30 to 80 Mhz Filter Helps Solve HF/VHF Interference Problems
Over the years I have used numerous strategies to combat interference to
my city based radio astronomy efforts. I've gone up in frequency to 1.4
Ghz, 700 Mhz, 600 Mhz, etc. and down the spectrum to 144 and 38 Mhz. All
of these experiments were hampered by strong interference from power lines,
electric motors, airline reflections, light dimmers, computers, military
satellites, and a slew of other demons. Perhaps my greatest enemy has been
a huge tower of pager and other VHF transmitters which is located only
two blocks from my house. These signals were so strong that they swamped
nearly every front end I have ever built.
Signals from transmitters like the one near my home can overwhelm
typical coil and capacitor tuned circuits and place voltages on the first
rf amplifier stages that drive them into nonlinear amplication ranges and
sometimes complete saturation. Thus even though my most recent receiver
at 38 Mhz was about 120 Mhz below the pager transmitter frequency, my drift
scans were riddled with regions where the indicated signals were off the
top of the chart or where the sensitivity to the desired signal was reduced
This problem seems to be no under control after the introduction of a three
stage helical filter which was purchased from Fair
Radio Sales in Lima Ohio. This military surplus unit costs less than
$50 shipped. It consists of three independently tunable stages with a range
of from 30 to 80 Mhz. It looks as though the filter was designed to be
used with a transmitter and includes a small watt meter with 6 and 60 watt
scales which can be read in forward and reverse directions for SWR indication.
The watt meter is not used in my application but would be handy if the
unit were used for transceiver operation on the 6 meter ham band (50 Mhz).
There are two connectors leading into the unit. One is a standard BNC connector
which I have taken to be the receiver or transmitter connection. The antenna
connector is an odd fellow which I am not familiar with and which was easily
replaced with another BNC as shown in the photo below.
The added BNC connector is shown in this image at the output
of the filter. A couple of big washers were used to mount the connector
in the oversized hole which previously served to mount the original connector.
My guess is that this unit cost the military in excess of $1000. The helical
cavities are silver plated as are the tubing helical coils. The filters
are slot coupled to one another, that is, there is no wire or capacitor
connecting the stages. The tuning knobs are vernier connected to what appears
to be glass or ceramic piston capacitors. The dials seem to be accurate
to within about 1 Mhz and each knob has a lock down. I didn't measure the
passband of the filter but it must be good. I tried to measure insertion
loss but because there seems to be some impedance transformation from 50
ohms at the output (probably a reduction to some lower value), it looked
to my test set up like there was a gain instead of a loss, which of course
This photo shows the input connector to filter. There is a small
circuit board which probably serves as the coupler to the watt meter in
the camber below the BNC. You can see the slot on the wall of the silver
plated chamber which serves to couple the signal to the next helical stage.
With the side cover removed you can see the heavy duty vernier
mechanisms which tune the capacitors in each chamber. The circuit board
for the watt/swr meter is visible above the rightmost vernier.
After four days of scanning the meridian with a 4 element YAGI at 38 Mhz,
I can definitely attest that my swamping problem is greatly reduced if
not completely eliminated. Of course I still have problems with broadband
interference from intermittant sources, but at least the graph produced
now looks like it should with a definite galactic plane buldge at the appropriate
time of day. This is a recommended buy if you are doing radio astronomy
in the 30 to 80 Mhz range. If anyone has any background on these units
please email us with what ever you know about them.
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