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MFJ-1278B MULTI-MODE ADVANCED OPERATION
DETAILED HF PACKET OPERATION
Here we will go into further detail on the HF Packet mode of operation. We will discuss
modem center frequency, HF tuning indicator, etc. So without any further delay lets get into
HF Packet operation.
Radio Setup for HF Packet Operation
Setting up your Radio interface will follow exactly the same procedure as for VHF NBFM
operation with the exception that you will probably not need to set the modem output level.
This is because linear mode radios typically have a transmit audio level control on the front
panel and an indicator that tells you when you have set the level correctly. You will adjust
the audio level just as you would for SSB operation.
If you are using an auxiliary audio input connector, such as an accessory port, you MAY
need to adjust the transmit audio level from the MFJ-1278B modulator. This is because
many auxiliary audio inputs are intended for phone patch audio. These levels are typically
much higher than microphone levels. If you do adjust the output level of the MFJ-1278B to a
relatively high value for this type of application, do not operate the MFJ-1278B with a VHF
NBFM radio unless you take steps to assure that the audio level for the FM radio will not
produce excessive deviation. If both radios are to remain connected to the MFJ-1278B, you
can set the audio level for each radio independently by using the audio output level
adjustments, R157 & R158, on the left side of the MFJ-1278B with a small screw driver.
The modem center frequency used by the MFJ-1278B for all of the narrow shift AFSK
modes including HF packet operation is 2210 Hz (2125 and 2295 Hz tones). Most
frequencies listed for packet operations in the HF bands were established using a modem
center frequency of 1700 Hz (1600 and 1800 Hz tones) in lower sideband mode. This is a
520 Hz offset (2220-1700) between the two modem standards. So a listed frequency of 7093,
for example, will cause you to have a frequency display on your radio of 520 Hz PLUS the
listed frequency (IF you are also using lower sideband mode). Thus you should find the
7093 KHz packet activity centered around 7093.52 KHz.
Your tuning indicator, when properly centered, is easily capable of 10 Hz alignment accuracy
per segment. It should be used as the tuning reference on any individual signal regardless of
the frequency indicated on the radio dial.
NOTE: The signal actually transmitted by your SSB transceiver ( F(emission) ) when using
the MFJ-1278B for AFSK HF packet operation using lower sideband will cover a
band of frequencies approximately 400 Hz wide and centered at the transmitter
indicated carrier frequency (F(ind)) minus the modem center frequency (Fc).
So you would use: