Roof rack effect on ground plane

[KK4JUG]

I have a Larsen NMO 2/70 mounted in the middle of the roof. I also have a roof rack with cross members fore and aft of the antenna. Each cross member is about 2½ inches above the roof and 8 inches laterally from the antenna. The radio is a Yaesu FT-8900. Presently, I only use 2 and .70 meters. My question is, does the roof rack have any affect on the ground plane?

 

[prcguy]

If you raise the feedpoint of the Larsen antenna to the roof rack it will not tune up very well and you will loose performance. I've seen it dozens of times.

I have a Larsen NMO 2/70 mounted in the middle of the roof. I also have a roof rack with cross members fore and aft of the antenna. Each cross member is about 2½ inches above the roof and 8 inches laterally from the antenna. The radio is a Yaesu FT-8900. Presently, I only use 2 and .70 meters. My question is, does the roof rack have any affect on the ground plane?

 

[k6cpo]

I've used a mag mount antenna on vehicles with roof racks. It's placed between the crossmembers of the rack and I've never noticed any degradation of received or transmitted signals.

 

[prcguy]

Your talking magnet stuck on the roof, right? Its when you get the antenna feedpoint up off the roof sheet metal like an NMO mount on the roof rack itself is when you get into trouble. The roof rack being in close proximity to a roof mounted antenna is nothing to worry about.

I've used a mag mount antenna on vehicles with roof racks. It's placed between the crossmembers of the rack and I've never noticed any degradation of received or transmitted signals.

 

[KK4JUG]

I had never heard of roof racks causing a problem but they're metal and metal in the wrong places can complicate and difficulties with a ground plane can also be an obstacle. I'll just leave things alone.

Incidentally, the antenna is attached directly to the roof.

 

[mmckenna]

Incidentally, the antenna is attached directly to the roof.

…like God intended.

I ran a VHF 1/4 wave whip on my wife's Ford Escape with a roof rack. Cross bars were about 18" (1/4 wave!) from the antenna both front and back. Never had any issues. Antenna swept just fine, SWR nice and low across the band.

 

[mass-man]

Incidentally, the antenna is attached directly to the roof......

EXCELLENT and your post is timely for me...NMO mount quarter wave going on this weekend on the roof of my Outback between the rack rails! The rails were less of a hassle than the damn sunroof!

 

[prcguy]

You should not have any problems. The majority of the roof rack is horizontal and at a right angle to the antenna so not much interaction. Its also very low in wavelengths at VHF/UHF so not really an obstruction.

I had never heard of roof racks causing a problem but they're metal and metal in the wrong places can complicate and difficulties with a ground plane can also be an obstacle. I'll just leave things alone.

Incidentally, the antenna is attached directly to the roof.

 

[KK4JUG]

That settles it. It's gonna be the status quo.

 

[ra7850]

I've also been curiou sabout this. My issue is my 2021 Ram 1500 has the panoramic sunroof. I want to have a hole drilled for a scanner NMO mount but there not too much metal in which to drill. One of my ideas was to get a roof rack, mount the nmo mount to that on one of the crossbars. Thoughts about the effectiveness if I choose this route.

 

[KK4JUG]

I've also been curiou sabout this. My issue is my 2021 Ram 1500 has the panoramic sunroof. I want to have a hole drilled for a scanner NMO mount but there not too much metal in which to drill. One of my ideas was to get a roof rack, mount the nmo mount to that on one of the crossbars. Thoughts about the effectiveness if I choose this route.

You don't have a whole lot of ground plane even without the sunroof and probably less if you mount it on a roof rack cross member. The only thing worse would be a convertible. Larsen (I think) make an antenna mount adapter that attaches above the middle brake light (above the cab rear window) for some Ford PUs. You might check with them about something for the RAM.

 

[mmckenna]

I've also been curiou sabout this. My issue is my 2021 Ram 1500 has the panoramic sunroof. I want to have a hole drilled for a scanner NMO mount but there not too much metal in which to drill. One of my ideas was to get a roof rack, mount the nmo mount to that on one of the crossbars. Thoughts about the effectiveness if I choose this route.

For scanner use, I wouldn't get hung up on the ground plane. Yes, ideal scenario would be to have a full 1/4 wave ground plane at your lowest frequency, but remember, it's a hobby and doing the best you can with what you have is often good enough.
I'd not mount a roof rack on the cab just for a scanner antenna. Do the permanent install NMO between the sun roof and the rear of the cab and go with it. It'll still work well.

As @KK4JUG mentioned, there are companies that make really overpriced mounts that go between the cab and the third brake light. It's not Larsen, as in the antenna manufacturer, it's a company call LarsOn. It's an expensive solution for those that don't want to drill a hole in the cab, but it doesn't magically fix the ground plane issue. Putting an NMO on the roof with a little ground plane is still going to work better -and- be cheaper.

 

[mmckenna]

Larson Electronics - Vehicle Mounting Plates

Vehicle Mounting Plates

www.larsonelectronics.com

 

[mmckenna]

I've also been curiou sabout this. My issue is my 2021 Ram 1500 has the panoramic sunroof. I want to have a hole drilled for a scanner NMO mount but there not too much metal in which to drill. One of my ideas was to get a roof rack, mount the nmo mount to that on one of the crossbars. Thoughts about the effectiveness if I choose this route.

These prices are stupid for what they are, but they attract the guys who really want an antenna, but don't have the cajones to drill the hole.

Antenna Mounts

Our third brake light antenna mounts make installs quick and easy! No holes to drill in your roof, no damage to your truck. Shop now for a variety of models!

bulletproofdiesel.com

 

[ra7850]

For scanner use, I wouldn't get hung up on the ground plane. Yes, ideal scenario would be to have a full 1/4 wave ground plane at your lowest frequency, but remember, it's a hobby and doing the best you can with what you have is often good enough.
I'd not mount a roof rack on the cab just for a scanner antenna. Do the permanent install NMO between the sun roof and the rear of the cab and go with it. It'll still work well.

As @KK4JUG mentioned, there are companies that make really overpriced mounts that go between the cab and the third brake light. It's not Larsen, as in the antenna manufacturer, it's a company call LarsOn. It's an expensive solution for those that don't want to drill a hole in the cab, but it doesn't magically fix the ground plane issue. Putting an NMO on the roof with a little ground plane is still going to work better -and- be cheaper.

Guess that settles it, I'm going to find a local radio/dealer installer to drill the hole in the roof.

Thanks everyone

 

[mmckenna]

Guess that settles it, I'm going to find a local radio/dealer installer to drill the hole in the roof.

Thanks everyone

If you were local, I'd offer to do it for you, but I'm on the opposite coast.

Having a shop do it comes with some benefits:
They will have the right tools and knowledge.
They'll route the cable well.
They should cut the coax to length. (Less coax, less loss)
They'll install the coax connector of your choice. (avoid unnecessary connections/adapters/failure points)
If you bring the antenna with you, they'll probably even sweep it for you to confirm proper operation.

In the end, you'll be happier with it. You'll know it's installed correctly and you'll never have to wonder if it's done right. The performance and comfort of knowing you didn't cut any corners is really nice.

 

[tweiss3]

I have the same 2/70sh antenna on my wife's car, and I don't notice any issues (dead spots/directions) with the roof rack bar 6' away.

 

[merlin]

As long as the ground radials are not touching any metal, any effect of things like racks or AC boxes, you will not see any significance.
Presuming the antenna is above such obstacles.

 

[ra7850]

If you were local, I'd offer to do it for you, but I'm on the opposite coast.

Having a shop do it comes with some benefits:
They will have the right tools and knowledge.
They'll route the cable well.
They should cut the coax to length. (Less coax, less loss)
They'll install the coax connector of your choice. (avoid unnecessary connections/adapters/failure points)
If you bring the antenna with you, they'll probably even sweep it for you to confirm proper operation.

In the end, you'll be happier with it. You'll know it's installed correctly and you'll never have to wonder if it's done right. The performance and comfort of knowing you didn't cut any corners is really nice.

If you were local, I'd offer to do it for you, but I'm on the opposite coast.

Having a shop do it comes with some benefits:
They will have the right tools and knowledge.
They'll route the cable well.
They should cut the coax to length. (Less coax, less loss)
They'll install the coax connector of your choice. (avoid unnecessary connections/adapters/failure points)
If you bring the antenna with you, they'll probably even sweep it for you to confirm proper operation.

In the end, you'll be happier with it. You'll know it's installed correctly and you'll never have to wonder if it's done right. The performance and comfort of knowing you didn't cut any corners is really nice.

Thanks for the thorough response, and the offer. I haven't found anyone local yet, closest I know of is 90 minutes away from me. My thought was to to make a small aluminum bracket to mount inside the truck with a bulkhead connector, short cable from the mount to the bracket, then BNC to BNC patch cable to the scanner. I also thought having them drill 2 holes and having them install 2 nmo mounts, a left side and right side.

 

[mmckenna]

Thanks for the thorough response, and the offer. I haven't found anyone local yet, closest I know of is 90 minutes away from me. My thought was to to make a small aluminum bracket to mount inside the truck with a bulkhead connector, short cable from the mount to the bracket, then BNC to BNC patch cable to the scanner. I also thought having them drill 2 holes and having them install 2 nmo mounts, a left side and right side.

Using the antenna inside will work if it's a really strong signal you are trying to listen to. If it's not, it's going to be a poor performer. All the metal surrounding the cab is going to be problematic. Some automotive glass also has a metal film tinting that will really block RF. If you have a rear defroster, it's even worse. You'd have similar performance just leaving the antenna on the back of the scanner.

Two NMO mounts might be a good idea. It would let you add a CB, GMRS, ham radio, another scanner, or cellular booster down the road. Probably cheaper to have the second one installed at the same time rather than doing it later.