Antenna placement advice, and impacts on front edge.

[W7GEL]

Need some advice and assistance.
I know enough that I don't know enough so looking to learn and grow. Hoping other maybe have some experience as well.

I suspect most of us have seen the Larsen dB Antenna placement chart, basically showing Dead center roof line is the best location in terms of dB numbers.

I have an SUV and where I put the red X the car already has a NMO mount location from the factory "old school analog cellphone antenna" that I could easily repurpose the hole. Anyone have an educated idea of the potential dB loss I might see? In terms of ground plane would the hood continue a ground plane or do ground planes have to be the same level and at the base of the antenna?
I suspect by this chart and other numbers it would be about -.02 loss and would loose some forward range.

I could go center of roof on the SUV but I have a Rack on the entire top, and the body has a sunroof on the SUV.

• If I want it dead center it would have to be mounted on the rack.
  • I won't say the rack is Bonded to the roof but it is bolted together aluminum.
  • I suspect this would suck as a ground plane, due to the construction of the rack.
    • I assume this is the worst option.
  • I could install a solid flat piece of metal on the rack with the antenna centered - basically its own custom ground plane.
    • no idea how big it would need to be.
• If I need it mounted on the roof of the SUV.
  • Rack does have open slats space design, about 3 1/2 inches between slats.
    • Would this affect the antenna? I'm not sure how metal close to an antenna impacts the radiation.
  • I can mount to the rear of the sunroof and between the open space in the rack.
  • I would estimate antenna would be centered right to left and about 2/3 to the rear of the SUV to clear the sunroof.
    • I suspect this would be the best option. range should be decent rearward still with the front being a little better.
• Or I could repurpose the factory hole up front.
  • I assume loosing the ground plane of the roof would impact the front range.
• Pretty sure most people say the NMO2/70B is the antenna to go with.

Design goals are.

• Overlanding in the forested areas of the Pacifica North West, Canada areas.
• Simplex to other rigs but also repeater use in the area as needed.
• Maybe some Cross band repeater.
• APRS

Above if you look ever so closely you will see a short inch antenna


Link to the Larsen db chart of ant placement, on page 17 of the pdf.

https://www.pulseelectronics.com/docs/library/Amateur%20Short%20Form%20Catalog%20Volume%202.pdf

 

[mmckenna]

It'll work. Had to put an antenna fairly far forward on an SUV once. Not ideal, but if it's your only option, it's what you have to do.

The hood isn't going to act as a ground plane as it's not directly under the antenna. That's going to result in a lopsided radiation pattern. I think the impact is going to be similar to the -2.4dB shown on the side of the hood in the Larsen document.

Depending on how much stuff you have on the roof rack, that would be a better location. If you run with a lot of stuff up there that's metallic/conductive, then it's going to impact the antenna. But if you only put non-conductive stuff up there, it should work better. The lack of perfect DC ground to the vehicle body won't be an issue. It'll give you a better radiation pattern and better performance.

But for short range simplex work, hitting repeaters, etc. the front of the roof may be 'good enough'.

 

[krokus]

As mmckenna mentioned, the safari rack will make a good RF ground plane. Use it, if you will be able to give a decent amount of space around the antenna.

 

[W7GEL]

Depending on how much stuff you have on the roof rack, that would be a better location. If you run with a lot of stuff up there that's metallic/conductive, then it's going to impact the antenna. But if you only put non-conductive stuff up there, it should work better. The lack of perfect DC ground to the vehicle body won't be an issue. It'll give you a better radiation pattern and better performance.

mostly non metal stuff at this point, weight lower is always better so limited metal heavy items for sure.

So the rack is just a bunch of pieces. I'm ok with drilling a hole in the body or rack. just want to pick the better of the two. I suspect if I mounted to the roof the coil would be about flush with the top of the rack with the ant above.

I was under the impression ground to the radio and ant ground plane is important.

As mmckenna mentioned, the safari rack will make a good RF ground plane. Use it, if you will be able to give a decent amount of space around the antenna.

I could install a flat piece of metal. "aluminum" on top, of the rack never see it unless I roll over Might actually make installing easier. I assume that would best option. If I go on top.

Part of me just wants to find the perfect spot on the roof and space on the rack. Especially if the rack is low on the antenna then why not and wont cause issues. Rack would actually protect the antenna coil - the whip well would be at mercy of whipping around.

 

[mmckenna]

Part of me just wants to find the perfect spot on the roof and space on the rack. Especially if the rack is low on the antenna then why not and wont cause issues. Rack would actually protect the antenna coil - the whip well would be at mercy of whipping around.

As long as the whip won't touch the metal, it would probably be easier to just mount to the roof of the vehicle and have it between the slats on the rack. That'll make routing the coaxial cable a lot easier. Usually the biggest issue with mag mounts or any sort of non-drilled mount is getting cable safely inside the vehicle.

 

[W7GEL]

As long as the whip won't touch the metal, it would probably be easier to just mount to the roof of the vehicle and have it between the slats on the rack. That'll make routing the coaxial cable a lot easier. Usually the biggest issue with mag mounts or any sort of non-drilled mount is getting cable safely inside the vehicle.

My intial worry was the rack would hinder the ant performance but I can easily install both and not have the antenna touch the rack. There is 3 1/2 inches in the open areas.

 

[mmckenna]

My intial worry was the rack would hinder the ant performance but I can easily install both and not have the antenna touch the rack. There is 3 1/2 inches in the open areas.

Should work just fine. Just check your SWR when installation is complete. It may impact it a bit.
I probably should have asked what the spacing is between the vehicle roof and the rack?

 

[WB9YBM]

Need some advice and assistance.
I suspect most of us have seen the Larsen dB Antenna placement chart, basically showing Dead center roof line is the best location in terms of dB numbers.

In addition to the db numbers, consider the radiation pattern: an antenna radiates best in all directions when mounted in the center of the ground plane (like the middle of the roof).