2m/440 dual band tonneau cover installation

[KD5ITM]

I’v mounted a Comet SBB-7 dual band antenna to the top of my trucks Leer fiberglass tonneau cover on the right side right behind the passenger side back glass using a custom made stainless steel mounting bracket. I fully understand that this installation doesn’t have the most efficient ground plane. Sticking with this antenna installation, what can I do to maximize what ground plane there is?

Should I run a copper ground strap from one of the mounting bracket bolts (bottom side of the tonneau cover) to the inside top lip of the bed rail using something similar to a star washer to insure that the ground strap has a solid bond connection to the bed rail lip? I’v also thought about lining the compleat underside of the tonneau cover with aluminum window screen, making sure that it has a good solid bond to the antenna mounting bracket, which will act as a ground plane.

As the curre dual band antenna installation is, I can hit a local club repeater from located on the south side of Houston, Tx from my QTH drive way 45 miles south of me. The issue I have is that where the antenna is located on the truck, when talking on this repeater located 45 miles south of me, my RX/TX signal is somewhat directional depending on where the truck is pointing, but when talking on repeaters located with in 15 - 20 miles from me, the issue with the RX/TX signals being somewhat directional is not a very noticeable issue, or not an issue at all. It’s only an a bit of an issue when talking to stations/repeaters located at further distances . Which is caused by not having the most efficient of a ground plane. For almost 25 years, my 2m/440 dual band antenna has been mounted in the same location on 2 prior trucks with no issues with the RX/TX signals being somewhat directional. Both of those trucks, the antenna was mounted with the same stainless steel bracket to the side of a drop in bed diamond tool box. The antenna mount did not have any metal directly under the base of the antenna so therefore what ground plane that was below the antenna was not the most efficient. The first pictures below is how my Comet SBB-7 dual band was mounted to the side of the tool box on both prior trucks. The second picture was taken when I was installing the antenna which is how it’s currently mounted now.
Thanks
Tim - KD5ITM

 

[mmckenna]

A strap from the mount to the bed isn't likely to help. There's a difference between an RF ground plane and DC ground. The strap is really only going to give you that DC ground, which you probably already have with the current mount.

Really the only way to improve that would be to have a 1/4 wave of conductive material in all directions under the very base of the antenna. That's not going to be possible off to the right side of the truck. Adding it in other directions is still going to result in a lopsided radiation pattern.

Even if you went with a 1/2 wave 'ground independent' antenna, the lopsided ground plane is still going to be an issue.

One of the reasons why we mount antennas dead center on the cab roof.

 

[KF0NYL]

I would do like @mmckenna suggested. Get a sheet of metal that is at least a 1/4 wave all around and install the NMO mount in the center of the sheet. You can epoxy the sheet to the underside of the tonneau cover. In the center of the cove will be best. I know a lot of people do this with camper shells/toppers for their trucks.

You will want a sheet of metal that is 3 ft by 3 ft for the 2m band and place the NMO mount dead center of the sheet.

 

[KD5ITM]

A strap from the mount to the bed isn't likely to help. There's a difference between an RF ground plane and DC ground. The strap is really only going to give you that DC ground, which you probably already have with the current mount.

Really the only way to improve that would be to have a 1/4 wave of conductive material in all directions under the very base of the antenna. That's not going to be possible off to the right side of the truck. Adding it in other directions is still going to result in a lopsided radiation pattern.

Even if you went with a 1/2 wave 'ground independent' antenna, the lopsided ground plane is still going to be an issue.

One of the reasons why we mount antennas dead center on the cab roof.

Well with my previous 2 mobile installations, the Comet SBB 7 dual band was mounted to the side of the tool box in pretty much the same location it’s currently mounted on the tonneau cover. I know the top of the tool box acted as a ground plane, but you can see that there’s no ground plane to the right of the antenna. It worked like a champ and twice on Sundays. I never had any issues with the antenna TX / RX signals acting in more of a somewhat directional way depending on how the truck was pointed.
I also have a 20m Hamstick mounted on top of the tonneau cover behind the left side back glass. With my two previous mobile installations, the Hamstick was mounted on the left side of the tool box. I’v been using a 20m Hamstick on the mobile since around 2001 - 2002, mounted to the left side of the tool box, with very good success. It’s very common for me to get signal reports saying that I sound like Im running 400w - 500w into a 3 element beam at 50 ft rather then 100w into a Hamstick on the mobile.
What Im asking in my original post is, will lining the compleat bottom side of the tonneau cover with aluminum window screen help as a ground plane? Assuming it will, and assuming that the aluminum screen needs a good bond with both antenna brackets via the bracket mounting bolts, do I need to also connect a copper ground strap from the screen to the bed of the truck? If so, will 1 ground strap work or should I connect a ground strap at each corner of the screen?
The Comet SBB 7 dual band is 55 inches tall which is why I’v never mounted it on top the truck cab on my 2 previous trucks. And of course a 20m Hamstick with whip is over 6 1/2 ft tall.

 

[KD5ITM]

I would do like @mmckenna suggested. Get a sheet of metal that is at least a 1/4 wave all around and install the NMO mount in the center of the sheet. You can epoxy the sheet to the underside of the tonneau cover. In the center of the cove will be best. I know a lot of people do this with camper shells/toppers for their trucks.

You will want a sheet of metal that is 3 ft by 3 ft for the 2m band and place the NMO mount dead center of the sheet.

The thickness of the fiberglass tonneau cover is too thick to mount the NMO mount the way your saying. The bottom side of the cover in lined with carpet

 

[mmckenna]

It’s very common for me to get signal reports saying that I sound like Im running 400w - 500w into a 3 element beam at 50 ft rather then 100w into a Hamstick on the mobile.

On HF, the toolbox won't supply enough true ground plane to be really effective, so it probably doesn't matter.

But if you are getting good reports with the existing setup, you'll probably still see an improvement by giving it even more ground plane.

What Im asking in my original post is, will lining the compleat bottom side of the tonneau cover with aluminum window screen help as a ground plane?

Yes.

Assuming it will, and assuming that the aluminum screen needs a good bond with both antenna brackets via the bracket mounting bolts, do I need to also connect a copper ground strap from the screen to the bed of the truck?

That would help bond everything to the truck, which isn't going to hurt. On VHF/UHF, the size of the tonneau cover is going to be sufficient for your needs. For HF, the more the better.

If so, will 1 ground strap work or should I connect a ground strap at each corner of the screen?

I'd probably do both. Your milage may vary.

 

[mmckenna]

The thickness of the fiberglass tonneau cover is too thick to mount the NMO mount the way your saying. The bottom side of the cover in lined with carpet

They make thick mount NMO's for these sort of applications. I've used them on UTV roof tops for VHF radios.

Up to 1/2 inch: Thick Surface Mount up to 1/2" - Thick Surface Mounts

Up to 1": Thick Surface Mount up to 1"

 

[KF0NYL]

The thickness of the fiberglass tonneau cover is too thick to mount the NMO mount the way your saying. The bottom side of the cover in lined with carpet

As @mmckenna said, there are different lengths of NMO mounts for different applications.

You are also going to want some type of ground plane under the tonneau cover. I've seen people use thin sheet metal. Window screen or even a sheet of faraday cloth would work too.

There are no ground plane VHF/UHF antennas available, but they will still benefit from a proper ground plane.

HF is another ball game as you will be hard pressed to get a good ground plane on any vehicle. HF antennas still work on vehicles even though there is not a good ground plane available, especially for the lower bands.

 

[KD5ITM]

As @mmckenna said, there are different lengths of NMO mounts for different applications.

You are also going to want some type of ground plane under the tonneau cover. I've seen people use thin sheet metal. Window screen or even a sheet of faraday cloth would work too.

There are no ground plane VHF/UHF antennas available, but they will still benefit from a proper ground plane.

HF is another ball game as you will be hard pressed to get a good ground plane on any vehicle. HF antennas still work on vehicles even though there is not a good ground plane available, especially for the lower bands.

Iv had very good success using a 20m hamstick for HF mobile since the early 2000s. I tried a screw driver antenna back around 2007 for about a year. It worked ok on 20m (but not as good as the Hamstick), the band with wasn’t as wide as it is with the Hamstick. The screw driver had higher SWR then I cared for on 40m and 15m. It worked good on 10m which is where I ended up leaving it tuned for and went back to using the 20m Hamstick with a coax switch so I could switch between 20m and 10m. After about 1 1/2 years, the drill motor inside the screw driver froze up and I ended up removing it and went back to what I was doing before the screw driver for HF mobile. Storing my 17m, 15m and 10m Hamsticks in a 1 1/2 inch PVC pipe with caps on both ends and storing that in the bed of the truck under the tonneau cover. It’s easy enough to swap antennas when needed for the band I want to operate on. I do have a 40m Hamstick with the WD4BUM Inducti-Match IM-1 coil that mounts at the base of the Hamstick. This is supposed to help with SWR as well as give the antenna more bandwith. It did help to get the SWR down to a 1.1 - 1.2 but it only slightly widened the band with. So I rarely ever use it other then for POTA where I can tune the hamstick‘s whip for the 40m frequency I want to operate on. A lot of people say think that the Hamstick antennas are poor HF mobile antennas. I’v had nothing but very good success on 20m through 10m. More times then not someone is telling me that my signal sounds like I‘m running 400 - 500w into a 3 element beam at 50 ft rather then 100w into a mobile 20m Hamstick.

With the Ham stick antenna being mounted on the tonneau cover, I just need to figure out the proper way to install the aluminum window screen so that the antenna has a proper RF bond as well as the truck to the aluminum screen. Im not worried about the screen its self working. I know several hams that use aluminum window screen as there ground plane when operating portable / POTA, I just need to know the proper way to install it so that it actually works as a ground plane. Once I get that figured out, I know that the aluminum screen will provide a much better ground plane then what I currently have or currently lacking.

 

[KD5ITM]

They make thick mount NMO's for these sort of applications. I've used them on UTV roof tops for VHF radios.

Up to 1/2 inch: Thick Surface Mount up to 1/2" - Thick Surface Mounts

Up to 1": Thick Surface Mount up to 1"

Well Unfortunately where the two antennas are currently mounted behind the left and right side back glass is where there going to be mounted and is what I have to work with and need to make the best of. The Comet SBB 7 dual band is too tall and the 20m Hamstick is for sure too tall to mount then on top of the roof of the truck. Like I said in my post, Iv been using this same set up since the early 2000s with the hamstick mounted right behind the left side back glass and the SSB 7 mounted right behind the right side back glass, which both antennas worked just fine. The only thing I can figure with my current set up with both antennas being mounted on top of the tonneau cover behind the left / right side back glass, is the lack of a ground plane. When the antennas were mounted to both sides of the aluminum tool box in the past, Im assuming that the tool box lid was enough metal to act as a ground plane to allow both antennas to have decent performance. Not the best of ground planes but enough to work.

 

[KD5ITM]

As @mmckenna said, there are different lengths of NMO mounts for different applications.

You are also going to want some type of ground plane under the tonneau cover. I've seen people use thin sheet metal. Window screen or even a sheet of faraday cloth would work too.

There are no ground plane VHF/UHF antennas available, but they will still benefit from a proper ground plane.

HF is another ball game as you will be hard pressed to get a good ground plane on any vehicle. HF antennas still work on vehicles even though there is not a good ground plane available, especially for the lower bands.

Yep thats why in my post I said that I was going to line the underside of the tonneau cover with aluminum window screen. And was asking how the screen should be properly installed in regards to RF bonding to the antenna mounting bracket as well as to the body of the truck bed. Im assuming that if a mag mount attached to the body of a car, the connection point between the mag mount and sheet metal acts as the RF bond, then I would need to RF bond the aluminum screen to both antenna brackets as well as the truck bed. The ground plane doesn’t need to be perfect, it just needs to be good enough to work.