Can I use a roof rack with my antenna?

Submitted: Monday, Aug 24, 2015 at 06:07
ThreadID: 130080 Views:3153 Replies:5 FollowUps:7
This Thread has been Archived
I've got a troopy with a UHF radio and with a ground plane dependent antenna fitted to a hole in the roof just behind the driver. Is there any compatibility with this and fitting a roof rack? I don't think I need the ultimate performance, but I don't want to screw it up either. As far as leaving the antenna alone the options seem to be either no roof rack, because it will screw up the ground plane, or else fit a short rack, either 3/4 or 1/2 length. I don't know if that would help or not. Obviously if it was a full length rack it would poke up between the mesh. Oh, around town I use a stubby 12cm antenna, but I have a longer 60cm or whatever one if I need it.
Back Expand Un-Read 0 Moderator

Reply By: vk1dx - Monday, Aug 24, 2015 at 08:15

Monday, Aug 24, 2015 at 08:15
We have a full length ARB tourer roof rack on our 100 series on which we mount a roof top tent. There is heaps of room in front of the rack for a roof mounted antenna.

Being metallic, the rack will affect the radiation pattern of the antenna. However, in saying that, it will still perform better than being placed down on the bull bar or mudguard, and thus lower than the roof line.

How far is the antenna from the top of the windscreen? Are you just assuming that a roof rack foul the antenna? Measure it and see if you can get a rack without fouling the antenna.

I just haven't bothered because the UHF CB is only for short comms anyway and ours works okay from the mount on the right hand mudguard.

Phil
AnswerID: 589588

Follow Up By: Chris B39 - Monday, Aug 24, 2015 at 12:52

Monday, Aug 24, 2015 at 12:52
If you've got plenty of room in front of your rack, maybe it's not full length. On the Troopy the full length rack is 2200, which is pretty massive. If you know the troopy, the roof is higher than normal and slopes from the windscreen to just behind the driver, at which point it then becomes flat. The full length extends even over the slopy part, so it's quite radically long. My antenna right now is on part of the roof where it just starts becoming flat.

The 3/4 length racks are about 1800 which might possibly fit right behind the antenna. My measurements say it would be close. The 1/2 length are 1100 which seems too short. Iron man make a 1400 model which is an option.

The question becomes, ok if it's going to affect the radiating pattern to the rear, how badly, and should I go for the 3/4 length, or hold back for 1400 length.
0
FollowupID: 857492

Follow Up By: vk1dx - Monday, Aug 24, 2015 at 14:23

Monday, Aug 24, 2015 at 14:23
I just measured ours, it's 1800mm long. The distance from the rack to the top of the windscreen is 450mm. Yes in my case it would be better in front of the rack but then that would mean another hole in the car for small gain. And even better still with a ground independant antenna which would lift the base of the active antenna element close to the height of the top of the roof rack.

Then you put a metal gizmo (read any metal based load) on the roof rack and stuff it up.

Personally, I wouldn't bother. On the angled bracket using existing bolts on the RH mudguard has been quite effective for many years.

Phil
0
FollowupID: 857497

Reply By: Life Member-Doug T NSW - Monday, Aug 24, 2015 at 08:47

Monday, Aug 24, 2015 at 08:47
I think UHF don't need a ground plane as did the 27Mhz , anyhow here is a website that should answer all your questions.

Antennas For CB Radio

.
gift by Daughter

Lifetime Member
My Profile  My Blog  Send Message

AnswerID: 589590

Follow Up By: Chris B39 - Monday, Aug 24, 2015 at 12:56

Monday, Aug 24, 2015 at 12:56
If you've got a ground plane dependant antenna you definitely need a ground plane. Of course I could switch to a ground plane independant antenna, but then they tend to be taller, and bigger (height in car parks is already touch and go with the troopy, and the stubby 12cm one works a treat), and also tend to perform worse. But maybe that is the direction I should go with a roof rack, I'm just not sure.
1
FollowupID: 857494

Reply By: allein m - Monday, Aug 24, 2015 at 10:15

Monday, Aug 24, 2015 at 10:15
I had some problems with my UHF antenna few weeks back and have done a lot of reading and one major thing is the higher the better

also put it up there and see how it goes test it with local repeater station
AnswerID: 589591

Reply By: Bob Y. - Qld - Monday, Aug 24, 2015 at 10:31

Monday, Aug 24, 2015 at 10:31
Maybe just go for a 3/4 length roof rack Chris. As Phil said, it may affect the radiation pattern, say less to the rear, but should still be all good to front and slides.

It's possible, if you're handy with a welder, to make a bracket that goes under the upper mounting bolts on the factory snorkel. We used to do that and it worked well, though running the co-ax wasn't as neat as we'd like. Have a photo somewhere that I could scan to show you.

Bob

Seen it all, Done it all.
Can't remember most of it.

Lifetime Member
My Profile  My Blog  My Position  Send Message

AnswerID: 589592

Follow Up By: Chris B39 - Monday, Aug 24, 2015 at 13:01

Monday, Aug 24, 2015 at 13:01
Seems to me mounting on the snorkel means switching from a ground plane dependant to an independant antenna. Which is fine if that's the best option. But I'm concerned how much that would add to the height of the already tall troopy compared to the little 12cm ground plane dependant stubby I use around town, and also the relative performance. I've heard of fold down ones etc, I guess that's also an option. The question is if I get the 3/4 rack, and it affects the pattern to the rear, just how badly.
0
FollowupID: 857495

Follow Up By: Bob Y. - Qld - Tuesday, Aug 25, 2015 at 21:54

Tuesday, Aug 25, 2015 at 21:54
We didn't mount the aerials ON the snorkel, but on a metal bracket that used the upper snorkel mount's captive nuts. So in fact this bracket became part of the roof, or ground plane, of the vehicle.

You can buy aerial bases pretty cheap, and set up a 2nd 150mm stubby off the snorkel bracket, if you found the roof rack fouled the centre roof mount.

As for signal to the rear, might depend on length of aerial, and the amount of gear you've stacked on the rack.

Bob

Seen it all, Done it all.
Can't remember most of it.

Lifetime Member
My Profile  My Blog  My Position  Send Message

0
FollowupID: 857555

Follow Up By: Chris B39 - Wednesday, Aug 26, 2015 at 12:22

Wednesday, Aug 26, 2015 at 12:22
Yes but to work a ground plane needs to extend in all directions. If you mount it near the snorkel it will only extend towards the back / left of the car, so you'll have directional issues
0
FollowupID: 857568

Follow Up By: Bob Y. - Qld - Wednesday, Aug 26, 2015 at 13:14

Wednesday, Aug 26, 2015 at 13:14
You may be right, Chris.

However the aerials we used were a Polar extended gain unit, ground independent, similar to the one below, 2nd from the left. Excellent performance from a 900mm aerial.



Bob

Seen it all, Done it all.
Can't remember most of it.

Lifetime Member
My Profile  My Blog  My Position  Send Message

0
FollowupID: 857573

Reply By: Bazooka - Tuesday, Aug 25, 2015 at 21:24

Tuesday, Aug 25, 2015 at 21:24
Had a setup similar to that for a 27 MHz CB on my swb Patrol years ago Chris. It had a metal cabin roof and resin roof at the back. Steel roof rack was 3/4 length and the aerial sat in the middle of the metal roof at the front. Worked very well.
AnswerID: 589652

Sponsored Links

Popular Products (13)