Help with GMRS Antenna Placement

[Delivers1234]

So in this figure Im trying to place the Tram-Browning BR-6140 GMRS antenna. It has those typical clamps for a pole. Could I add it to X1 or X2 in the diagram? It is a pole with CB above it? X1 would be higher but closer to CB antenna. Or would I get interference? etc. The antenna are about 3.5 ft apart. Its not to scale and I'm on the second floor. Any issues? Its a side view. The powerlines are far in the back.

 

[vagrant]

The metal mast will interfere with an antenna mounted against it at position X1 or X2 and who knows what your SWR will be.

Alternatively, replace your Tram BR-6051 antenna with a dual band antenna that will handle GMRS and MURS. Something like the Comet GP-9NCA and then connect whichever Comet 416 diplexer in the house to split your signals to the two radios, if you have two. The GP-9NC is rather long, but this is radio. I'm sure there are shorter alternative antennas, but getting the antenna away from the mast and using a diplexer will work fine.

I currently use a dual band Diamond X50 antenna with a Comet CF-416 diplexer. It splits it to Motorola VHF and UHF handheld radios. I mostly use them for amateur which is why I use the Diamond X50, but the antenna SWR is under 2:1 on MURS and GMRS. The gain is not super duper, but the repeaters around here are easy to hit and simplex GMRS/MURS is not an issue for me.

Update: It looks like there is also a Comet GP-6NC which is a bit shorter and less cost. Thinking further, if you have a discone laying around you could use that instead, but it won't have the same gain.

 

[Delivers1234]

Thank you. I want to keep murs for a project. I guess I’ll add another j mount for the gmrs.

 

[Delivers1234]

Also, when does desensitization occur? When the tx radio antenna is next to another antenna? Does the other antenna need to be on to be desensitized? If it is off when the tx radio transmits, is it protected?

 

[vagrant]

Much depends on the radio. Still, even when the radio is off energy can still travel from the antenna through the coax to the radio. Too much and game over. The factors to consider are the power output (watts), the gain of the antennas and distance from one antenna to another, both vertically and horizontally. If all three of your radios, GMRS, MURS and CB are putting out five watts or less, your J mount spacing should be fine as far as protection.

A way to test is to put an SWR meter on each coax line with a 50 Ohm dummy load on the other side. Then transmit on the other two antennas and see if any energy is coming back in. Do this for all three antennas. Just to give you an idea of spacing, I have three separate TX radios and antennas on my vehicle and I don't have an issue after testing using 50W. Your antenna 3.5' spacing is similar to my vehicle, but your antennas have more gain.

As to desense, if you're talking on one radio you're not listening to the other, but it will probably occur. You'll need to test your particular setup.

 

[TomLine]

Use a horizontal stand-off from the cb mast or one of the other masts? (that cb mast may be part of it's circuit). Your proxmity to those plenty enough to blow off your feet Kv power lines make me feel fortunate that I have only a few phone drops to deal with.

 

[Delivers1234]

The power lines are about 30-40 feet away.

 

[mmckenna]

You need to stand the antenna away from the mounting pole if you cannot mount it on top of the mast.

You can buy fancy brackets that will do this easily, or you can go to Home Depot and get what you need.

Get two of these:

Superstrut 2 ft. 14-Gauge Half Slotted Metal Framing Strut Channel - Silver Galvanized - Strut Channel (ZB14HS2EG) ZB14HS2EG - The Home Depot

The Superstrut 2 ft. 14-Gauge metal, half slot Channel is designed for use to support various building construction including trapeze systems, shelving and other conduit supporting applications. Has an

www.homedepot.com

Get two of these to match the existing pole you are mounting to:

Superstrut 1-1/2 in. Universal Strut Pipe Clamp - Silver Galvanized (Z703 1 1/2EG-10) Z703 1 1/2EG-10 - The Home Depot

Visit the Home Depot to buy Superstrut 1-1/2 in. Conduit Clamp Z703 1 1/2EG-10

www.homedepot.com

Those are 1 1/2". Your mast may be a different size.

Get two of these:

Superstrut 3/4 in. Strut Fitting Universal Strut Pipe Clamp - Silver Galvanized (Z703 3/4EG-25) Z703 3/4EG-25 - The Home Depot

Visit the Home Depot to buy Superstrut 3/4 in. Conduit Clamp Z703 3/4EG-25

www.homedepot.com

Those are for 3/4" conduit.

Get a short length of 3/4" EMT conduit (aka: Thin Wall conduit). You only need 2 feet or so.

Use the strut and clamps to make two stand off brackets. One side will attach to the mast you already have. The other side will hold the 3/4 EMT conduit. Mount your GMRS antenna to that.

That'll get you about 2 feet separation between the supporting mast and your antenna, which should work fine. For a small antenna like the GMRS antenna you have, that should be more than sufficient to support it.

Based on your location, you should have no problem hitting some of the GMRS repeaters in the area.
I peeked at your amateur license, and realized I was over there last weekend. Met my sister and her family at Elli's for dinner on Sunday night.