Batboard

Are ARES/RACES group has been given a motorhome that was used as the County's mobile Command Post. This motorhome was a stock motorhome that they converted to a mobile command post. They have about 8 antenna's VHF/UHF. Some gain, other 1/4 waves. Motorola 3/4 mounts are used. The antenna's are mounted to a metal channel that runs the entire lenght of the vehicle. RF power out for each radio is 60 watts or less.

Should the antenna's be mounted a 1/4 wave lenght apart to avoid intermod?

We want to re-due this correctly, and wisely seeking the good advice from this board. I don't want to do this twice.


Jim Duram, K8COP
Muskegon County, MI.

I believe at least a 1/4 wave length for the lowest frequency.

They should normally be a 1/2 wave apart.

At least 1/4 wavelength apart based on the lowest frequency. If you can go further that's even better.

Yes the farther the better. I have 2 quarter waves on my roof about 2 1/2 feet apart and still get knock out one the one when I transmit on the other.

Yes, the farther the better, take advantag of that big ground plane.
You may still get desense and intermod, so your operators need to be aware of when others are transmitting, and if they notice it, they can have the other transmitter stand by....
Train them to use LOW power as much as possible.

If you have 2 radios for the same band, put them at oposite ends.
Or put the lower priority one down on the cowel, set at low power.

Keep any lowband as far as you can from any metal in the pattern.

I agree with Max-trac and there will be a learning curve for the operators.

I think Tessco still sales a adjustable MNO mount for varied roof thicknesses also. Try using the same type of mounts so the antennas can be moved if needed. A patch panel will make this easy.

Jim, the channel you describe would not be a good ground plane. I am thinking the "channel" is just a narrow piece.

Even with a good all metal roof there is going to be interaction with some antennas and their location(s). I allow the cables from all the mounts to be able to reach all the radios so the antenna position can be changed arround easally. Also use a good well shielded coax cable, not the cheep stuff, there will be cross coupling between the cables where they are bundeled together.

I recently had an example of coupling;
100 watt 6 meter Maratrac with a tri-band Maldol antenna.
Scanner on a VHF 5/8 wave (which works great on 6) 3 feet away.

I measured 20, YES TWENTY, watts when I put a watt meter at the scanner end of its coax!
Needless to say, the scanner now needs fixing.