Low pass filter for 70 MHz
In need of a low pass filter for my 70 MHz transverter, I decided to build the filter as published on the OZ2M website. The filter published there is working without problem and needs no or very little tuning. The original design can be found here:
http://rudius.net/oz2m/70mhz/lpf.htmhttp://rudius.net/oz2m/70mhz/lpf.htm
I build this filter several times for low power applications, using normal ceramic capacitors.
Now I needed a high power version, I didn’t have the correct capacitor but obtained 2 ceramic doorknobs each 50 pF. It needed some adjustments of the inductors to get it working.
The whole filter was build in a available metal box.
So I ended up with the following values:
C1 and C2 50 pF doorknob
L1 and L3 3 turns on 20mm diameter 50 mm long
L2 3 turns on 20mm diameter and 25mm long
Results:
Feed through loss measured with my signal generator and mW meter 0,2dB
140 MHz -44 dB
210 MHz around -55dB
Layout of the filter
Measuring the filter with the tracking generator, centre frequency 125 MHz, span 250 MHz
