A 23 cm Filter having medium Q

A small Filter tunable to anywhere within the 23 cm Amateur band is described here. I use it in front of the ATV preamplifier to get rid of 13 cm spure from the ATV transmitter and limit QRM from radar signals. Tree serial L-C circuits are used. Inductance (L) is made from brass rods, Tuning Capacitors are 4 mm brass screws which are fixed with lock nuts after tuning.

The box size is 60x60x20 mm. The brass rods are 6 mm diameter, the outer rods are 52 mm long while the middle rod is 50 mm. The rods are screwed to the box using M4x20 screws. Actually, it would be better for performace (filter loss, Q) to use a milled box but I soldered the box together using brass sheet material, it turned out to be good enough for my purpose.

Solder lugs are pressed between the box inner wall (A) and the outer brass rods. At a distance of about 3 mm the solder lug fins are bent upwards and connected to the BNC (or whatever connector type is used) inner conductor using sivered copper wire (B).

The above picture shows the M4x10/15 tuning screws and the lock nuts. Nuts to guide the screws are soldered to the outside of the box.

Snapshot of the analyser screen, horizontal span is 5 MHz/division, vertical span is 1 dB/div. The green line marks the 0 dB calibration point.The filter is tuned to 1278 MHz center frequency. At center frequency the thru loss is 0.6 dB. The 3 dB bandwidth is 32 MHz, resulting Q is 1278/32=40 which is expexted.

last changed: Jan, 29 2004 by DF2CK