In our continuing quest to evaluate the output purity of various QRP kits, Willy W1LY constructed a Bill KA1QYP Half Pint Kit.
The assembled kit produced a solid 600 mW output and was one of the cleanest radios we’ve seen. The worst case was the 3rd harmonic which was -59 dBc.
The Half Pint uses a Cauer Filter, similar to a 5-pole LPF but two of the sections are tuned to suppress the 2nd and 3rd harmonic frequencies.
The Elsie Predicted response is shown below. Note the pronounced dips at the harmonic frequencies. Loss at 7.040 MHz is about 0.9 dB, and the 3 dB knee is about 8.1 MHz:
The measured performance is shown below. The 2nd harmonic is down about 63 dBc, and the 3rd harmonic is down about 59 dBc.