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MFJ-1278B MULTI-MODE ADVANCED OPERATION
If your radio has no provision for IF shift, you will have to determine the center frequency of
the audio which is passed through the filter and realign the modem to the center frequency of
the filtered audio. If you are lucky, this may turn out not to be necessary. The tuning
indicator and DCD LED can be used to give you an indication when the modem is aligned
with the filter similar to the indications described above. If the tuning indicator hovers
around the center of the display when the modem is listening to noise being passed through
the narrow filter as described above, realignment of the modem is unnecessary.
If you have determined that it is necessary to realign the modem center frequency to the radio
filter center frequency. You should refer to Appendix D at the end of this instruction manual
for the alignment procedure.
Operating HF Packet
Much has been said and written about the relative merits of HF versus VHF packet operation.
HF packet operation is made to appear more difficult than VHF NBFM packet operation by
several factors. Some of these are:
1. HF propagation is much more time variable and is more prone to produce intersymbol
errors than is VHF propagation.
2. On the "published HF packet frequencies" many more stations are trying to use a single
channel simultaneously than is the usual case on VHF.
3. Due to the wide area propagation characteristic of HF radio waves (sometimes
nonreciprocal) with relatively dead "skipped" zones, the carrier sense multiple access
(CSMA) feature of packet radio is a less than perfect arbitrator for time sharing the
channel between users.
4. Many of the commercially available Terminal Node Controllers (TNC) for use on HF
packet have data carrier detect (DCD) circuits which are of limited use or no use at all
on a HF packet channel. This further degrades the effectiveness of CSMA.
The DCD circuit in the MFJ-1278B has been optimized for the HF packet mode. It can be
adjusted to ignore background noise while still being able to promptly respond to a valid data
carrier. It has a "hang time" feature that prevents DCD dropouts when short multipath hits
occur or collisions put phase discontinuities in the received data carrier. Thus, most
multipath conditions will not cause the MFJ-1278B to begin transmitting before the other
station is in a listening mode. The DCD hang time also prevents the MFJ-1278B from
"piling on" a collision between 2 other stations on the channel. The DCD circuit in the MFJ-
1278B is NOT affected by the fact that there is a large amplitude difference between different
signals on the same channel. So it will not allow you to collide with a relatively weak station
which is transmitting immediately after a relatively strong station has finished.