, two antennas. Only one would be used
and turned on. My concern is that the unused antenna could couple as a
parasitic element and effect SWR of the operating antenna. Do I really
need to worry about it? If so haw far apart do the two antennas have to be?

These must be very small fishing boats, as ALL vessels over 65 Ft MUST
have two Vhf receivers operational while navigating. Marine ch16, and
marine ch13. this is required by The Bridge to Bridge Radiotelephone
Act.
It really isn't a big deal as long as the two antennas have some
seperation either vertical or horozontal. I have installed multiple
MORAD 156HD antennas on a Masthead Tree, using 24inch Standoffs,
and not had any problems with simulataious operations as long as the
radio's frequencies were seperated by 50Khz or more. Of course I
was working with good commercial type Vhf's like Modar55/75 or
SEA156's, and not the RayJeff consumer crap that one would usually find
on noncommercial vessels.

Bruce in alaska
--
add a 2 before @

Dan Andersson wrote in
:

Ted wrote:

I've been working with some fishing boats and these guys want to have
redundent VHF setups - two radios, two antennas. Only one would be
used and turned on. My concern is that the unused antenna could
couple as a parasitic element and effect SWR of the operating
antenna. Do I really need to worry about it? If so haw far apart do
the two antennas have to be?


Ted,

One way is to place the aerials at a "non 1/4 or 1/8" wavelength apart
from each other but VHF is really not that bothered if you are more
than a 1/4 away.

If you really want to be sure, just tilt the spare aerial 90 degrees
in the horizontal plane. That will limit the interaction even further.

And render it useless for calling the coast guard, since they are
vertically polarized (unless you hinge it to come up when deployed, but
that's one more thing to break). Basically just make sure they are more
than about 4 feet apart horizontally and that, if really close, the spare
radio is switched off the feeder unless it's needed. If they are mounted
vertically, one over the other, you don't even need to do that. But the
lower one will not be as efficient or effective as the higher one.

--
Dave Oldridge+
ICQ 1800667