36
Notes for Radio Hams and Experimenters
This receiver has a single intended purpose: to help newcomers, young and old alike, to
enjoy tuning the sheer magic of shortwave radio. You'll quickly gain the skill of finessing
the regeneration control to choose AM shortwave broadcasts, SSB or CW/RTTY. The
MFJ-8100 just might be the very best regenerative receiver ever designed, but it is not
intended to be all things to all people.
The MFJ-8100's PC board is a very roomy "platform'' which may seem to invite countless
modifications. However, please remember the original purpose of the receiver before
"hacking''. It's a beginner's first shortwave receiver, designed for the most popular SWL
broadcasts plus a SAMPLING of our ham bands. If you expect to pass it on as a gift or
resell it, it would be prudent to accomplish any desired modifications on the outside of
the receiver itself. For example, the function of the RF gain control can be duplicated
with a pot at the antenna connector with no drilling required. Similarly, alternative DC
power, additional audio amplification or audio filtering can be provided externally. If you
change any values in the L-C tuning in order to try a band of special interest, be sure to
note such changes in this manual. Remember that changing C3 or any inductance affects
the tuning range of ALL the bands.
[Author's Hint: To save you time as well as wear and tear on the PC board, here's what
my own curiosity showed: reducing C3 can easily bring in 12 and 10 Meters on Band E.
However, attempts to increase this capacitance beyond 220 pF without ALSO increasing
total inductance are pointless if you're looking for good 160 Meter performance.]
While it's not intended as a communications receiver, the very fact that the MFJ-8100
covers all or part of so many different popular CW bands might intrigue some QRP
enthusiasts interested in multi-band portable trans-receiving. The author conducted a few
simple tests using an HW-9 for transmitting, side by side with the MFJ-8100 on 30, 20,
17 and 15 meters. You will want T-R switching to short the receiver antenna input to
ground and also to mute the audio. Plan on a separate keying sidetone. Frequency
spotting must be done with a very low RF level. Receiver stability when switching the
antenna input is amazingly good even at 21 MHz. While the MFJ-8100 is not represented
as a communications receiver, you indeed can have some multi-band QRP fun with it.
And FUN is exactly what this great new receiver is all about!
Look at it this way. If you take your multi-band QRP mini-transmitter and MFJ-8100 on
a trip and happen not to work DX or anybody else, you can still dial up BBC, VOA,
Moscow, and many other places of our ever-shrinking global community!