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.
UHF is for groups. Tours and truck Contact on highways.
Anything off road. Hire\buy a Sat Phone. There are combination Sat\Mobile sets avail. 2 for price of one.
Worlwide. Get a HF Licence and set. (Provisional? used to cover everything,) unless you got serious.
I had one 50 odd yrs ago.
B4 I went to Marine. 0-30 meg Basically..
They tended to be too up themselves for me.
I still carry my 6in City antenna Taxi's use, in dash.
Differing propagation waves. High Round rather than flat oval.
-- Edited by macka17 on Sunday 3rd of September 2017 08:57:08 PM
-- Edited by macka17 on Sunday 3rd of September 2017 08:58:26 PM
-- Edited by macka17 on Sunday 3rd of September 2017 09:00:42 PM
Snippy said
08:08 AM Sep 4, 2017
Thanks Mike,
Very informative.
aussie_paul said
07:12 PM Sep 4, 2017
Excellent Mike, thanks.
Aussie Paul.
C00P said
10:42 PM Sep 4, 2017
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
C00P
ABNER said
11:59 AM Sep 6, 2017
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 .
UHF CB radio and its limitations
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.
Good Clear and comprehensive explanation Mike.
And after all that reading..
UHF is for groups. Tours and truck Contact on highways.
Anything off road.
Hire\buy a Sat Phone. There are combination Sat\Mobile sets avail.
2 for price of one.
Worlwide. Get a HF Licence and set. (Provisional? used to cover everything,)
unless you got serious.
I had one 50 odd yrs ago.
B4 I went to Marine. 0-30 meg Basically..
They tended to be too up themselves for me.
I still carry my 6in City antenna Taxi's use, in dash.
Differing propagation waves. High Round rather than flat oval.
-- Edited by macka17 on Sunday 3rd of September 2017 08:57:08 PM
-- Edited by macka17 on Sunday 3rd of September 2017 08:58:26 PM
-- Edited by macka17 on Sunday 3rd of September 2017 09:00:42 PM
Very informative.
Excellent Mike, thanks.
Aussie Paul.
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
C00P
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 .
Bob.