FM Broadcast Bandcut Filter (RX/TX)
This notch filter has been built to avoid the loss of sensibility that any handheld radio experiences when connected to a good gain antenna rather than to the series rubberduck. Usually the presence of big broadcast signals in the adjacent bands is responsible for the apparent deafness. In Italy FM broadcast stations transmit in the band 88-108 Mhz, so the easiest solution to the problem is to use a notch filter to cancel that band. The filter I built is a Chebyshev filter with N=3. The spectrum analyzer analysis shows that the attenuation is about 10dB@88MHz and about 14dB@108MHZ.
The construction of the notch filter does not have any criticity; it is very important to build the filter inside a metallic box. The capacitors must be good quality variable capacitors; the more the current they can bear, the more the power you can use on transmission. If use on transmission is not important, normal variable capacitors can be used. Coils have been built using 1mm diameter enameled wire and the standard formulas. Since it is quite difficult to build perfect coils, and soldering can somehow influence the geometry of the coil, hence changing its value sligtlhy, it is strongly suggested to tune the notch using a spectrum analyzer, or even a VHF sine generator and an oscilloscope. Using the latter method you probably won't be able to smooth the center of the attenuation curve (around 98MHz), but you will surely be able to tune the beginning and the end of the bandcut. It is very important that you tune the filter with the box already closed, because of the parasitic capacitances introduced by the box cover; the best technique is to drill a small hole in correspondence of the capacitors so that they can be tuned from outside using a plastic screwdriver.