Does your club participate in contests on multiple Bands? If it does you are very likely aware of the interference that can be experienced from nearby radios operating on adjacent bands. Filters can be purchased from numerous commercial suppliers; however a set of filters for each band will set you back many hundreds of dollars. The Darwin Amateur Radio Club has been participating in contests for many years and has in the past purchased a set of commercial filters which work very well.
At our Club station the DARC operates two beacons one on 10 meters and another on 6 Meters we also broadcast simultaneous transmissions for the Sunday Morning WIA news on 3.555 MHz, 7.193 MHz, 10.137 MHz and 14.337 Mhz. As you can imagine there have been issues with interference with this number of closely located transmitters producing all manner of intermodulation products.
We decided to try our contest filters 80, 40, 20 and 10 meters and as expected most of the interference problems vanished, as 30 meters is not a contest band we had no filter for this band.
We contemplated the purchase of another set of commercial filters but first decided to investigate the manufacture of some home brew filters. The lids came off each of the commercial filters and to our surprise they seemed quite simple to replicate, the filters were simple two pole parallel tuned circuits coupled with a single capacitor. We also made measurement of each of the filters using a RIGOL spectrum analyzer with tracking generator. On average the insertion loss was measured to be about 0.5dB with the first adjacent band attenuated by approximately 25 dB with maximum rejection of other bands at least 30 dB down, Return loss was typically about 17dB a little above 1.3 : 1 SWR. We were sure we would be able to replicate the commercial filters but how about building filters for other bands like 30 meters. I did a search of the internet and was unable to find a filter calculator for this particular design. I then did some calculations on the component values used and again to my surprise the capacitor used in each of the parallel tuned circuits had a reactance at the operating frequency of about 11 ohms and each of the coupling capacitors had a reactance at each operating frequency of about 50 ohms. With this in mind the calculations to obtain the appropriate capacitor for any frequency filter: Reactance Xc =1/2πFC rearranging the formula C =1/2πFXc where Xc is the capacitive reactance in ohms.
All coils are approximately 1/2 inch diameter wound with 18 SWG enameled wire the coil turn spacing is adjusted to resonate each of the tuned circuits. Obviously adjustment is easier if you have access to a specan with tracking generator, however reasonable adjustment ca n be made using an SWR meter and adjusting the coils for best SWR. Insertion loss is more easily minimized when using a specan.

All capacitors used are high quality 630 volt silvered mica.

The circuit  CAD Version courtesy Terry VK5TM

VK8BN Filters