Fitment of HF Radio Auto Tune Antenna

Submitted: Saturday, Feb 25, 2006 at 01:06
ThreadID: 31129 Views:2106 Replies:9 FollowUps:2
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Hi Guys

Just wanted to know from those of you out there who have fitted there own Auto Tune Antenna how did you get around the the fact that you have to have a 160mm gap between the Antenna and the body of the 4wd.

I am fitting one onto a GU Patrol that has duel wheel carriers and the only place that you can fit it is on the outside of one of the spare wheels as this gives you the 160mm needed so as not to interfere with the efficiency of the Antenna.

According to the mounting procedures in the manual from Barrett the exact words are as follows " FAILURE TO MAINTAIN THESE CLEARANCES WILL NOT ONLY REDUCE THE EFFICIENCY OF THE 910 ANTENNA BUT MAY ALSO LEAD TO INTERNAL RF ARCING AND FAILURE.

I do not want to risk any damage to the Antenna as to replace it would not be cheap and the other half would not be happy so as i said the only place i can think of is on the outside rear of one of the spare tyres, but where do you run the cables and the fact that when you need to use the spare you have to remove the Antenna does not excite me at all, any help would be appreciated.
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Reply By: Member - Phil G (SA) - Saturday, Feb 25, 2006 at 08:40

Saturday, Feb 25, 2006 at 08:40
Hi Martin,

Can I suggest that outside one of the dual spare wheels is not a good place unlesss you can be sure it won't get hit by bushes and trees. Maybe you could make a bracket that comes out from one of the wheel centres, or between the wheels. Other alternatives I've seen are swing down brackets, and swing out brackets - all custom made.

I had Phil from Electric Bug in Adelaide explain the clearance thing to me. For best performance, the 160mm clearance from steelwork needs to be from the junction of the plastic covers, about 2/3 of the way down the aerial. Then you need clearance at a 45degree angle in all directions upwards from that point. So an ideal installation will usually have the antenna tilting backwards slightly, and given the rear of most vehicles leans forwrad, this is normally easily achieved. Its worth trying to achieve that clearance - best to come over as a strong signal than one of those scratchy ones you get out there. And you also have to treat tyres as if they are made from steel (because they are!).


AnswerID: 156959

Reply By: Member - Paul P (Bris) - Saturday, Feb 25, 2006 at 08:50

Saturday, Feb 25, 2006 at 08:50

These people may be able to assist with fabrication. UHF Bracket In Adelaide though. Click on the Accessories section


AnswerID: 156961

Reply By: Darian (SA) - Saturday, Feb 25, 2006 at 10:20

Saturday, Feb 25, 2006 at 10:20
Indeed re the warning - high energy levels in that antenna case, on TX. I've seen inside mine - large PCB with all sorts of gizmos on it - relays, coils, processors, dooflackers and tiny decraminators (I'm no tech as you can see) - soon as they play up, its back to the factory in Perth.
AnswerID: 156974

Reply By: signman - Saturday, Feb 25, 2006 at 15:03

Saturday, Feb 25, 2006 at 15:03
The RF is in fact radiated from the ' whip' portion of the antenna unit. It is imperative that it is in the clear. If you mount the Autotune on a bracket off your wheel carrier (between carrier & vehicle)- won't most of the length be running next to your rear glass, and by the shape of the rear of the vehicle (and the Autotune) you should have your 160mm clearance at the critical area!!
I wouldn't be having a heart attack about 160mm- so long as there is some clearance. If the Autotune is splattering that much RF around, that would be more of a worry.
Don't have anyone standing near the antenna when transmitting- and especially when tuning up.
AnswerID: 157007

Follow Up By: geocacher (djcache) - Saturday, Feb 25, 2006 at 22:20

Saturday, Feb 25, 2006 at 22:20
You are talking about a radio that puts out 100 watts of RF.

If I tune mine in the garage on 8 Meg I can turn all the sprinklers on via the computer controlling them several metres away.

Also your assumption that all the emitted RF is coming only from the whip is incorrect.

My reading of his post is that it would be mounted near the C pillar not in the middle of the glass. Therefore most of the body would be within 160mm of the corner pillar of the wagon.

This is a bit of gear that is very expensive and required to operate faultlessly in matters of emergency. It's an issue worth considering and trying to achieve the recommendations is worth the time.

FollowupID: 411262

Reply By: Member - Roachie (SA) - Saturday, Feb 25, 2006 at 18:11

Saturday, Feb 25, 2006 at 18:11
If you're running dual spare wheels on a Kaymar or Opposite Lock rear bar etc, you have exactly the same set-up as me. I have a Barret auto-tune and I have a bracket that utilises the bolts in the large back door which were previously used to hold the OE spare wheel bracket. Indeed, I actually have a spare (home made) bracket, powder-coated black, which you'd be welcome to for $20- if you want to pay for the postage as well.
I have no problems with reception on RX or TX ......well nothing that I could ever put down to the position of the antenna anyway. It is worth noting that I also have a full length steel roofrack, so a portion of the whip is adjacent to the steel rack.
AnswerID: 157030

Reply By: Mike Harding - Saturday, Feb 25, 2006 at 18:33

Saturday, Feb 25, 2006 at 18:33
The clearance doesn't have to be 160mm or else the whole thing blows up. Barrett are picking a sensible minimum number but you'll probably get away with less - how much less? No idea.

You would need to go very close to the vehicle metal before the voltage on the antenna creates an arc but beware of the antenna bouncing around if you _ever_ plan to transmit whilst driving.

The bigger problem, I suspect, will be the detuning effect of the proximity of the vehicle metal to the antenna which may reduce its ability to autotune to a low SWR. It's a hard thing to test without properly mounting the antenna but if you can jerry rig it in the location you wish to fix it and provide a _really_ good earth to it via a short piece of braid or copper strap as you can (don't use wire) and still tune to a low SWR then it should be OK for a permanent fix in that location.

Personally I would avoid autotune antennas like the plague – too expensive and too prone to failure or damage on a 4WD.

Mike Harding
AnswerID: 157031

Reply By: Mike DiD - Sunday, Feb 26, 2006 at 08:17

Sunday, Feb 26, 2006 at 08:17
Barrett, Codan and Moonraker Autotune antennas are fundamentally different.

The Barretts have electrical components inside that do not radiate, but will still have some electric field. The main radiation comes from the whip. The 160mm clearance will be more of a problem on a straight-sided vehicle like a bus or a van. For rear-mount on a 4WD where the top of the housing is at roof height, there isn't a problem due to the slope of the rear window.

Codan Autotunes have a vertical coil inside the housing which does contribute to the radiated signal. So it really helps to keep the top part of the housing as clear as possible.

Moonraker Autotunes have the coil higher up from the base so they are less sensistive to nearby metal around the base of the antenna.

AnswerID: 157098

Reply By: Wayne (NSW) - Sunday, Feb 26, 2006 at 09:04

Sunday, Feb 26, 2006 at 09:04
On this subject and a question for the experts.

If the Auto tune antenna was on a swing away wheel carrier, would it work better if the wheel carrier was opened? That way the antenna would be way from the vehicle.

AnswerID: 157101

Follow Up By: Mike DiD - Sunday, Feb 26, 2006 at 12:05

Sunday, Feb 26, 2006 at 12:05
Yes, you will get a better signal.

BUT you must connect copper brais at least 25mm wide across any hinges on the Tyre Carrier as well as the Door hinges.

FollowupID: 411353

Reply By: Member - Martin- Sunday, Feb 26, 2006 at 19:20

Sunday, Feb 26, 2006 at 19:20
Thanks for all the info guys that is why i love this site you get good sensible answers. I think i will go with the back door bracket option and thanks for the offer Roachie but i enjoy fabricating my own gear i think it is half the fun.
AnswerID: 157186

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