Guide to UHFs in RVs

You're 40km out of town and the rough track is narrowing fast when you realise you've taken a wrong turn. You start to backtrack,but within seconds you're axle deep in soft sand. It's only after hour of digging in searing heat when you've taken your last sip that you realise you haven't seen another vehicle since leaving town. You dial 000 but get no reception.

Minor issue can have dire consequences in remote areas and communications are vital. A UHF radio could be your only friend.

Many accuse CB radio of being of short range, patchy and unreliable but according to Phil Pullem, UHF radio installation specialist,that's often due to poor set-up. Phil owned Electric Bug for 32 years, is an advisor to the SA Government, and has designed many communications products. Phil says that out of all the vehicle radio antenna installations he's seen only 20% realise the radio's full potential.

UHF radio transmits best in a line of sight, in other words if your antenna can "see" a point then your radio can potentially transmit to it, therefore, antenna location and antenna type impact a radio's performance.

"Many people attach their CB aerial to a roo bar, but in doing so their own vehicle will create a shadow for the signal, spreading out behind the car, since the top of the bar is level with the bonnet," says Phil.

"It also can be shadowed by any vehicles or vans in front. Even expensive, tall antennas projecting above the roof level suffer from this problem as antennae tips radiate little energy."

The rule of thumb is to place the bottom half of the aerial at a point where it can see the greatest distance, even to the horizon if possible, and the ideal place to achieve that is on the vehicle roof line or roof racks.

Phil says a good CB antenna for a vehicle needs to be only a metre long and some mounting systems are hinged so the aerial can fold down as it comes in contact with overhanging foliage. A short antenna designed to take the knocks of overhanging branches works much better than a long antenna mounted low down.

It's also best to fix your antenna is vertically. A sloping antenna may look cool but its efficiency is low, as your antenna's broadcast is largely flat, like a dinner plate, radiating out forma central point at the antenna base. If you tilt the antenna, one edge of that imaginary plate rises up into the air, while the opposite slopes down towards the ground. You're only achieving maximum range at that notional horizon at two points at right angles to the highest and lowest points. The closer the antenna is to being vertical the better it will be, and any UHF antenna which bends as you drive may lose performance in motion, but should be fine while stopped.

EMERGENCY SITUATIONS

If you can't attach your antenna to the roof, another option is to optimise your broadcast during an emergency.

A simple method is to uncouple the vehicle from your trailer and drive several hundred metres away so that the camper represents a reduced shadow to the signal, or even drive up onto higher ground.

Alternatively, choose an antenna that is ground independent, in other words, one that can be taken away from its mounting point on the vehicle and still work while connected with an extension cable.If you attach the mounting point to the end of an extension pole with sufficient cable, you can raise the antenna to a higher point.

At ground level, on flat terrain, the horizon is 5km away. If there is nothing between your antenna and the horizon that would be the limit of your broadcast range, but if you raise your antenna by just 2m you double the range to 10km, and so on.

One broomstick or even two joined together will help you gain the maximum height.
"You just need something light and simple, like sleeved plastic conduit and a length of coaxial cable (50 ohm RG58), and since it only has to be a few metres long it doesn't have to be expensive,"explains Phil.

ANTENNA CHOICE

Your choice of antenna is significant, too. For convoy style work,where you only wish to talk to and hear from vehicles within a kilometre or so in front of or behind you, a short rubberised antenna, 150mm to 300mm long, known as a quarter- or half-wave antennae, is fine and less likely to strike overhead foliage, and be damaged when they do.

For long distance work, a full length aerial, up to a metre in length, is preferred. Most times the longer antenna can be stowed in the vehicle for emergencies or when setting up camp as a base radio for others in your group.

However, when you get into rugged mountain country those shorter quarter- or half-wave antennae have another benefit. As the antenna shortens in length, the major transmission in the antenna is squashed up and down, inducing the transmitted signal to transform from a largely flat plate-like form to one which radiates up and down as well as horizontally, like a globe. This means it can bounce the signal off higher rocks, cliff faces and other landforms above and below, potentially transmitting to the far side of a ridge or hill, or in a valley below, well out of that line of sight. These bounced signals travel less than a line of sight signal but in a valley, a signal from a big antenna just may not"get out".

Phil also advises having a radio that is easily removed from the vehicle so that if necessary it and its ground independent antenna can be carried, along with the vehicle's battery, to the highest ground around for the maximum range.

If given the right altitude and a clear line of sight, your UHF CB radio can have a range of over 100 km, covering 31,000 sq km,greatly enhancing your chance of finding someone to come to you raid.

It's good to share this information with your fellow travellers as the most important person in this equation might be sitting next to you.

RADIO SIZE

Don't be fooled into thinking that a small hand held CB radio can do this job for you. They generally only cover a radius of about a kilometre, Phil experimented with a 5W hand held and managed to contact someone over 300km away from the top of the Grampians in Victoria. Like we said, altitude is very significant.

However, in general a well-set up CB system in a car can cover upto 3500 sq km of area, as long as you set it up well.

All of this is ignoring the benefits of the repeater station network, which might give you coverage of up to 20,000 to 70,000 sqkm. To achieve this while travelling you will need a comprehensive understanding of your radio's capabilities and the repeater system.But that's another story.

"You can use UHF CB to call someone when you have a problem when you're in a remote place," explains Phil.

"Use it as a bridge between yourself and the locals, other travellers or 4WDers, or between you and the trucking community.Don't be afraid of truckies, although a few maybe grumpy, they know what it's like to spend a long time at the wheel, all about local road conditions, and can be a big help when you want to get around them or they want to get around you.

"These are all people who can come to your aid if you can contact them with a well set-up UHF radio."

Source: Camper Trailer Australia #38