A IF filter was my first thought and I started to fool around with them.
First, I cut a small piece of PCB and prepared for "dead-bug" soldering style.
I prepared also two small piece of RG58U to connect the filter to a Marconi IFR analyzer.
In my box I had two type of cristals.
One type is a 4 MHz and the second one is a 38.9 MHz, both of them HC-49U format.
First I started with the 4 MHz ones but I didn't noticed a nice shape on the analyzer so I quicly moved around to 38.9 ones.
These are overtone cristals so I watched to fundamental frequency which had to be around 12.6 MHz.
And it was!
I don't want to bother you with math stuff. Instead, I will put the self-explanatory pictures.
The two SMD resistors are 470 Ohms and the small SMD capacitors are 1nF.
Here it is the filter on the tests. After tests, it can be put on a smaller and nicer board. I didn't had capacitors, I will play with them monday, HI.
There are 9 cristals. At less than 1 USD, I believe is a good alternative!
Indeed, one had to choose from a big bunch, but I had an already opened bag of cristals with around 25 pcs. I observed on the analyzer that there was two type of shape for that bunch.
The insertion loss is around -4db wich is not bad at all!
The center frequency was set at 12.96400 MHz. This might be somehow unusual for elder hams but with modern DSS it's easy to set the IF to this value.
-22db attenuation at +1.5 kHz
-27 db attenuation at - 1.5 kHz
It has a passband of around 1.8 kHz at -15 db. Maybe I will send it to some experimenter who want to play with because I am very curious about how it sound in the real HF traffic!
Other pictures:
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