MFJ MFJ-1270C, Системное руководство

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MFJ-1270C/1274C MANUAL ADVANCED OPERATION
Page 103
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 TNC 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 TNC 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.
Set the audio input to the TNC so that the DCD LED on the TNC just flickers
occasionally. Set it so that the DCD duty cycle is between 10 and 20 percent.