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MFJ-1278B MULTI-MODE ADVANCED OPERATION
5. Many of the stations on HF packet have no tuning indicator at all and many others have
an indicator that is only marginally useful. Unlike VHF FM operation, frequency error
between transmitter and receiver cause frequency errors in the modem tones. This is the
reason that a good tuning indicator is absolutely essential for HF packet operation. The
tuning indicator makes it possible for the operator to reduce the modem to modem
frequency error to near zero. Reliable HF packet operation requires tuning errors of +\-
30 Hz or less.
6. Many of the stations currently operating HF packet do not have their radio bandwidth
matched to the characteristics of a 300 baud 200 Hz shift FSK signal. This causes a
severe degradation in modem performance resulting in unnecessarily increased numbers
of retries and drastic reduction in data throughput for the SHARED channel.
Serious HF packet operation should not be considered without a 500 Hz filter in the
receiver IF. An audio filter is NOT an acceptable substitute.
"Squeezing" the edges of SSB filters together with so called variable bandwidth tuning
(VBT or PBT) to produce a 500 Hz bandpass is difficult to properly align and results in
operation near the edges of the filters where the phase response and group delay
characteristics are particularly poor. So while this offers some marginal improvement
over wide filters, it is not a good substitute for a real 500 Hz filter.
In spite of the above factors, HF packet communications can be a reliable and enjoyable
mode. The trick is to operate in a manner that allows you to avoid the problems mentioned
above. Your purchase of an MFJ-1278B has already helped you avoid the DCD and tuning
indicator difficulties. Configuring your radio bandwidth to match the spectral characteristics
of the HF packet FSK signal will allow you to avoid the modem performance degradation
that results from excessive radio bandwidth. However, you will still be affected by other
station's problems in these areas when operating on a crowded packet channel.
If you do have your radio configured for the proper bandwidth, you will notice that many
stations you connect to will seem to not be able to copy you as well as you copy them. Don't
worry, your transmitter output isn't low, you are just observing the difference between a good
demodulator behind the correct radio bandwidth and any kind of demodulator behind a radio
which has far too much bandwidth for the mode.
The actual mechanics of operating the MFJ-1278B in the HF packet mode are the same as for
the VHF packet mode. The main differences that will be apparent to you will be the slower
baud rates, the higher incidence of propagation related effects and QRM, and the requirement
for accurate tuning. It will take some practice before you can rapidly acquire another
station's signal and initiate a contact.