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Mount antenna to roof rack or back hatch?

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anthodel

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#1
I am looking to install a Yaesu FT7900 & Larsen NMO2/70B on my Subaru Outback but the antenna situation is a little tricky because I have a roof rack that I am not willing to part with. I am trying to determine the best location to mount an antenna. In the attached photo, I highlight two possible mount locations: (A) highest point on the back of the rack. (B) lip mount off the back hatch.

From what I understand, higher is better, but having a good ground plane is also essential. My questions are as follows:

1) Will the rack mount (A) be okay for 2m/70cm, assuming I have good grounds between the rack, antenna mount, and vehicle?
2) Is location (B) a better option because it is closer to the vehicle instead of elevated like (A)? Or will location (B) suffer because the rack (with or without luggage) will interfere with the signal?
3) Can you think of any other location to mount that will get me better performance?

I've read a lot of threads discussing this topic but I've found no real definitive answers. Maybe there are none? What would you do in this situation?

Thanks,
Antho
 

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W8RMH

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#2
I think the A position would be best, above the obstruction of the rack should it be mounted on the body. Proper grounding should alleviate any ground plane issues. I have seen installs on light bars, roll bars, and other type racks work very well. Just be aware of any clearance risks.
 
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#3
Ideally you want to keep the area surrounding the antenna 19" (1/4 Wave Length) from any metal objects. Provided the rack is metal and you don't have the ideal minimum clearance I would mount it to the rack itself. The rack appears to have more than enough metal in it to suffice as a ground plane.
 

anthodel

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#4
Thanks for the quick responses! Looks like on the rack is the best way to go. Just curious, will my radiation pattern be biased toward the rack (front), e.g., if I am trying to hit a distant station, I should point my car in that direction?

Thanks,
Antho
 
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#5
Thanks for the quick responses! Looks like on the rack is the best way to go. Just curious, will my radiation pattern be biased toward the rack (front), e.g., if I am trying to hit a distant station, I should point my car in that direction?

Thanks,
Antho
These are less than ideal systems and they're not easy to model, so any answer you get stands a very good chance of being inaccurate.

My guess is that you'll have some tilt to the radiation pattern. How much is hard to say. I wouldn't be surprised if your best signal strength comes off of the sides of your vehicle rather than front or back. However, this difference shouldn't be particularly noticeable. In general, if you're using a 1/4 or 5/8 wave antenna, that difference shouldn't amount to more than a few dB. It's hardly even worth turning your car around for that.
 

N9JCQ

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#6
I have an Acura RDX and I mounted my antenna on the cross bar of my roof rack. I am using an FT-7800 myself. It works quite well for me.
 

mmckenna

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#7
So, mounting locations aside, the issue you are going to have is getting the coaxial cable inside the vehicle.

While it's handy to have external items to mount antennas to, you'll always run into the same issue that those with mag-mounts have, getting cable to the radio without damaging it. Routing coaxial cable through doors or windows is going to lead to issues. You'll either damage the coax over time, or you'll create a path for water to enter the vehicle.
Since it looks like a fairly new vehicle, I doubt you want water intrusion.
If you are concerned about "hitting distant station"s then you need to be concerned about your coax.
Pinching coax in doors or windows is going to lead to it's early demise. Using sub-minature coax to sneak it in leads to losses, especially on UHF.

The NMO-2/70 is a 5/8'ths wave antenna on VHF, so it's about 48 inches tall. A bit of blockage by the rack isn't going to be a big deal.
Grounding is also important for these antennas, as they are not "ground independent" half wave designs.

The -right- way to do this is to drill the hole and install a permanent NMO mount in the roof. This addresses the water ingress issues ~and~ it will eliminate the issues with feed line getting damaged in doors or windows. It also provides a proper ground plane, which will allow you to get the most out of your antenna.

It's your car, it's up to you, but I've personally never regretted doing a proper install of a permanent mount. That, coupled with a good antenna, will really get the most out of your system. It also reduces the cheesy "bolt on" after thought antenna installs.
 
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#8
Personally, I would go with a fender/ hood mount left front . On 2/440 using repeaters you won,t notice the difference. I had antennas on roof with a roof rack and have to remove them occassionally to use the full rack. Even in remote areas I have had enuff antenna to make reliable contacts .
 
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#9
Don't forget the airbags

So, mounting locations aside, the issue you are going to have is getting the coaxial cable inside the vehicle.

While it's handy to have external items to mount antennas to, you'll always run into the same issue that those with mag-mounts have, getting cable to the radio without damaging it. Routing coaxial cable through doors or windows is going to lead to issues. You'll either damage the coax over time, or you'll create a path for water to enter the vehicle.
Since it looks like a fairly new vehicle, I doubt you want water intrusion.
If you are concerned about "hitting distant station"s then you need to be concerned about your coax.
Pinching coax in doors or windows is going to lead to it's early demise. Using sub-minature coax to sneak it in leads to losses, especially on UHF.

The NMO-2/70 is a 5/8'ths wave antenna on VHF, so it's about 48 inches tall. A bit of blockage by the rack isn't going to be a big deal.
Grounding is also important for these antennas, as they are not "ground independent" half wave designs.

The -right- way to do this is to drill the hole and install a permanent NMO mount in the roof. This addresses the water ingress issues ~and~ it will eliminate the issues with feed line getting damaged in doors or windows. It also provides a proper ground plane, which will allow you to get the most out of your antenna.

It's your car, it's up to you, but I've personally never regretted doing a proper install of a permanent mount. That, coupled with a good antenna, will really get the most out of your system. It also reduces the cheesy "bolt on" after thought antenna installs.


Also, if you're going to do it right, as MMcKenna suggests, be aware that there are usually side curtain airbags and such that you want to make sure your coax clears. You won't want the coax interfering with the bags so that they can deploy as designed.
 

anthodel

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#10
Thanks, all! I didn't expect this big of a response so quick!

Personally, I would go with a fender/ hood mount left front.
...
geartow, I was considering the fender / hood mount but then I got to thinking about RF exposure. Everything online tells me that the windshield SHOULD block enough RF from that antenna @ 50 watts. I guess I'm a bit paranoid and getting the antenna up higher is a bonus.


So, mounting locations aside, the issue you are going to have is getting the coaxial cable inside the vehicle.
...
mmckenna, you bring up a very good point. I have a few ideas for getting the cable in through the back hatch, hopefully, without causing too much damage or letting water in. I may end up taking your advice if I find that I can't get the coax in without pinching.


... In general, if you're using a 1/4 or 5/8 wave antenna, that difference shouldn't amount to more than a few dB. It's hardly even worth turning your car around for that.
ab3a, thanks for the advise. It'll definitely make my life easier if I try not to worry about which way my car is facing. :)


I have an Acura RDX and I mounted my antenna on the cross bar of my roof rack. I am using an FT-7800 myself. It works quite well for me.
N9JCQ, thanks! It's always encouraging to hear that others have experienced good results with similar installations.


Also, if you're going to do it right, as MMcKenna suggests, be aware that there are usually side curtain airbags and such that you want to make sure your coax clears. You won't want the coax interfering with the bags so that they can deploy as designed.
romanr, you have a very good point for anyone doing a mobile installation. My plans don't involve running coax near the side curtain airbags, but my planning phase didn't even take them into account.
 
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