Epson Multi-mode Data Controller MFJ-1278B Benutzerhandbuch

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.