Antenna placement: Separation by Wavelength?

[Leadenwah]

I need to mount 3 antennas of different bands on 1 metal plate which is 8-1/2" x 13". This is the only surface which can be used for this base station application as it is high and in the clear.

The antennas: #1- Larsen NMO-UHF colinear ( 450-470 ) TX & RX. #2-VHF Broadband Quarterwave ( 150 -160 ) TX & RX. #3-Larsen NMO800 (800-866) for RX only.

I know that separation based on wavelength is important, but I just can't figure out the best distance between them or how to group them, that is, 3 corner triangle, straight line, etc. How can I best orient these 3 antennas to minimize interaction?

[HLA]

that space isn't big enuf. they need to be spaced so they can never accidentily touch eachother. plus that isn't a big enuf ground plane for the vhf. i don't know what your situation is but if it's on a tower like you're making it sound you could put the uhf and 800 antennaes on the top and off to the sides and put the vhf upside down underneath in the middle? i've had to rig 2 antennaes on 1 ground plane on opposite sides before but not 3.

[tvsjr]

Suffice it to say, you aren't going to be able to do this electrically "right". Right and working are two different things. Observe that most any squad car install has antennas that are technically too close together.

Do you have access to hang stuff underneath? How sturdy is the plate? Any height restrictions?

Assuming you can mount things wherever you want, I'd then go on a search for no-ground-plane antennas. Something like:
Maxrad MWV1322HD(S) - VHF
Maxrad MUF4502(S) - UHF
???? - 800 (don't feel like searching for one!)

That eliminates your ground-plane issue. I would then mount the VHF on the top side, dead center.... the UHF on the bottom side, dead center... and the 800 absolutely as far away as possible.

If you can't mount to the bottom side, I'd suggest a right triangle:

Code: Select all

     8.5"
:----------:
:VHF       :
:          :
:          :
:          : 1
:          : 3
:          : "
:          :
:          :
:UHF    800:
:----------:

Such a configuration uses the properties of the various bands, plus the fact that you are RX-only on 800, to your advantage.

You might also consider the VHF and UHF on diagonal corners, and the 800 hanging off from underneath.

Alternately, have you considered bolting a larger plate of sheet metal (1/4" aluminum would do nicely) to the existing plate?

[Leadenwah]

Thanks, folks,

These are good suggestions. Thanks also for the diagram. I knew that I was on thin ice with this plan. The VHF is very ridgid as is the NMO-800, but the UHF has some flex to it since it's a 5/8 over 1/2 type antenna.

This is actually on a very high brick chimney, but I'm limited to the 13 x 8-1/2" because that's extent of the opening down which the cables could be routed. The rest is SOLID brick. The aluminum is 1/8 inch thick and the total plate area is 44" x 24"...but, I'm stuck with that limited mounting area since the plate mounts flat on the surface. There is no overhang, the plate exactly fits the surface on which it is mounted.

I'm using NMO low loss "bulkhead" mounts with "N"connectors on the underside for the cable connections.

I'm limited to this height and this configuration is the only way to get around restrictions on visible antennas on the building. The difference between Not Correct and Not Working is something I've come to understand in quite a few other areas as well. I've got lots of thing which Ain't right, but do work to some extent and...some of the time.

Thanks also for the suggestion on antennas not requiring a ground plane. I'l l check out that Max Rad.

Many thanks !

[HumHead]

If your situation is actually that specific, then why don't you just use a single VHF / UHF dual-band mobile antenna, run one feedline down, and install an appropriate triplexer at the far end to split out VHF, UHF, and 800?

[Jim202]

You might want to look into using a Sti-Co triband
antenna. Comtelco Antennas has several antennas
that work good for this application. They are not that
expensive. The problem here is the small physical
size of the ground below the antenna. What ever
you do will be a compromise.

Like has already been said, use a multi band antenna
and then run a single coax cable dow to the radios.
At the radio end you put in a "triplexer" and then run
a cable to each of the radios. One each for VHF, UHF
and 800 MHz. Works real good on all the applications
I have been involved with.

We have started using this solution on a number of
mobile command vehicles. They come to us and want
more radios installed than the original design. As you
can't get into the ceiling of most of the MCV, you need
to use the existing coax feeds. Just replace the
antennas and go for it.

Jim

[Leadenwah]

I'll check out the Sti-Co ( only know their disguised antennas ) and the Comtelco Antennas. It's worth looking into.

The asset is the height otherwise I wouldn't be trying to pull off such a compromise. My next best location is 15 feet lower and under a roof of fiberglass shingles which are impregnated with metallic ( probably pulverized copper slag ) particles. This seem like effective shielding and thus not an option.

From a previous suggestion, I'm fiddling with elevating one of the three original antennas above the other two. It may have to be the smallest for appearance sake which would be the NMO800.

Thanks again for the help.

[N4DES]

Austin makes really nice multi-band antennas
http://www.austinantenna.net/multiband.htm
but they require a really good groundplane.

Mark

[Leadenwah]

Thanks, Mark,

I'll check out the Austin line. I recall hearing about an Austin Condor, but I'm sure that they have a good offering.