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WB8WFK Active Attenuator with Field Strength Meter

I originally built this unit for use at the 1999 region II ARDF championships in Portland Oregon. Old CD4000 series CMOS logic (which may now be getting harder to obtain) was used because it could operate directly off of the battery with out a regulator.

Another option if you are willing to experiment would be to replace the whole osc circuit with a CMOS clock osc. The Digikey catalog has a large selection of them. If you did that you would also need a regulator to generate +5 or 3.3 depending on the clock you find.

This would make it a more modern design as you would be doing away with an old part and all of the components that make up the 1Mhz osc.

The schematic can be downloaded by clicking this link (opens PDF file)

I built my unit dead bug stile on double sided PCB. This allows good RF connections between the BNC's. This is very important for proper operation. L1 is 20 turns of number 22 AWG wire on 1/8 inch form. Inductance is ~ 3.3 uhy. Diodes D4 and D1 where HP 5082-2835 microwave diodes in my version. Other hunters have used 1N914 type diodes. Depending on the transmitter power the FSM starts working around 15 to 10 feet. I used the 3 element tape measure beam with the unit in Portland (HP diodes).