(An article originally written for the Berkshire County Radio Association News, Aug 1993.)
This is a derivation from the Handi-Finder project from the May 1993 issue of QST. I didn't happen to have the IC they called for, but did have a 555. I also preferred to reverse bias the off diode instead of letting it just float. In the circuit shown almost none of the values are critical, the only thing that is required in fact is the 555 IC timer. Construction is not real critical either, but I do recommend putting the whole thing in a metal box and keeping the leads from the antennas to the diodes short, preferably using coax. The antennas should be spaced 1/4 to 1/2 wavelength apart for decent operation. Be sure to use an FM receiver, this will not work with an AM or SSB receiver... Although it can DF an AM signal.
This circuit operates by rapidly switching from one antenna to the other. This creates a phase shift that FM receivers can turn into a tone. When both antennas are the same distance from the transmitter there is no phase shift so the tone disappears.
WARNING! Do not transmit through this circuit. It might be able to handle low power on an HT, but more than a watt or so may blow the diodes. Also, the SWR that the radio sees may be radically different than 50 Ohms! so your radio may not like it either.
R1,R2 1.8 KOhm
C1 .1 uF (pick for nice tone)
R3 - 7 3.32 KOhms
C2 .05 uF (bigger may be better)
C3,C4,C5 .01 uF
D1,D2 SK9150A/555 PIN Diode (any good switching diode should be OK, maybe 1N914)
V+ 6-12VDC (<10 mAmp load if my meter is working right) >
I used ceramics for C2-C5, and mica for C1, but just because they were handy. The resistors are all 1/8 Watt carbon, but only because they were handy also. The only restrictions I would put on playing around is to keep D1 and D2, R5 and R7, and C3 and C5, identical. This doesn't mean use what I did! just keep them the same so that the paths from each of the antennas to the junction of D1 and D2 are identical. Actually, C3 and C5 are optional.
Added note on easy construction techniques:
> > Not sure if I finished this thought last night -- but on the unit that
> > I built, I used a 36" wooden yardstick for the boom and used two of the
> > cheap radio shack twinlead/rabbit ear TV antennas for the elements. The
> > plastic housing can easily be used to mount the antennas to the
> > and the twinlead can be removed leaving the remaining little brass
> > solder tabs to connect the coax and PIN diode to the elements. The
> > advantage of using the yardstick for the boom is that you can collapse
> > and fold your elements and easily re-lengthen them to the proper length
> > when it's time to use it. No guesswork or estimating!
> > One does have to look pretty hard for the rabbit ears at RS -- they have
> > the more expensive ones in plain view, but the cheap $2.99 ones are
> > -- the cheap ones are the ones you want.
> > --Tim