UHF Radio Antenna Location

Submitted: Sunday, Oct 05, 2003 at 17:50
ThreadID: 7616 Views:3927 Replies:6 FollowUps:2
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Hi all

A question regarding placement of UHF aerials..

Is there any difference in performance between bull bar mounted and over the bonnet.

Don't know much about reception issues and I'm getting conflicting reports regarding placement.

There is no reasonable spot for the aerial on my Defender regarding the bonnet configuration due to the way the vehicle is put together ... a door hinge is a possibility.

The question is this.. do the aerials work better over a flat metal area v the bull bar or is this only an issue when in motion.

Ta
Rosco
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Reply By: Truckster (Vic) - Sunday, Oct 05, 2003 at 18:03

Sunday, Oct 05, 2003 at 18:03
Look here for the answer.

http://www.gme.net.au/land/choose_ant.html
AnswerID: 32825

Reply By: TravellingVegemite - Sunday, Oct 05, 2003 at 23:27

Sunday, Oct 05, 2003 at 23:27
The dealer I got mine from recommended mounting on the roof gutter rather than the bull bar so that the car (and caravan) is not in the way of the signal in the rearward direction. It is also better when higher.

Roger
AnswerID: 32850

Reply By: Old Soldier - Monday, Oct 06, 2003 at 08:35

Monday, Oct 06, 2003 at 08:35
G'day Rosco,

I would suggest you take note of the link provided by Truckster. For the average usage you will not get better advice.

Re gutter mounting, if gutter mounted your maximum radiation will normally be to the opposite side of the vehicle, so in reality that nullifies the advice to place it there so you can transmit back over the caravan you are towing.

In this position the weakest lobes of the emission would be to the front and rear.

Examples of all radiation patterns are shown in the link from Truckster.

The thing to remember here is that this is all working on the principles of Antenna Theory. and that's just what it is - Theory!!!

From time to time you will get situations where the characteristics of your transmission and reception defy all theories - and take it from a former proffessional radio operator and instructor, that happens more than the theorists would like to admit :) :)

Enjoy the bush

DennisN



AnswerID: 32858

Reply By: Phil G - Monday, Oct 06, 2003 at 13:53

Monday, Oct 06, 2003 at 13:53
The radiation patterns are irrelevant when using a ground plane independant antenna. I cannot see any reason to purchase an antenna that requires a ground plane. That leaves one main variable - antenna height, and for identical antennae, a gutter mounted always outperforms a bullbar mounted.

I do a lot of 4wd convoy work, and the bullbar mounted antennae give the poorest range, unless one of those very large, expensive antennae are used.

My recommendation is to use a grount independant base and purchase two antennae to go with it - a short rubber stubbie for around town, and for most bush work and the normal 6 dB for desert or open country work. I end up leaving the stubbie on 90% of the time as it is indestructable and performs extremely well when roof mounted. A swing down base is useful if you intend using the longer 6 dB aerial as well.

Also, the most common cause of poor UHF performance, is poor installation with shorting or core breakage at the plug - make sure its soldered well, and get the shop to do it if you're unsure, or not handy with a soldering iron.

Most roof mounts are best positioned above the drivers door, and the cable is run down the channel and into the cabin thru the rubber taking the wiring to the front door (May not have one on a Landrover!)

Cheers

Phil
AnswerID: 32882

Reply By: Boxa - Monday, Oct 06, 2003 at 18:53

Monday, Oct 06, 2003 at 18:53
Ross,

Plain and simple. UHF's mode of tx/rx is LOS (Line of Sight). What you see is what you get. Obviously the higher the better and the more radiation conductor beneath (metal) the better the reception in the opposite direction will be.

It works much the same way as holding someones hand while you touch an electric fence, you feel a tingle while they receive the brunt of it.
The AE is you touching the fence with the furtherist part of the car being the unfortunate shock victum. Mounting the AE on the bullbar will use the entire car to transmit better from behind.

To answer your question,
the more further back you have your AE from the bullbar, the further forward you can both tx and rx.

Its my job, I have to know this stuff.

Happy trails,
Box
AnswerID: 32903

Follow Up By: Member - Ross- Tuesday, Oct 07, 2003 at 08:10

Tuesday, Oct 07, 2003 at 08:10
Thanks Box

Makes alotta sense ... as does most of the above.

Be mindful of the fact my knowledge of radios would slide around on the back of a 5 cent piece.

So what about Phil's suggestion regarding a non ground plane dependent antenna?

Do these work as well as ground plane dependent?

The bull bar is obviously not the best location but I am concerned about leaving it in a low hanging tree branch if roof mounted... "out of sight is out of mind etc etc"

I guess it all boils down to whether the two types of antennae perform equally.

RoscoRosco
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FollowupID: 23553

Follow Up By: Boxa - Tuesday, Oct 07, 2003 at 19:41

Tuesday, Oct 07, 2003 at 19:41
Ross,
You need to consider your needs and of course your usage. If you do alot of bush work then you need to have an aerial that you know will be out of the way or at least tucked in and not pertruding from the roof. If comms are a major priority for your trips then you will need to look into Phils suggestion with running the ground independant rig.
A good 6dB antenae will have a working radius of 15km+ on its worst day.(no hills)
Personally, I run a 6dB antenae on my bullbar as I work in tightnet convoys and I like to be able to use the aerial as a visual height detection for the GU in the tight stuff. Other than that, I only use it for road reports.
This suits my needs.
Hope this helps some what.

Happy trails,
Box

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FollowupID: 23600

Reply By: Eric Experience. - Tuesday, Oct 07, 2003 at 23:21

Tuesday, Oct 07, 2003 at 23:21
Ross.
The best place for the antenna is the centre of the roof but if that is not on then a bracket of the spare wheel mount works best on the defenders, make the bracket at a hight which allows the bulk of the antenna to be above roof hight and towords the centre line of the vehicle, it will not be damaged by branches that are at the normal 45degree angle. Eric.
AnswerID: 33073

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