Mobile grounding and magnetic mounts…

[BMDaug]

So I’ve got a manpack radio system that I’d like to be able to use in the car as well. I have a compactenna and for now, just a mag mount (while I’m considering my permanent options).

I’m assuming to get the SWR to usable levels, since I’m using a mag mount, I need to tie the radio ground to the vehicle chassis ground to get the capacitive coupling effects of the body panel even though the manpack system is self contained… I’m placing the antenna near the corner of the roof panel as the compactenna manual recommends, but the SWR is over four according to the rig expert… I’ve never used a mag mount with a radio that’s not powered by the car battery.

Also, is it acceptable to place the compactenna about 18” from the car stereo antenna? The car stereo antenna is center back so the only way to avoid placing the compactenna near it is to place it in a front corner above the driver’s or passenger’s door…

Any advice is appreciated!

Thanks for reading,
Brian

 

[mmckenna]

RF grounds and DC grounds can be different things. Grounding your radio pack to the vehicle shouldn't make any difference.

The counterpoise/ground for the magnetic mount is through the antenna base via capacitive coupling. In other words, at RF frequencies, the small gap between the bottom of the magnet and the vehicle roof is essentially invisible.

If you are getting SWR that high, there's probably something else going on.

You want the antenna far away from the AM/FM radio antenna. The radio pack probably puts out a few watts, maybe more, and will desense the receiver, and if strong enough, will damage the AM/FM radio. I'd suggest putting a few feet between them.

 

[BMDaug]

RF grounds and DC grounds can be different things. Grounding your radio pack to the vehicle shouldn't make any difference.

The counterpoise/ground for the magnetic mount is through the antenna base via capacitive coupling. In other words, at RF frequencies, the small gap between the bottom of the magnet and the vehicle roof is essentially invisible.

If you are getting SWR that high, there's probably something else going on.

You want the antenna far away from the AM/FM radio antenna. The radio pack probably puts out a few watts, maybe more, and will desense the receiver, and if strong enough, will damage the AM/FM radio. I'd suggest putting a few feet between them.

Thanks! See that’s what made sense to me because I know people use a mobile antenna on a mag mount with an HT and it’s fine. I don’t understand what could be causing that kind of SWR. The compactenna is supposed to provide an SWR of less than 2.5 across the 136-174mHz 380-520mHz and 760-870mHz bands… what else could it be I wonder? The cable is about 15’ and is directly wired to the nmo mag mount which is factory terminated to an N connector, which is directly connected to the analyzer, so I’m not sure where the issue may lie.

The manpack is set at 30W so it is a significant amount of RF. I wanted to keep the antenna as far away from me as possible for this reason. Maybe I’ll just remove the car stereo antenna and use the existing hole for the compactenna…

Thanks again,
Brian

 

[mmckenna]

There's a couple of nice things about NMO mounts:

1. They are standardized, so you can take a simple 1/4 wave VHF antenna and pop it on there. Test again and it should show a lower SWR. If it doesn't, it probably means there is an issue with the connector or the mount itself. Don't assume that just because it was made by someone else, that they didn't screw it up. Go at it with a continuity tester and make sure you have continuity all the way through, and no shorts between center and outside shield. Wiggle all the connections to see if it's an intermittent fault.

2. They are standardized, and like all good standards, some manufacturers take liberty with the design. Make 100% sure the center pin on the antenna is making contact with the center button on the NMO mount. I've run into cases where certain antennas won't screw down far enough to make contact and it'll show up as a whacky SWR.

If you have used this magnetic mount before, check the coaxial cable for damage. Getting slammed in doors, rolled up windows, etc. will damage the coaxial cable.

And if the antenna is new, don't assume it's automatically OK. Manufacturing defects happen, and it really sucks when it happens to you. Try another antenna.

 

[WA0CBW]

Another way to check cables is to get a NMO to UHF adapter and a dummy load. Screw the dummy load on the adapter and connect the adapter to the NMO mount. Check the SWR. This can identify cable, connector, and installation issues.
BB

 

[BMDaug]

Another way to check cables is to get a NMO to UHF adapter and a dummy load. Screw the dummy load on the adapter and connect the adapter to the NMO mount. Check the SWR. This can identify cable, connector, and installation issues.
BB

That’s a great idea! Using a multimeter I can confirm that there is continuity of both conductors (0r shield to nmo mount threads and 0r center conductor to center conductor) as well as no connection at all between them. The antenna was good a month ago and has been sitting on the shelf until a few days ago…

I guess I’ll order an adapter.

Thanks guys,
Brian