During some recent 4WD Training Courses, it became apparent that there are some myths and ideas out there that aren’t quite right regarding UHF radios, antenna’s and installations, and it became clear that perhaps an article about them might be well timed.
UHF Radios are a great accessory in our 4WD vehicles enabling us to contact others in our group or convoy, other road & track users, truck drivers and possibly someone who can come to our aid, especially in remote regions and if we can contact them with a well set-up UHF radio.
So if your UHF CB radio reception is patchy and unreliable, that’s often due to its poor set-up.
A UHF radio transmits best in a line of sight – in other words, if your antenna can ‘see’ a point, your radio can potentially transmit to it. Therefore, antenna location and type have a huge influence on a radio’s performance.
What Radio type to get?
Whilst a small hand-held CB radio might be convenient don’t be fooled into thinking it’s all you need. From our experience we’ve found that they generally only cover a radius of about 1 to 5 kilometers at best and that depends on conditions including dust. Having said that, in recent years’ hand-held models have improved and you can now get 0.5 watt to 5 watt units, 5 watts being the maximum legal power output, which do get slightly better coverage but they are limited by a very short aerial. Some of the better hand-held models can be switched from 1 watt, 3 watt to 5 watts allowing the user to conserve battery life when in close range, by setting the unit to 1 watt, but being able to switch the unit up to 5 watts to help increase its transmission range when you’re further away from others. Of course being on 5 watts reduces the battery time available before it needs to be recharged. Some units can also be connected to an external antenna via a coax cable (RG 50 or RG 58). Hand-held units are ideal for close quarters communication such as from a spotter directing a driver through a tricky situation or for general camp communications.
Standard in car units are generally 5 watts and there are several reliable brands to choose from. GME (Australian owned), Uniden and Oricom are all well regarded brands and there are other new players on the market now too. You also have a choice of a traditional larger unit with all the controls on the face of the unit or a smaller one with the controls on the hand-piece with an extended lead and a slimline unit that can be installed out of the way.
It’s also important to consider where you’ll install your UHF radio. Traditionally (years ago) the radios were commonly installed on the side of the gearbox tunnel or at the bottom of the dashboard with the microphone clipped within easy reach. Some also installed them under one of the front seats with the microphone clipped to the center console. Whilst the lower locations seem good, some people have discovered that their door seals leak in water crossings and as a result the units have been drowned and are beyond repair or salvage.
As UHF units have become smaller and more streamlined in more recent years, it’s become fashionable and practical to install them with the Radio/CD Player higher up in the dash itself. Overhead consoles provide the ultimate solution because your radio/s can be installed above your head with no impact on the dashboard at all and the microphones are still within easy reach.
Antenna Type and Coax Cabling
Your choice of antenna is also very important. For close convoy-style touring, where you only wish to talk to and hear from vehicles within a few kilometers in front of or behind you, a short rubberised antenna, between 150mm and 300mm, is fine. These low gain antennae are less likely to strike overhead foliage but for long distance work, a higher gain antenna is much better.
Gain refers to an antennae’s ability to improve reception and transmission and is measured in decibels (dB).
It refers to its radiation pattern with low dB in the shape of an apple and better for hilly terrain and as dB increases so does distance and the transmit pattern becomes more the shape of a flat dish.
One important thing to consider and often not understood is that coiling excess antenna coax cable in the engine bay or under the dash is also a problem. It causes the cabling to become an inductor which can produce a voltage spike down the cable and back into the transmitter and blow up the output stage transistors.
The Solution: Shorten the antenna’s coax cable at one end to the desired length, (either aerial base or radio end), fit the plug to the coax and then plug the coax into the UHF radio. This will not damage the antenna or its signal regardless of what ‘dB’ the aerial is. If you aren’t able or don’t want to shorten the coax, the next best option is to run it in long loops under the dash, to help avoid it possibly creating that inductor issue.
Location is really a compromise of best transmit location compared to where you can mount it on the vehicle. The best position is in the middle of the roof, but who wants to drill a hole in their roof. For ease of fitting, traditionally many people attach their CB aerial to their roo bar, but in doing so their vehicle creates an uneven radiation pattern spreading out behind the car and to one side if fitted to one side of the bull bar. Your signal forwards can also be shadowed by any vehicles in front of you. Ironically poor antenna location means that only a low percentage of user’s realise their radio’s full potential. Even an expensive high gain antenna projecting above the roof level from a roo bar suffers because antenna tips radiate very little energy. The general 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. In other words, the ideal place to achieve this is up on the vehicle’s roof line or roof racks.
If you’re at ground level, on flat terrain, the horizon is about 5 km’s away. If there is nothing between your antenna and the visible horizon, that would be the limit of your transmission range, but if you raise your antenna by just 1 m (i.e. from your bull bar up on to your vehicles roof) you may increase the range over that visible horizon.
If having an aerial high up on your vehicle is a concern, you can get hinged mounting systems so the aerial can be folded down going into a shed or carport and when it comes in contact with overhanging foliage. Even a short antenna higher up that can take the knocks of overhanging branches works much better than a long antenna mounted low down. Aerial mounts with an artificial ground plane can help improve transmissions by evening out signal radiation patterns.
The antenna pictured is a 4dB mounted on a hinged
mounting system. It also has an artificial ground
plane which evens out its radiation pattern.
It’s also best to fix your antenna vertically, this sounds obvious but the closer the antenna is to vertical, the more effective it will be. A UHF antenna that bends as you drive may lose performance when travelling, but should be fine when you’re stopped. A sloping unit may look good, but its efficiency will suffer because an antenna’s broadcast is largely flat, like a dinner plate, radiating out from a central point at the 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, impeding its efficiency.
With a higher altitude and a clear line of sight, your UHF CB radio can have a range of more than 100km, greatly enhancing your chance of finding someone to come to your aid.
From our in vehicle unit, I once spoke to a bloke with a 5W hand-held unit and we managed to stay in contact with him more than 130km away from a high point in the Flinders Ranges, but our altitude (over 600m) was an important factor.
So in an Emergency Situation, one option to optimise your transmit capability is to drive up onto higher ground if it’s available. Of course all of this is ignoring the benefits of the repeater station network, which will often give you further coverage also. There are a lot of repeater stations in the Australian Outback operated by station properties. Only use these repeater channels in case of emergency as they are used for property operations. This is a gentlemen’s agreement not to operate on the repeater channels.
Where to in the future?
A few years ago the UHF service was expanded from 40 channels (wide band) to 80 channels (narrow band) utilizing the same 477MHz spectrum by slotting 40 more (new) channels in between the existing 40 channels. This has created a small problem many are not aware of but is noticeable when you are communicating with others using the old 40 channel units when using your 80 channel unit. Because the band width has been halved it equates to an audio lower in volume, which is why reception sounds softer or louder depending on who’s transmitting on what units. It becomes a pain in the proverbial when you have to constantly adjust the volume to compensate when two different aged radios are being used in a conversation, but there is light at the end of the tunnel. The ACMA have mandated a phase out of 40 channel UHF radios, so by around 2022 all UHF radios will have to be 80 channel units and the current ‘volume issue’ will then be resolved. If offered a second hand 40 channel unit for sale or free, it might be a wise idea to politely refuse the offer.
…. and to finish …. A couple of Handy Hints.
If you have thin steel aerials on your vehicle, corrugated roads have a tendency to shake the aerials to the point that they snap off at the base (particularly aerials mounted on bull bars).
The Solution: Try using self-vulcanising tape wrapped around the aerial base and part way up the aerial. It seems to limit the whip affect at the aerials base that creates the right harmonics to shake the steel until it breaks and falls off.
The antenna pictured is a 6dB with the self-vulcanising tape
covering the locking grub screw and only a short distance up.
The tape could be wound further up the antenna.
UHF Radios & Operational Etiquette
Always listen before transmitting as the channel may be in use by other persons. This helps avoid people talking over each other which leads to a squawking noise and can be quite annoying.
Always wait for 1-2 seconds after you’ve pushed the microphone button in, before talking. This helps avoid those listening, missing the first part of your conversation or reply.
Always leave a three second break between “overs”. This allows others in the group, or other parties, to put in a “breaker”. The “breaker” may only be to add comment to the discussion or it may be because of an emergency situation.