[HCDX] MFJ-306 SW Converter

[David Hodgson <daveahodgson@xxxxxxxxx>]

MFJ SW CONVERTER

  Here are some impressions of the car SW converter
model MFJ-306.  
  Why buy a SW converter to use with the vehicle?s
existing RX? For me, there were several reasons. I did
not want to invest $200+ in a car stereo with SW
bands, and did not want to lug around a portable radio
which would not provide enough volume with the windows
rolled down. I also did not want a set up which would
be a distraction to driving. And finally, I didn?t
want to leave a radio lying on the seat just waiting
to be stolen.  All these problems are surmounted with
the use of a SW converter, which I placed under the
driver's seat out of view.
  There are however, several disadvantages to this
piece of equipment. You don?t get a SW frequency
readout. I don?t care about this however, because I
find looking at a radio while driving a distraction. I
do not want to take my eyes off the road to make
adjustments. I am able to tune the converter just
using the car radio?s scan button. Another
disadvantage of the MFJ-306 is the requirement of a
clarifier on the front panel of the unit in order tune
 stations between the car RX?s 10 kHz MW digital
tuning steps. 
  The biggest problem with this product however, is
it?s susceptible to extreme overload from nearby FM
stations, particularly in the FM BC band and VHF LOW
TV channels. This problem is made worse because the
vehicle?s antenna is cut to be specifically resonant
at 1/4 wave-length around 100 Mhz. Since I live in an
area in with a number of very powerful nearby FM
transmitters, this was a major problem. Even the most
powerful SW signals would be severely degraded due to
the heavy FM QRM. When I drove out in the countryside,
reception was fine, but I spend 99% driving time in
the city, so that meant the converter was unusable, in
fact for awhile, I forgot I even had it in the car.
The way I fixed this problem was by putting a low pass
filter between the vehicle?s antenna and the
converter. Again, I did not want to spend money, so I
built one for free out of some scrap material I had
lying around.  I made an inductor by winding a length
of copper wire onto about a 6 inch section of plastic
PVC pipe, and made sure to maintain the continuity of
the coax shielding by jumping a wire between the input
and output feedline shielding. Then I placed it inline
(actually in the glove box) between the antenna and
the converter. The next step was to readjust the
trimmer capacitors in the converter. This required
making very fine adjustments with the converter on. 
The final complaint is that the MFJ-306 is overpriced.
With shipping it runs close to $90. I think somewhere
in the order of $40 dollars would be more in line with
its performance, especially considering its problem
with local FM image rejection 
  The homemade low-pass filter is very effective, and
now the unit works as advertised, even in the presence
of very strong FM signals. To my surprise, there is no
problem with QRN from the vehicle?s alternator or
ignition system, despite the inductor being
unshielded. Presumably, the body of the car is
effectively shielding electrical noise from the engine
compartment. The only source of noise is the turn
signal relay (located inside the vehicle), and it is
very minor. The audio is excellent due to the use of
the vehicle?s good quality RX/audio amplifier in
conjunction with a 4- speaker system.  One is not able
to do any Dxing with this setup, but it does provide
simple, very enjoyable, good quality SW listening
while driving. 
  So in conclusion, I do not strongly recommend the
MFJ-306 considering the price MFJ is asking,
especially to anyone who lives in an urban area and
who does not want to make the necessary modifications.
With the use of a low-pass filter and proper
adjustments to the trimmer capacitors, the unit does
provide decent performance, and hearing SW stations
with nice audio quality, is a HUGE improvement over
the garbage these days on the local AM and FM bands.
Who knows, maybe you can find a used MFJ-306 somewhere
for under $20-$30.In that case it is well worth the
effort of installation and modifications.

David Hodgson
Nashville,Tn. USA


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