The Band Pass Filter schematic you find here comes from an old Dutch magazine called Free Radio Magazine and is designed by VEROBB-DX. The filter is build symmetrical and has both IN- and OUPUT impedance of 50 Ohm. So it can be used after each buffer-, driver- or amplifier-stage.The dimensions of the coils' diameter is the INNER-diameter of the coils!!! Make sure that the coil's end-wire is in a axial line with the coil and is as short as possible. You should use CuAg (Cupper Argentum) wire for the coils, with a diameter of 1,3 mm. The connection between L1 and L2 (same for L4 and L3) is at 1,5 turns of the grounded-side of L2 (same for L3). This inner-connection determines the overall bandwidth so we advise you to experiment a little.
At a frequency of 105 MHz the attenuation of the filter is less than 1[dB] at a bandwidth of 10 MHz (thus from 100-110 MHz). When going beyond these frequencies the attenuation slides very fast to 30[dB]. The filter is tested with FM transmitter powers up to 35 Watt. If you would like to use this filter with more RF-power, you should use thicker wire. AND you should adjust the length of the coils, because the inductance changes when using thicker wire. (Once again: for best performance experiment, too bad not everyone has the right test/measure equipment (=RF spectrum analyser)). For C2 you should also use a good turnable capacitor from about 100[pF] (Mica Type MT-31 or equal).
The filter should be mounted in a metalbox with shielding plates between every coil (see figure below). C1 and C3 are pass-through-capacitors with minimum capacity (some picoFarads... the less the better). In the figure BNC-connectors are suggested, but of course you can use SO239 (=PL259), or even better: N-connectors, too.Capacitors:
C1,C3 see text
C2 100pF variable ceramix trimmer (see text)
Coils:
L1,L4 diameter 13[cm], length 16[cm], 4 turns
L2,L3 diameter 6,3[cm], length 13[cm], 4 turns. Visit: Pulsar FM
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