The AM Forum

On Dec. 21,'08 I defined a problem with my GK-500C and asked for feedback. All comments and suggestions were considered and used as a basis for my diagnostic attempt. A strong harmonic output (measured at 28 - 30 DB below the fundamental) was measured. Transmitting on 3885 KHZ (300 W carrier) included an approximate 3 to 5 W sig on 3770 KHZ.

I carefully measured and inspected every connection, component, ground return, switch, coil, chokes and capacitors in an attempt to find a component or connection that was causing this problem. Although I was given advice on how to modify the original WRL design, I wanted to keep the original as such. The successful resolution incorporates a series tuned 75M RF trap located after the "L" matching network (needed to match the 300 OHM output of the rig to the 50 OHM antenna). This trap consists of a 100 pf air tuned capacitor and a 24T (21 gage formvar) close wound coil on a varnished toilet paper roll (I'll let the reader think of some interesting things to call the "trap"!). To lower the "Q", I added a 150K OHM resistor in parallel with the coil.

The trap easily nulls out the 75M harmonic. Using the spectrum scope function on my 756 PRO II, I measured the nulled harmonic signal to be better than 50 DB below the fundamental signal.

I believe all GK-500C's have this condition - but, my "dual" 160/75M antenna provides a clear path for the 75M harmonic to radiate and exacerbates my problem. I want to thank all who provided ideas and encouragement and to AB2EZ and WA1HLR for their specific technical advice. WA2UJX