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Antenna tuning aid.
pa0nhc 20150304

A selective field strength indicator, tunable to 80m an 40m. Useable for tuning to max output of a loop antenna or fox hunt (ARDF) transmitter.
Good sensitivity, large sensitivity range. Made with easily obtainable and cheap components, and a minimum number SMD.
Power 9V / 2mA.

CD is half a double varicap. D1 is a RF detection Scottky diode pair, with very low threshold voltage. The value of R4 also determents the good max. sensitivity, as T1 just opens when P2 is at "Max". Stopper R3 prevents parasitic oscillations.

The internal gate diodes inT1, and the fairly large source resistances (R5+P2) guards T1 against overload. D2 protects the meter.

Single sided PCB (red boundary). Right yellow = meter and battery.

Mount P1 and P2 from the PCB bottom side up. All other components are on the (copper less) top. Except SMD components C2, CD, D1 and T1. They must be soldered AS LAST on the bottom copper side (static sensitive).
C2 is adjusted for mid-tuning-scale at 3.65MHz.

Cabinet: ABS box Hammond 1591C.
Use the top silk as drilling template.
Place the antenna BNC bus on the left side, under "ANT".

Glue the meter on the top of the lid.
Mount the PCB under the lid, using the switch nuts.
The 9V battery fits under the meter, fixated with foam plastic.

Print PDF drawings as 100% or "Original size".

MASK
600DPI

Drilling
template
Bottomsilk XRAY Materials