NOTES: If only one beacon will be used, C2 can be replaced by
a 33pF fixed cap. Otherwise, insert C2 without soldering
then connect a freq counter to C1 (using a x10 scope probe for isolation if available) and adjust C2 for 3.579545
MHz. Measure C2 then replace with fixed ceramic or mica caps.
Actual board size is 3.5" x 2.0".
A rogue trace on the bottom of the board (duplicate of one on top) shorts
to the Keyline. It runs from U3 pin 1 to a
via on the bottom of the board. It should already be cut on either side of the heavier Keyline.
The metal crystal case must be raised above the board to avoid shorting out the traces underneath.
Caps C4-C8 are all in parallel to resonate the loop. A capacitance decade
box is very helpful in tuning. With
the loop away from all metal objects, tune for minimum current. Use a real 12V battery (Gel, AGM, or
NiCad) to power the beacon. A "Lab" supply, even one rated at several amps, may cause strange effects such as
drifting DC current. Current should be roughly 0.5 amps for any of my loop designs. Most of the installed
capacitance should be polyproplyene, although the smaller "C8" trim caps can be Mylar.
Caps C4-C8 can be mounted on the loop. This will allow use of a 2-wire feedline, and will increase Q slightly.
To prevent overheating in case of accidental loop detuning in the field,
a small heatsink formed from a strip of sheet
aluminum ~3" long can be fastened under Q1 and bent in a "U" over Q1. Note that this heatsink is "hot" with +12V
and output signal.
Note that the LED will only light if all 3 connections to the loop are
intact and a real signal is being
transmitted. It is real "BITE"!