HAM radio topic: EM based design of a 432 MHz high pass with 144 MHz rejection

Hello,

there are some HAM radio operators around here, so maybe this is of interest.

In our multi operator VHF/UHF contest location, we had an issue with the 144 MHz fundamental (!) signal beeing picked up by the 70 cm antenna which was driving the 432 MHz LNA into nonlinearity. That?s why we decided to add an additional high pass filter in the 432 MHz receive path, which indeed solved that problem.

Starting from an existing design by VE2ZAZ, I created a modified 70cm high pass filter that requires no wirewound coils and no tuning. Detailed design and simulation report, including measurement results and PCB files:
https://muehlhaus.com/support/ham/hpf432

Enjoy!
Volker

Thank you Volker. This is a simple to build but high performing filter that I'm sure many of us will find useful, even in non-contest situations.

Brian.
GW6BWX

Volker,
Can you upload ADS Project File here, I will try something..

This is an impressive 50dB narrow-band rejection.

To stay in the same area of smart printed filter designs, here is an interesting BPF for 2m band which gives 62dB rejection in 73cm band.
https://www.hobbypcb.com/index.php/p...nd-pass-filter
There is no information about component values, but I think is not very complicated to do a bit of reverse engineering using an EM simulator.

For anyone wanting to experiment with it on FR4 line length of 43mm, width of 2mm and spacing of 1mm with 68pF capacitors is a good start.

Peter

Sure, the entire ADS workspace is attached.Highpass_70cm_wrk.zip



Hi Peter,

what length do you mean with 43mm? If you follow the link in my post, I have given the dimensions that I used. Substrate was FR4 in may case. The ground cutout helps to reduce the length of the shunt line to 18mm, for the given target value of 18nH.

73 de
Volker DL5DAW

For who is interested, I played a bit with this tiny BPF and I found the values of the 3 capacitors, which is 82pF for each of them.
The dimensions of the microstrip lines are easy to get, because there is a ruler in the picture.
I used: line width=2mm, spacing between lines =1mm, PCB=FR4 double plated, Dk=4.4, h=1.5mm

Hi Volker

I've not been online over the weekend, hence the delay in replying.
I should have been clearer in my post that I was referring to the Hobbypcb circuit. The line length of 43mm was the length of the 3 coupled lines. Like vfone I used standard 1.5mm dielectric FR4.
I used Genesys Momentum for the simulator.
Attached, I hope is the layout and simulated response.
I did build one on a piece of board using cut and strip, I'm not particularly proud of it, the copper did not come off very well. It does however show that the design is relatively forgiving of errors, the line spacing ended up around 1.5mm and the line widths are not exactly 2mm, a bit wider if anything. The response is a bit narrow with higher loss than predicted. Also attached is a plot of the measured response of filter with and without a cover. The grounding was just 'wire worms' to the ground side of the board.
If I can hack one together anyone can, and with a little care get good results without test equipment.

Peter

Just now I see the post #5
I used lines with length of 40mm, and that is the reason I got higher values for capacitors. This has minor importance from the moment the results are pretty similar.

Hi vfone

I guessed that you had used shorter lines, that will give anyone who wants to try and build one another set of values to use.
I put soome tinned copper wire on the tracks on the one I built to shrink the gap and widen the bandwidth and reduce the loss a little.
It looks a useful little filter that will go into the 'it will come in handy one day' box.

Peter

Peter

Any news on your experiment?

I tried to place another notch just besides to first one to enlarge the rejection but there is a coupling ( due to length ) between two notches and therefore no success..

Great, thanks!

I thought you might want to replace the "printed inductor" by an SMD one. Initially I thought that will fail because of Q factor, but it seems that high Q SMD inductors are a possible alternative, if size of the circuit matters.