MFJ-8621 Data Radio
5
Antenna:
For best packet performance, use a high-quality antenna adjusted for minimum VSWR
at your operating frequency. Omni-directional types--like the MFJ-1740 1/4-wave
ground plane, MFJ-1750 5/8-wave ground plane, or MFJ-1764 dual 5/8-wave Super
gain
-should provide excellent area coverage. If you need added punch to gain reliable
access into a busy regional node, install a directional antenna--like the MFJ-1763 3-
element 2-Meter beam. When installing the antenna, be sure to feed it with high-
quality low-loss coax and keep the cable run as short as possible. In many cases, a
lower antenna fed with good cable will outperform a higher antenna fed with lossy
cable. To cut line losses to a minimum, avoid adding unnecessary in-line connectors,
power meters, and coax switches.
TNC Connection:
The MFJ-8621 plugs directly into any MFJ TNC starting with the MFJ-1270B. Any
other DCD equipped TNC, such as the PK-96 and many others, will also work with this
data radio. Use a 5-pin DIN-to-5-pin DIN cable, MFJ-5100, for connection to MFJ
TNCs or the PK-96.
Caution:
Not All TNCs have DCD (Data Carrier Detect) circuitry, and your MFJ-8621 --
which has an unsquelched output---will not work with those TNCs.
Pin Function
1
AFSK IN (150 mV pp / 800 mV pp)
2 GND
3
PTT (ground or logic-low for TX activation)
4
AFSK OUT (2.2 V pp into 600 Ohms)
5 N.C.
RFI:
After completing initial adjustments, always reinstall the covers on your MFJ-8621 and
TNC. Without shielding, the receiver may be swamped with RF interference generated
by microprocessors in the computer and TNC. For added RFI protection, many
veteran packet operators recommend installing ferrite RFI chokes on interconnect and
power cables--and keeping the radio as far as possible from the TNC and computer.
The MFJ-701 RFI kit provides 4 snap-on chokes for this purpose, along with
instructions. Snap-on RFI chokes are also available at Radio Shack (273-104 and 273-
105).
