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Introduction
to Data Communications - Continued
SLOW
SCAN TV
Remember seeing this mode on a five-inch mono screen with a high
persistence tube. It seemed to take ages for the picture to form, QRM
could mess it up very well, and it was a complicated mode to run. Lots
of amateurs persisted with it, and still use it today, but again, the
computer has made it easier to use. JVFAX will run SSTV, but there are
a number of programs to find, and a search on the Internet will provide
a good starting point. Again, this is a mode that has to grab you. I
could not see much point in sending a picture of Aunt Nellie's cat to
another amateur, but if that is what floats your boat then give it a
try!
CLOVER
Clover is a rather complex system invented for the purpose of transferring
files and text on the HF bands at higher speeds than packet, with faster
throughput and versatile, self-adjusting parameters. In this respect,
it is somewhat akin to the Pactor mode, but it offers improved data communication
in the HF spectrum by using a high-speed, bandwidth efficient modem and
an error correction protocol designed to deal with changing propagation.
It derives its' name from the cloverleaf type of waveform produced on
a monitor scope. The only method of using this system is to buy a PCI
card for the PC from the USA. It has on-board digital signal processor,
control processor and protocol processing. One real advantage of using
Clover is the bandwidth it occupies, namely 500hz, as against a nominal
2000hz for Packet. It is expensive however, a board from the USA will
cost around $1000, which, when imported into the UK is usually pound for
dollar.
FREQUENCIES
These
frequencies are all amateur band frequencies, and normally, activity can
be found, depending on propagation.
CW 1.810 - 1.838
MHz.
3.500 - 3.580 MHz.
7.000 - 7.035 MHz
10.100 - 10.140 MHz.
14.000 - 14.070 MHz.
18.068 - 18.100 MHz.
21.000 - 21.080 MHz.
24.890 - 24.920 MHz
28.000 - 28.050 MHz.
DIGIMODES 1.838
- 1.842 MHz. ( RTTY is the preferred mode on this band, no
AX.25 operation is allowed )
3.580 - 3.620 MHz ( AX.25 confined to 3.590 - 3.600 MHz )
7.035 - 7.045 MHz ( excluding AX.25 )
10.140 - 10.150 MHz (excluding AX.25 )
14.070 - 14.099 MHz
14.101 - 14.112 MHz
14.225 - 14.235 MHz SSTV / FAX
18.100 - 18.109 MHz
21.080 - 21.120 MHz
24.920 - 24.929 MHz
28.050 - 28.150 MHz
Within these segments,
it is left to the amateur to sort out where to put the various data modes
themselves. Unfortunately, due to the popularity of Data modes now, it
is becoming very crowded and sometimes it is difficult to find any clear
frequencies at all, and a certain amount of "spread" does take
place, sometimes outside of the recommended range of frequencies. However,
it really is in the interest of everybody to try to maintain some discipline
regarding band-plans.
I mentioned earlier that CB stations are active on Packet. They obviously
should not be, but having said that, some very interesting servers have
emanated from that section of radio enthusiasts. I run one on my BBS,
along with a number of others, the Teletext server on FBB BBS, a well-used
server. If you are inclined that way, why not take up amateur radio and
use the legal system; we could always use extra talent.
Having also mentioned band-plans, it would be very helpful if the CB operators
would also respect their band-plans. Listening on the amateur 28MHz band,
it is becoming increasingly used by CB operators. This is mostly from
Italy, Spain and other countries. If you are using CB, please ask those
operators to desist from using amateur frequencies!
Running data does
require a stable receiver. It is also helpful to have a selection of filters
available, including DSP. The more modern transceivers should be eminently
suitable, but some of the older ones may not perform as well. Obviously
antennas also play a large part in your ability to use the HF bands. The
higher the better is applicable today just as it always was. Enjoy whatever
mode you try, and I hope you work lots of DX.
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