Building a 6m band pass filter
Written by Administrator
Googling the net, I found a very interesting document written by YU1LM about building a 50MHz band pass. He describes three filters a wide, an average and a narrow one. All of them 9th order filters.
I decided to build the average one and made a SPICE simulation on it. Here is the schematic:
and here is the SPICE result:
As I found some used Huber&Suhner filter modules on ebay some time ago, I thought it would be worth a try to fit the filter
into one of these little boxes, already equiped with N connectors. But first thing to do was to make the air coils. The
easiest way to do that is on the lathe. First select a spiral drill with the right diameter for the core and then mount the drill
together with one end of the wire on the lathe. Now turn the lathe by hand. With the other hand hold the wire tense.
This way you can make very eyen coils.
All the capacitors I used are SMD 1206 or SMD MELF 0207. Now I was ready to solder the coils and capacitors into the little case:
Finally I was ready to test the filter. First thing I realized was that there is a very strong coupling between the air coils and
the case. Causing a severe frequency drift and insertion loss when the top cover is open versus it's closed. Here is a
picture showing the frequency response with and without the cover. The right line (higher center frequency) is taken
with the cover on.
To reduce the center frequency I made new coils with5 instead of 4 turns each. With this I almost hit the spot. The next picture
shows the change in frequency response by increasing from 4 (right) to 5 (left line) turns:
All I had to do now was a little squeezing on the coils to fine adjust the center frequency. The next picture shows the frequency
response before and after squeezing. The center frequency of the spectrum analyzer is 51.3MHz. The reading of 49MHz is 2 Megs
off. My Tek 492 will turn 30 next year :-)
So the lesson learned is: The concept is good. The circuit can be tuned easily, but I have to rebuild it in a much larger case.
The insertion loss is larger than 12dB and results only from coupling between the air coils and the case. This can be seen in
the first spectrum picture. Without the cover the insertion loss is almost 10db less. So now I know: The Huber & Suhner cases
are pretty, but useless for short wave applications. Another issue is the coupling between the coils. It is highly recommended
to mount the air coils as far as possible from each other and mount them perpendicular to each other. This I knew before,
but It was a foggy Sunday and I simply wanted to know how worse things can get.
Tomorrow I will find a larger box and start from scratch.
Here is version 2 of the band pass. I mounted each filter stage on a small PCB:
and all three of them in a small box:
Here is a frequency response of version 2 of the filter f0 is set to 51,4 MHz (ignore the 49MHz reading on the CRT)
Attenuation at f0 is 2,8dB. 3db points are at 49,9MHz and at 53,5MHz, so we have a bandwidth of 3,6MHz.
Mission completed. We can install the filter in the city repeater.