VHF Tx Issues and Antenna Location

[danletkeman]

Wondering if anyone has some insight to an issue I am having with very limited TX ability with our VHF radios on some equipment. I can't seem to narrow down the issue and replacing everything results in the same problem so I don't believe it is a hardware issue. The only other thing I can think of besides the antenna location is the coax cable length or perhaps in some cases coiling of the excess coax cable.

Taking one issue at a time, this problem is on a 2004 Peterbilt 378 semi. The antenna is mounted on the mirror of the truck beside a CB antenna. The other mirror also has a CB antenna and there is a factory AM/FM antenna in the center of the roof towards the front. This truck can receive signals at a fair distance, but can only transmit within a few Km (10km) of our antenna mounted on a 68' tower. To compare to another truck, which has the antenna located at the backside of the roof of the cab, this truck can transmit and receive at very far distances (80-100km).

Does the location of the VHF antenna make that much of a difference or would there perhaps be some other interference or unwanted deflection of signal that I should be testing for?

 

[tweiss3]

Yes. The more metal you can put under the antenna, the better. On the mirror, the antenna may not be match well, and the cab blocks 180 degrees of the transmitted signal. Antenna type/design also has a big factor in it's performance and effectiveness.

 

[rivardj]

Wondering if anyone has some insight to an issue I am having with very limited TX ability with our VHF radios on some equipment. I can't seem to narrow down the issue and replacing everything results in the same problem so I don't believe it is a hardware issue. The only other thing I can think of besides the antenna location is the coax cable length or perhaps in some cases coiling of the excess coax cable.

Taking one issue at a time, this problem is on a 2004 Peterbilt 378 semi. The antenna is mounted on the mirror of the truck beside a CB antenna. The other mirror also has a CB antenna and there is a factory AM/FM antenna in the center of the roof towards the front. This truck can receive signals at a fair distance, but can only transmit within a few Km (10km) of our antenna mounted on a 68' tower. To compare to another truck, which has the antenna located at the backside of the roof of the cab, this truck can transmit and receive at very far distances (80-100km).

Does the location of the VHF antenna make that much of a difference or would there perhaps be some other interference or unwanted deflection of signal that I should be testing for?

Being mounted next to a CB antenna on a mirror mount likely places the VHF antenna too close to the CB antenna. I agree with @tweiss3 that insufficient ground plane is also a likely contributing factor in poor performance.

Coiling the coax cable has no effect on range. Length of the coax cable can be an issue if over 100 feet or more, not likely an issue if using the coax that came with the VHF mobile antenna mount.

 

[mmckenna]

Most antennas want a ground plane under them to work properly. Half wave antennas will work without a ground plane, however, they will work much better with one.

The mirror bracket, while likely grounded to the truck chassis, provides no ground plane. RF ground plane, and a DC ground are different things. Even if you were running a half wave antenna, the lopsided ground provided by the limited mirror bracket isn't going to help.

Close proximity to other antennas can be an issue, also. If not spaced properly, it can result in a directional pattern.

Your best option is to move the antenna off the mirror bracket and place it on the truck roof. Ideally you want about 18" of conductive material in all directions under the antenna mount to give it a suitable ground plane, so plan the location accordingly. It does not need to be center of the roof for it to work, just at least 18" in all directions around the antenna of conductive material. Make sure it's well spaced away from any other antennas on the roof, or any other large metal objects (trailer, etc).

Coiling up a few feet of coax should not be an issue. All coax has some amount of loss, but a few extra feet of coax cable on VHF isn't causing your issues.

You can easily test this out with a magnetic mount antenna to confirm performance. Once you've done that, install a permanent mount.