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QRP transmitter PA output and lowpass filter? Nov 23rd 2014, 16:30 4 4,845 on 23/2/15

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QRP transmitter PA output and lowpass filter? KA8SYX on 23/11/14
Hi Everyone -
How can I determine whether a certain PA transistor used in a QRP CW transmitter in combination with a particular lowpass filter design will need an output transformer to match the output impedance to a standard 50 ohm impedance? I have a transmitter I built from a kit a while back that ended with a simple pi-type (two caps/single inductor) filter and based on some reading I did was leery about it meeting spectral purity requirements. Furthermore, more reading I did on the transmitter design suggested that the lowpass filter was not well-suited to the PA in its stock configuration, with the parts supplied. So I read a piece on RF amplifiers by NA5N and used his equations to figure out a new pi-type filter. The stock filter made the PA transistor run very, very hot, which to me meant a poor match. Output was very low, too. The filter I devised, still a pi-type so I could use the existing PC board, used a hand-wound toroidal inductor and silver mica capacitors instead of the ceramic caps and solenoidal coil provided in the kit. The new filter, based on new observations, permitted cooler PA operation, higher output and it sounded better in a nearby receiver. However, it is still a three-element lowpass filter. But my attempts to build a filter with more poles keep falling short even when I use published designs. Where I am looking for an appropriate cut-off above 10.5 MHz (30M transmitter), I cannot get the impedance of the filters I build to depart from transparent (antenna analyzer right to filter input to 50 ohm dummy load attached right to the filter output) above about 9.9 MHz. Anyway, I'd like to be able to use this transmitter in regular operation, as the improved 3-pole pi output seemed to work well. Any advice? Thanks very much! 73, Ray KA8SYX

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