70MHz (4M) Low Pass Filter

So to go along with my original 4m Tranverter System which had a decent PA I needed a LPF (Low Pass Filter) on the output.  There are lots of designs online [1, 2, 3] (thanks), take the construction methods from those sites.

2013-10-08 10.08.55

Above is my final LPF but when I came to making the first LPF I never had the right value, or type, of capacitors to hand so I decided to make it using the PCB Capacitor method.

2012-07-02 18.06.03

The only issue with a PCB Capacitor design are the losses are higher unless they are made perfectly and with good quality PCB material, they do work  as you can see from the plot below but with 0.71dB through loss.


This plot is from before the 2 screens, seen in the above image, were in the middle of the filter. As you can see the rejection at 144MHz is –40dB which I think could be better.  So I fitted the screens and the performance at 144MHz improved by 1dB and at 432MHz by 5dB.  But the through loss increased to 0.76dB.


So for a while I used it but at the next rally (which I believe was the Dutch Autotron) I got the required 56pF ‘door knob’ capacitors.

2012-07-02 18.40.48

These type of capacitors have screw mounts on each end as seen below left. One thing to note if looking for such capacitors is they usually come from Russian surplus equipment, as such, due to the Russian alphabet, the letter P is shown as an n as is seen on this capacitor, which is actually 22pF, not 22nF.

 2013-10-06 20.14.18 2013-10-06 20.14.57


As you can see in the above that using door knob capacitors I get much better through loss numbers, 0.07dB! and still keep –40.5dB at 144MHz and an improved –71dB at 432MHz.  The LPF is usable at 50MHz also where it has a through loss of 0.35dB.

By adjusting the inductors you can achieve the best characteristics.

2013-10-08 10.08.33

Note the 56pF capacitor is marked in Russian as 56n

2013-10-08 10.09.07

Make sure you use decent N-Type connections or you will increase your through losses.

The through loss over the whole of HF is acceptable, but the filter is of little use for those frequencies as even at 29.7MHz the 2nd harmonic reduction is about –0.2dB 🙂


My new 4m Transverter, project write-up to follow, uses an off the shelve 300w 70MHz LPF which has built in forward power detector and reverse power [high VSWR] shutdown.  This is mounted in the transverter, and along with my LPF externally I now have about –100dB rejection at freq*2 (140MHz).

2013-01-19 20.14.34

Leave a Reply

%d bloggers like this: