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 to nil.

The Filter Unit
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 Filter Unit
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 is impossible.
The Filter Unit input
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.
The Filter Unit input
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.