My Pi-Filter page



To provide matching for a wide range of load impedances on most of the HF amateur bands, it is normally necessary to use tuning capacitors with maximum capacitance as high as 350pf or even 500pf. For these capacitances, it is difficult to find tuning capacitors with plate spacing greater than 0.25mm and hence breakdown voltage is limited to around 1000 volts. However there are gang capacitors around which have up to 220pf maximum capacitance with spacing of 0.5mm providing 2000V breakdown and power of 400 watts.


- Making of! -
The first primary coil is wound free standing, diameter 57mm, and pulled out to a length of 84mm. I use 2mm copper earth wire. The secondary coil to the antenna is of a diameter of 67mm and a length of 24mm. I sprayed the coils with car paint.



The ends were fixed onto 8mm wooden poles of about 100mm long. After filing grooves for the taps I stuck these with a hot glue gun. On the rear side of the coil I fixed with the glue gun onto the other pole all the windings, see diagram. The secondary coil is placed over the primary coil, so that all distances are the same between primary and secondary coils. I made two crosses and fixed, with glue the end of the coils fixing poles to the crosses. The switch S1 must be made with ceramic wafers and of top quality. On the output to the transceiver an SWR meter must be used. As I have mentioned the voltage can rise to over 2000V so please use with care. I have used the Pi-filter on my 40m open Delta-Loop and can only say it was a complete success.


Back to the Ham page Back home