The science behind UHF aerial placement?

[gumboot Avid PP Poster! Find all posts View Profile Close gumboot]

Morning,
I am interested to hear from others about UHF aerial placement (and the science behind it) for best reception.
Traditionally it was just put it on the bullbar and be done with. But is this the best option?

There seems to me three main options generally. The bullbar, the roof gutter/roof rack or on the rear say on a kaymar or rear bar bracket. Each have general pros and cons. IE easy to mount on a bullbar, a tall aerial on the roof rack will cause height issues around town and may collect trees in the bush etc
But reception must be one of the the most important drivers. ie How much extra range and senstivity to do you get if your aerial is above your roof line. I always thought the higher the better?
I would also expect, good quality coax, length of coax, not wrapped around a power lead, etc also make small differences as well
Then on top of all this there is the various different types of aerials and their db gain for the different terrains you drive etc.

Mine is a GME 4703 on the bullbar ,the tip of it is barely over the roof line. and was thinking the other day about the benefits of mounting higher up on the vehicle
I would be interested in your thoughts and advice. Any commerical experts in Brisbane?
Cheers
Robert

[JoshF Advanced Member Find all posts View Profile Close JoshF]

from the old CB days, centre of the roof was the best place as your radiation pattern was circular and your car assisted in being part of the ground plane. If you mounted the antenna forward or backwards of this your radiation patten became an elipse accroding to which end your were attached too.

I don't think this has changed alot with UHF

[AJ120 Out of control poster! Find all posts View Profile Close AJ120]

Hieght is the real key, the higher you can get it the better. I while ago I was sitting on top of Billygoats ridge (the day before the GTG started) and called for fellow PPers, I ended up talked to VKprado who was around 60-70kms away from me.

I know you cant have an arial that high but it highlights the point, height is might!

Other than that, where is the most convenient place to mount, run the cable etc etc are usually the determining factors.

Cheers Andrew

[toddy Member Find all posts View Profile Close toddy]

Most aerials these days you buy are ground independent so the car is more of a physical interference I believe. Happy to be corrected as one of my friends was a radio geek. If mounted on bullbar then it will be hard to pickup signals from behind and likewise transmitting.

I use a mini aerial for around town, and screw on a 6.5dB taller one for bush driving to get above the car.

Photo in link.

aerial by chews04, on Flickr


aerial 3 by chews04, on Flickr


aerial 2 by chews04, on Flickr

[120D4D Avid PP Poster! Find all posts View Profile Close 120D4D]

I've said 'height is might' before on this. Mine is mounted to the top rail as I dont have a rack - and dont have the balls to drill though the roof skin - but I know reasonable performance can still be gained by mounting on the bullbar.

A UHF expert once told me to think about the aerial as a (vertical) fluroescent tube. If you can see the tube in the dark then great, if you can only see the surrounding light then it will still work - just not at its best.

Yes im sure there are some star wars fans that will dust off their light sabre's tonight and test this out, so may the force be with you...

[Johnboy67 Lurker Find all posts View Profile Close Johnboy67]

Hi Guys,

Having been a bit of a Radio Geek in the past, I have to agree with earlier posters, and say that "Height is Might". This is basically the truth. UHF is line of sight, so the higher you are or the flatter the terrain between you and the reciever the better chance of you getting reception. I can give you an example, some years ago I was on the North East tip of Dirk Hartog Island and with a hand held UHF radio (approx 3 watts power) I was able to clearly talk to people in Carnarvon about 95km over the sea, a very nice flat terrain.

There are also some odd atmospheric conditions that can assist UHF, such as inversions, where a warmer air mass traps a cooler air mass close to the surface of the earth (I think that's right I am not a meteorologigist) and UHF waves can bounce or skip off the inversion layer. I have heard of UHF signals being received over several hundred kilometres because of this bounce. This also used to happen on AM side band radios and was often referred to as skip. It also definitely happens with HF where you can speak to someone 3,000km away but not 200km away. End of the day however you can't count on these situations.

In my opinion there are 3 things to consider.

1) The power output of your radio. This will help how far you can send your signal, but not what it can hear (your antenna does that and it is crucial with transmission also). The maximum Power output from a CB radio, legally, is 5 Watts and in practice many "5 Watt" radios only put out 3 to 4 Watts. Most Radios can be tweaked and adjusted to put out more power but you will reduce battery life if you do this. (It's really only important if you are using handhelds which rely on inbuilt batteries) You need to find a friendly radio geek who can tweak your radio and make sure it is putting out 5 watts, they'll usually do it for free cause its kinda nice to talk to some real people every now and again. No disrespect I am a closet radio geek!!!!!!!

2) The type of antenna you use. If you use a dipole antenna (omni directional like the ones most 4WD use) you can get an equal spread of signal both for transmision and reception in all directions. Imagine a doughnut shape eminating from your antenna where your antenna is like an arrow shot through the center of the doughnut hole. You can also use uni directional or Yagi antenna which extends the range of the antenna by changing the doughnut shape to a cone shape eminating from the tip of the antenna in the direction that you point it. This will increase the range at least two or three fold but only in one diection. Then there is the db rating and which without going into too much detail, basically it means how tall or long the antenna is, meaning how much of the UHF frequency wave that the antenna can catch. Db can help but the position of antenna is more important.

3) The location of your antenna. The best place on a vehicle is to put the antenna smack bang in the middle of the roof, using the roof as a ground plane and using the highest point of the vehicle as the lowest point of the antenna. I have to confess that I have never done it and never will because it looks like crap and I'm never going to drill a hole in the center of my roof. So, a 'ground plane independent' antenna mounted on the Bullbar that pokes up above the roof will give you a good range forward of the vehicle and less range behind, vice versa for an antenna mounted on the spare wheel carrier behind. In truth, the higher the antenna the better and I mean the mount point and the tip. The shorter the run of coax will help but it is basically negligible over the distance of an average vehicle. Making sure that all connections are solid joins and neatly soldered is more important.

With UHF CB Radio assume that the range is 2 to 3 km and whatever you get beyond that is a bonus. To increase your range make your transmissions from the top of hills or even minor crests in the road. Assume that you will get limited range in twisty hilly conditions, and you will not be disappointed. Regardless of how much power your radio puts out you must consider your location, height (in relation to your receiver) and the terrain you are in. You can use UHF radiao to transmit over 100's of kilometres but it all depends on the power of the transmision and location of the tramsitter and receiver.

Didn't mean to write so much... Hope this helps J

[JoshF Advanced Member Find all posts View Profile Close JoshF]

Also worth considering is where you are going to be travelling, not a bad link to explain the various antenna types, DB's and how they broadcast

http://www.radiospecialists.com.au/m...20antennas.htm

[gumboot Avid PP Poster! Find all posts View Profile Close gumboot]

Hi Guys,

Didn't mean to write so much... Hope this helps J

That's very interesting reading, thankyou.
Cheers
Robert

[toddy Member Find all posts View Profile Close toddy]

Yeah, top read Johnboy67 thanks for the explanation, i do remember my radio geek buddy telling me this stuff.

[120D4D Avid PP Poster! Find all posts View Profile Close 120D4D]

Nice one Johnboy!

[Hazza Advanced Member Find all posts View Profile Close Hazza]

Very informative and on the money Johnboy67. I purchased my aerial (same as Toddy below) from Olbis at Rocklea (Brisbane) and was given the same info. A short aerial in the middle of the roof can perform better than a long one on the bull bar. As the roof it's out for me for obvious reasons, I opted for the bull bar and on the ARB deluxe bar (120 series) there is two mounting points. Gentleman at Olbis advised to place it on the driver’s side as this give a more direct line down the road and also keeps it away from trees when off road. He advised the top of the base (chrome section) and start of the whip should above the body being the bonnet in my case. I’m sure there are many styles of aerial but I purchased this one based on the success in the mines both short and long range. It also allows me to change whips for short or longer directional range.

Johnboy67 In the past with CB radios you could purchase aerial boosters that gave a huge range but illegal. Is this the same for UHF?

Hazza

[SOL Find all posts View Profile Close SOL]

So is it possible to mount an antenna front and rear wired in together?

[Bushbasher Out of control poster! Find all posts View Profile Close Bushbasher]

Be also mindful that in some states it is illegal to mount antennas in front of the driver so you'll have to put them on the left side unless you have a get out of jail free card. Recent legislation changes in WA have forced our corporate fleet to put the Codan auto tuning antennas on the rear of the car hanging from a bracket welded to the spare tyre mounting plate or alternatively from the rear ladder on the troopy roof rack. The UHF antennas are roof mounted. I like the front left of the bar for the antenna as you can see if it's going to get smashed by foilage. Put it up top and it's at the mercy of the trees, all dependent on where you're driving of course. Most of us have a limit on the height of the vehicle including roof bars and racks due to the height of the ceiling in the carport/garage.

[Bushbasher Out of control poster! Find all posts View Profile Close Bushbasher]

Ground independent antennas don't need to be mounted in the middle of the roof or middle of the bonnet. Most antenna manufacturers provide a guide to the use and operating paramiters of their antennas. Have a look at the Uniden, GME or other common UHF suppliers websites. They usually show a picture of the impact of terrain versus distance with various antenna lengths.

[Johnboy67 Lurker Find all posts View Profile Close Johnboy67]

Johnboy67 In the past with CB radios you could purchase aerial boosters that gave a huge range but illegal. Is this the same for UHF?

Hazza

Yes they are available, and yes they are illegal. Also bear in mind this only boosts your transmission not reception so if you want speak to some with a boosted signal, they will need one also so you, and everyone else in between, can hear them.

Oh I did not mention repeaters (usually channel 1 to 9) there are a number of these around and probably a list some where on the web so if you are planning a trip somewhere might be worth googling UHF repaters in that area.

Cheers