Antenna placement question?

[WSEZ903]

I have a Subaru Forester Sport that I have a Tram 5.5" mag mount with a Browning UHF gain antenna that I have cut to 467 MHz.  I have purchased direct mount to the roof but I want to drill once and be done.  Question is when I place the mag mount in the middle in front of the shark fin by a few inches I get an SWR of 1.7 of higher.  If I move it in between the shark fin and the side rails on passenger side it drops to 1.5 +/-.  If I move it to the same spot on divers side I get about 1.3 +/-.  The back of the vehicle does have a fiberglass/plastic cowling for rear window and more towards the front I have the "Moon Roof" as they call it.  I do have a roof rack that I put on once in a while and with it on or off I did not notice any gain or loss in SWR when testing with it on the sides.  The roof rack rail in attached picture in the back I have moved forward and all the way back and it did not really affect the reading.  I.e. 1.52 vs 1.56.

Any thoughts as to why it is lower on the drivers side vs center?  Attached is a screen shot of my roof.  Side note the passenger side black rectangle that looks like it is on roof rack it the thumb screw for 360 camera mount.

Edited Friday at 06:25 PM by WSEZ903
Mis spelled word.

[SteveShannon]

3 minutes ago, Willie said:

I understand that it will work. I am just curious to know how much (if any) the antenna placement affects the directional performance of a UHF mobile radio. I know it was an issue back in the day when I had a CB radio because of the lower frequencies. I don't have and have never dealt with a GMRS mobile radio before. So finding the best SWR is the only concern then?

No, SWR isn’t the only concern.  Depending on where you place a ground plane antenna there will be some directional bias.  Ideally you would place it in the center of the ground plane, but because of the short wavelength of UHF, a ground plane antenna isn’t nearly as sensitive to placement and a lot of people are satisfied even with a ground plane antenna mounted at the very edge of a ground plane, such as the edge of a tailgate or hood.

As @WRYZ926 says, as long as you have at least 6 inches of ground plane in any direction from the mount you should be fine.

[BoxCar]

I would place the antenna as close to the middle of the metal roof. That would be about where the rear crossbar is and in line with the fin. Your SWR could have measured higher on the right side because of something running just under the roof where the antenna was.

[WRXB215]

Not sure why you are getting those results but I would not worry about an SWR of 1.7.

[WRYZ926]

I would first figure out exactly how far back the sun roof goes inside the roof. You don't want the NMO mount and/or coax interfering with your sun roof. While as center as possible is ideal, you really don't need that much metal around the base for a good ground plane with UHF

[WSEZ903]

1 hour ago, WRYZ926 said:

I would first figure out exactly how far back the sun roof goes inside the roof. You don't want the NMO mount and/or coax interfering with your sun roof. While as center as possible is ideal, you really don't need that much metal around the base for a good ground plane with UHF

Good call. I don’t use it a whole lot but forget about that. I will have to pull the liner down in the back with it retracted to see when it stops. 

[WRYZ926]

As long as you have at least 6 inches of metal all the way around the antenna mount then you should be good to go. A 6 inch radius is the recommended minimum for a good ground plane. And even "no ground plane" antennas benefit from having a good ground plane. 

[Willie]

I know this discussion is focused on SWR, but I'm not seeing any mention of antenna placement vs radiation pattern. Is this not an issue with UHF? I'm just wondering what a VNA analyzer might tell you (if anything).

[WRYZ926]

Well you do want to put the antenna on a flat spot when ever possible and on the top of the vehicle. Sometimes that is not an option though. For the OP, anywhere on the roof that will not interfere with his sunroof and has at least 6 inches of metal all around the mount will work fine.

[WRUU653]

4 hours ago, WSEZ903 said:

Good call. I don’t use it a whole lot but forget about that. I will have to pull the liner down in the back with it retracted to see when it stops. 

Another option is a mount like this that combines the satellite antenna and an NMO.  What year is your Forester? I ask because we have a 2018 (with moon roof) that currently doesn't have a radio so I'm interested in how your install goes.  

[Willie]

2 hours ago, WRYZ926 said:

Well you do want to put the antenna on a flat spot when ever possible and on the top of the vehicle. Sometimes that is not an option though. For the OP, anywhere on the roof that will not interfere with his sunroof and has at least 6 inches of metal all around the mount will work fine.

I understand that it will work. I am just curious to know how much (if any) the antenna placement affects the directional performance of a UHF mobile radio. I know it was an issue back in the day when I had a CB radio because of the lower frequencies. I don't have and have never dealt with a GMRS mobile radio before. So finding the best SWR is the only concern then?

[WRTC928]

9 hours ago, WRUU653 said:

Another option is a mount like this that combines the satellite antenna and an NMO.  What year is your Forester? I ask because we have a 2018 (with moon roof) that currently doesn't have a radio so I'm interested in how your install goes.  

I'm intrigued. Does that replace the sharkfin?

[Willie]

1 hour ago, WRTC928 said:

I'm intrigued. Does that replace the sharkfin?

Appears to, yes. https://www.l-com.com/Images/Downloadables/Datasheets/ds_HG-GPSGLNMO195-SMA.pdf

[BoxCar]

8 hours ago, Willie said:

I understand that it will work. I am just curious to know how much (if any) the antenna placement affects the directional performance of a UHF mobile radio. I know it was an issue back in the day when I had a CB radio because of the lower frequencies. I don't have and have never dealt with a GMRS mobile radio before. So finding the best SWR is the only concern then?

The shape of the ground plane will have an effect on the radiation pattern as it will mimic the ground somewhat. The antenna will still be omnidirectional with a larger lobe in the direction of the greater area of the ground.

[BoxCar]

2 hours ago, WRTC928 said:

I'm intrigued. Does that replace the sharkfin?

I'd say it doesn't replace the shark fin as it only has the NMO mount and GPS antenna. Most shark fins have 3 or 4 antennas - AM, FM. Cellular, GPS and Sirius.

[BoxCar]

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[WRYZ926]

1 hour ago, BoxCar said:

I'd say it doesn't replace the shark fin as it only has the NMO mount and GPS antenna. Most shark fins have 3 or 4 antennas - AM, FM. Cellular, GPS and Sirius.

I would say that is true for most, but not all vehicles. I know it's not true on my 2023 Ford Escape.  The shark fin only has the GPS and Sirius antennas in it. The AM/FM antennas are actually built into the rear window

I forget which thread these mounts were discussed in on the forum. They are a viable option for those that only run one antenna or don't mind swapping antennas out regularly.

1 hour ago, BoxCar said:

The shape of the ground plane will have an effect on the radiation pattern as it will mimic the ground somewhat. The antenna will still be omnidirectional with a larger lobe in the direction of the greater area of the ground.

This is correct.  But the gentle slope of most SUV/CUV roofs won't make a big difference. I have to run short antennas on my Escape and have the GMRS antenna towards the back. I still have at least 12 inches of metal all the way around the base for my ground plane. I do notice a very small difference between my antenna in the middle of the roof versus towards the back. But it is not enough to worry about. I was more concerned about keeping the antennas spaced to minimize any interaction between them.

[WRUU653]

5 hours ago, WRTC928 said:

I'm intrigued. Does that replace the sharkfin?

I am unsure if the shark fin on our 2018 Subaru Forester is more than just GPS. I’ve been searching but nothing conclusive yet. Of course once you drop the headliner you will know. If I were to get one of the combo NMO/GPS antennas I might go with this one. I remembered that it has a metal mounting and housing, better for the ground plane. I’m not positive if the other one I posted is (though I think it said it was). I did take a look at the area the moon roof retracts to and at least on ours it looks like it comes back pretty far. I know it wouldn’t be in the most ideal position but I might opt for a lip mount that goes on the rear hatch myself. Sometimes good enough is good enough. I’ll definitely follow your progress. Oh here is a video I found on installing the EM Wave.

[WSEZ903]

Ok.  So after digging around some on the roof area and playing around with options I went about 7 inches in front of shark fin. Gave me a few inches of space as buffer from moon roof when opened. I was originally opting for passenger side area between rail and fin to give me more space when I put basket on but I am getting better mileage with it off and use it not like I thought I would. (Kids are bigger and not into camping anymore) the only thing I will add is a piece of loom on the inside and zip tie it to the cross member to be sure to keep it out of the way. 
 

Thanks for all the feedback back. It has been about 20 years since I did this and the vehicles back then did not have all theses side air bags and safety features. 
 

kevin

Edited 10 hours ago by WSEZ903
Added picture

[WRUU653]

8 hours ago, WSEZ903 said:

Ok.  So after digging around some on the roof area and playing around with options I went about 7 inches in front of shark fin. Gave me a few inches of space as buffer from moon roof when opened. I was originally opting for passenger side area between rail and fin to give me more space when I put basket on but I am getting better mileage with it off and use it not like I thought I would. (Kids are bigger and not into camping anymore) the only thing I will add is a piece of loom on the inside and zip tie it to the cross member to be sure to keep it out of the way. 
 

Thanks for all the feedback back. It has been about 20 years since I did this and the vehicles back then did not have all theses side air bags and safety features. 
 

kevin

That looks ideal. Nice job.