Weird SWR issue.

[super61]

I have a 1985 GMC G2500 vandura. I am upgrading the radio and antenna to a Cobra bearcat 880 and a Wilson little will. My issue is I can't put it on the top as it's fiberglass. The windows, however, are metal. So I stuck it to it and worked for a good 2 hours and I could not get the SWR to move at all. No matter what I did the SWR would not move. Finally I put it on the hood at the section between three panals and I have 1.0 across all channels. Can anyone tell me what the difference is? I know magmounts don't need a ground as it's built in but I am lost to this. I would like it on my roof but if I leave it on the hood I may not get good RX/TX.

 

[ko6jw_2]

Mag mount antennas need a ground plane. Hence, the need to stick them on something metal. They generally don't perform well otherwise.

 

[mmckenna]

There's a couple of ways to approach this. Fiberglass tops on vehicles isn't uncommon, so not a big deal.

Ideally, you want to ditch the mag mount. Install a permanent antenna base in the center of the fiberglass top. I'd strongly recommend a thick mount NMO. On the underside of the fiberglass top, you'll need to establish a ground plane. That can be as easy as using some wire, foil, sheet metal, copper flashing, anything electrically conductive. Make an X pattern radiating out from the center of the mount. Make it as big as you can, even connect it to the vehicle body where it meets.
On the NMO mount, install something like a Larsen NMO-27. I've used them for 30 years with excellent results.

Any through the roof type antenna mount will work, but the NMO mounts give you a lot more possibilties.

You can use the mag mount, but you'd need to install a piece of steel on the roof and still make the ground plane underneath. Then you still need to get the coax inside.

The hood will work, but as you expect, not ideally. The ground plane will be lopsided and make the radiation pattern directional, which you may not want.

 

[FiveFilter]

I use a mag mount on my Jeep Wrangler. It has a fiberglass top, so I place the mag mount antenna in the center of the hood, biased a little towards the windshield. I bring the wire into the passenger's side window.

Since I only use the radio for specific trips, I only install the antenna when I need it.

I get good range and performance. My antennas, however, are a bit longer than yours. I use either a 56-inch Wilson 500 or a 62-inch Wilson 1000, depending on what I feel like using at the time.

 

[K5MPH]

I have use the little will mag mount before and never had any trouble at all from this antenna.....

 

[paulears]

I think people get sidetracked with the notion of ground, as in ground plane, and then metal as in something to stick the mag mount to - but the real issue is attaching the magnet to something metallic is NOT a ground plane. Top be efficient, antennas have to be made to function by design - and if the ground plane is a critical part of the antenna, simply attaching them just isn't enough. Foil, wire, plates, big sheets of tin are all trying to be a ground plane. The perfect one is on boats - the antenna goes up, and the sea makes a pretty unbeatable ground plane. Anything smaller than an ocean isn't as good, but performance is still good, until the groundplane area just gets too small to function effectively. Generally, the larger the ground plane, the better the system performs.

 

[Rred]

Magmounts typically DO need a ground plane, and they typically accomplish that by a capacitive coupling to the large metal surface (roof or hood) that they are placed on.

I don't know the "Little Will" but it appears to have a large base loading coil, and that kind of design is often used where the base loading coil forms a counterpoise, acting as a ground plane for the antenna. So if that's what Wilson is doing with that antenna (there's no other need for a base loading coil) then this particular antenna would NOT need a ground plane.

One of the ways you can make a magnetic mount antenna stick to a fiberglass roof, is to go online and buy the biggest strongest (N50 or N52 grade) rare earth magnet that you can find. Hockey puck sized. That may set you back $50 or more, but if you put that antenna under your roof with a bit of duct tape (use glue if it works out) then the magmount should literally jump out of your hands and onto the roof.

And if you've never used those large rare-earth magnets? BE VERY CAREFUL THEY ARE DANGEROUS. If two of them snap together across a bit of your skin, you are going to have a very painful time trying to get them off again. They can literally JUMP and require two hands or a very clever mind to separate again.

If you want to try this approach--don't cheap out on the magnet.