UHF CB radio and its limitations

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This is a post I wrote and posted to another forum a while back but
I think it probably has more relevance here:

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: [I have to cheat here because I can't post links yet but
Google the following]

ve2dbe and/or Radio Mobile

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" (11km scale) 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" (24km scale) 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" (87km scale) 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" (130km scale) 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.

1508576198_hilly.jpg

1508576642_hilly1.jpg

1508576668_flat.jpg

1508576689_flat1_-_rx100m_6dbix.jpg
 

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