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Simplex and Duplex |
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You'd think it would be enough for radio users to
just use one channel at a time but oh no, they've got to be clever and use two
at the same time. This is very confusing, why do they need to do that? |
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With the most basic method of operation, each user has a
transceiver. A transceiver is a radio which can transmit on pressing the
PTT button and when released, it acts as a receiver. Two
people can talk to each other taking turns to transmit. While one is talking,
the other is listening and vice versa. When they have passed their messages,
they both go to standby and are both receiving the frequency. |
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Comms radio is not like the telephone we are all familiar
with where both persons can speak at once. It would be possible to make an
analogue handset act like a telephone but this would add expense, complication,
weight and size. |
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In its simplest form of
radio use, two users will transmit and receive on the same frequency. This
is called simplex operation. Some folks refer to it as back-to-back
working. Some examples of this are CB radio, PMR446, UK General, Channel
16 marine, Kinloss search and rescue and many on site users where all
radio users are within a short distance of each other. |
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Sometimes, the range of handheld radios is not enough to
achieve the coverage required. In these circumstances, a
repeater is used. A
repeater is a box of tricks that receives the signals from the mobile users and then
re-transmits their signal, usually at higher power and from a bigger aerial . In order to
carry this out, it needs to transmit and receive on
different frequencies or else it would be chasing its own tail. |
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All mobiles transmit on one frequency which is the
repeater input frequency, and all mobiles are tuned to receive another which is the repeater
output frequency. So in this way, all communications go through the repeater.
It's useful for you the scanner user because you'll hear handhelds much further away than
you would otherwise and they will all be the same signal strength so you
won't miss out on anything. |
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If you need to calculate a frequency pair, you can
download this excellent utility by mainsailman -
You will need to have a spreadsheet application
installed on you computer, or failing this, you can download a free Excel
reader from Microsoft. The url is on the links
page. |
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