A Transmitting Low-Pass Filter for 166.5 KC

by W5JGV

July 21, 2002

Since the transmitter at WC2XSR / 13 is actually a form of a switching power supply with a tuned output tank circuit, the harmonic content of the signal is fairly high. Even though the antenna system used here is a very narrow band system, it cannot be depended upon to suppress harmonics and spurious emissions. A low-pass filter is required to meet FCC regulations. Since various harmonics of the 166.5 KC carrier frequency will fall into the AM broadcast band, harmonic suppression is a real necessity.

A standard 5-pole filter low bandpass ripple was designed and constructed from available components. The filter is symmetrical, in that either end may be used as the input or output connection. The filter is comprised of two air-core inductors and three fixed mica capacitors, two of which are the same value. The filter is designed to cut off at about 275 KC, and is usable up to about 200 KC. Losses are low, and the filter as shown can handle 600 watts continuous RF power. Losses would be very slightly less if a larger cabinet had been used so as to allow for greater spacing between the inductors and the cabinet walls.

The schematic of the low-pass filter may be downloaded here as a one-page PDF file.

This is a view of the completed filter.

It is assembled in a surplus mini-tower computer case. Aluminum panels are placed over the front and back openings of the case frame for shielding. All connections are placed at one end of the cabinet. Although an all Aluminum cabinet may be used, a steel cabinet is preferred for this application due to its greater shielding ability at LF.

A view of the inside of the Low-Pass filter assembly.

The two inductors are mounted at right angles to each other so as to minimize the mutual coupling between them. The coils are mounted so that there is at least 1-1/2 inches of clearance between the coils and the walls of the cabinet. A little saw & chisel work removed surplus metalwork inside the cabinet to allow proper mounting of the components. The capacitors are genuine WW2 surplus transmitting mica capacitors rated at 2500 volts and 8 amperes. Interior wiring is point-to-point using #12 THHN copper wire.

The input and output coax connectors are mounted on the left panel, along with an earth connection bolt. The left hand capacitor is C1, the two center capacitors are wired in parallel to form C2, and the right hand capacitor is C3. Inductor L1 is on the left mounted vertically, and inductor L2 is mounted horizontally on the right side of the case. The inductors are supported with Plexiglas plastic supports. All coil mounting hardware and the coil taps are done with 1/4 inch steel nuts and bolts.

Here you can see the Aluminum panel covering the openings in the computer case. The input and output connections and the earth bolt are also shown.

A closer picture of the coils and their mounting points. The coil form is thin-wall PVC drain pipe. See the schematic diagram for full construction details.

Looking end-on to one of the inductors. The mounting bolts are shown at the top and bottom of the picture, and the coil taps are shown to the left of the picture. Note the use of backing washers on the connection points. These are used since the pipe wall is so thin.

A close up picture of a coil connection.

Plexiglas strips are bent into shape and glued together, then drilled and bolted to the coil forms and the case. Simple plastic "L" brackets could be used instead, but I already had formed plastic sections at hand, so I used them instead.

Here's the results of all the construction, a nice clean sine wave at the carrier frequency. If you compare this waveform to the transmitter output waveform, you'll see the improvement insignal quality.

The schematic of the low-pass filter may be downloaded here as a one-page PDF file.

73, Ralph W5JGV

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