The one point I what to get across in this post is that UHF CB must never be relied upon for emergency communications in the Australian bush.
The *only*, pretty much guaranteed, forms of emergency radio communications for bush use are HF radio, satellite telephone and PLBs and even they can have issues.
If you never travel more than a couple of kilometres from the bitumen then UHF CB is all you need but if you go much more than that then you should seriously consider one of the above.
From time-to-time threads arise regarding UHF CB radio and, invariably, there will be be a few responses recalling occasions when people have made contacts over distances of 200km and more. There are three ways this can be achieved and it's probable none of them will be available in an emergency - I'll explain more about them shortly.
For our purposes radio signals fall into two very broad groups: HF (High frequency) covers signals up to 30MHz or so. VHF/UHF/SHF (Very, Ultra and Super) high frequency covers all other signals.
Australian UHF CB at 477MHz falls, unsurprisingly, into the UHF band.
The major difference between these two groups is that HF signals bounce off the ionosphere and travel around the planet. VHF/UHF/SHF signals pass straight through the ionosphere into space and continue on their way to infinity.
UHF signals are "Line of Sight" and behave exactly like a torch beam, ie. the Rx (receiver) needs to be able to "see" the Tx (transmitter) in order to receive a signal and this is the basic limitation of the distance UHF CB can cover because the curvature of the planet means that once the Tx and Rx are more than about 5km apart (at head height) the transmitted signal will pass over the head of the Rx. This is not a definitive distance because of signal "scatter" but it's in the ball park. Indeed, if you are standing in a valley it will be a lot less and if you are standing on a mountain top it will be a lot more.
So: we know that height is one of the three things which will give us more range the second is tropospheric ducting.
Tropospheric ducting is a random event which we don't fully understand but is caused by changes in the troposphere which can carry a VHF/UHF/SHF signal for thousands of kilometres and then pop it out to some unsuspecting bloke in Darwin or elsewhere who suddenly hears someone from Melbourne on his UHF CB radio. It is totally unpredictable in its timing, duration, distance and strength and is of no use to the radio user other than scientific interest.
The third is a CB repeater: a repeater is a Rx/Tx pair usually mounted at a high location to maximise its coverage. A repeater receives signals on one channel and re-transmits them on another channel. CB radios may be set up for this dual channel operation. Repeaters are a great asset to CB use but *only* if you are close enough to reach one. And "Let's hope I'm in range of a repeater" is not a good emergency radio plan.
The propagation of radio signals may be mathematically modelled and this technique is now commonly used for determining the optimum place to locate mobile phone cells, police radio repeaters and the like.
The website:
http://www.ve2dbe.com/rmonline.html
Provides limited public access to some very sophisticated software for propagation modelling and I created the following plots there. I have based them on a Tx and Rx height of 2m (car antenna), a power of 5W with 1dB feedline loss and 4dBi antenna gain for an omni antenna, these are typical UHF CB car installation values. The exception is the last plot which I modelled as though the vehicle was communicating with a repeater at 100m of elevation.
Areas of green are where coverage is good and conversations may be held with little trouble. Yellow means there will be many dropouts and words will frequently need to be repeated or spelled phonetically. Beyond the yellow there is no usable coverage. I have added a red distance line to give scale.
What do we learn from these plots?
"Hilly.jpg" show how little coverage UHF CB will produce in the High Country of Victoria or any other country with hills and valleys. What little coverage there is is very patchy and most of that will be on surrounding high ground. To the NW there is almost zero coverage whilst the NE affords about 6km of patchy coverage.
"Hilly1.jpg" is similar. I know that area well and I can assure you the coverage you see there is all but useless as all that ground is dense bush and steep valleys - there won't be anyone there.
"Flat.jpg" is perfect UHF CB country in the flat desert area of NW Victoria - this is as good as UHF CB gets and decent coverage extends to 15km or 20km from the Tx.
"Flat1.jpg" is in the same area as "Flat" but this time the vehicle is communicating with a repeater at 100m of elevation and will be able to hit it from 40km or maybe 50km. However I doubt there are any repeaters at 100m in that area?
UHF CB is ideal for car-to-car communications and the like but it should never be relied upon for emergency use.
Nice work, Mike. As you say, on or near highways, UHF could provide comms very quickly, but in such cases a mobile phone might work too. The other advantage of the UHF CB is that it could warn others on the same highway that there is an accident nearby.
But as you say, while a sat phone might take a little while to lock on and make the call, at least you can talk directly to the emergency services. It's usually the best option. THe GPS-enabled personal beacon gives a precise location but is really only for life-and-death situations, and doesn't (in most cases) convey any info on the nature of the emergency. It's cheap to buy, but very expensive to use (to the taxpayer, at least).
We carry all three...
Cheers
Great post, people need to be aware that height of antenna's do not necessarily mean good signal either, it's the radiation pattern that counts especially on vehicles. I carry UHF 25w radio, VHf Marine and also an EPIRB that is registered to my boat and also my 4WD. having that and satellite signal coverage is the poor mans emergency tool. I would only use it in life threatening situation .
