2˜

3

is suspended above the ground piane and each componenfs leads are very short (sec FigurÄ™ 2). Tliis also allows componcnts of various sizes to be accommodated and provides the means to easily replace componcnts which is an a id to e.\perimentation.

Il carmot be stresseci morÄ™ that short leads must be used wherever possible. This. and tlić coppcr substrate will ensurc a very stable circuit that will provide drift-free operation over long periods of time. Never use wirewrap or other types of "neal constntction^,'. These types of construetion are very pleasing to the eye but ean produce copious ainounts of stray capacitance and inductance that form many "aceidental" tuned circuits which will afÅ‚cct the stability of the circuit.

Mount tlić eariablc eapaeitor such tliat the adjustment slot is facing away finom tlie ground piane so it can be tweaked from above through a hole drilled in the enclosure you seleet This allows finaÅ‚ frequeney twcaks to be madc alter tlie unit has been assembled and mounted into the enclosure of your choice. The same is true for tlie microphone element which shouki be the highest circuit componenL thereby allowing it to slightly proÅ‚rude through the enclosure top.

When selecting an enclosure. seleet one madÄ™ of plastie or other non-eonduetive materiaÅ‚ or you will find that the tuned circuit will be affected by hand capacitance. An exception to this would be a larg er metal enclosure in which the surface is fairÅ‚y

w

distant from tlie tank circuit. The author has tried to house the unit is smali aluminum "minibox" enclosures without success and for this reason recommends a plastie enclosure. Anothcr benefit of tliis materiaÅ‚ is the ease with which it can be machined: an xacto-knife can be used as a drill bit-a few twirls aets tlie hole started and tlie

V

tapered naturÄ™ of tlie blade allow s the hole to be enlarged as you go. The same knile can also be used to de-burr the holes atter the correct size has been achieved.

The circuit card should be mounted to the ease asmg nylon or other non-conduetivo machinÄ™ screws or fastened with adliesÅ„ e from a hot-glue gun. Screws are preferredL siÅ„ce this allows the card to be removed for

senieing if tliat need should ever arise.

-

FigurÄ™ 3 depicts the authofs prototype using a 9V transistor radio battery.

CHECKOUT AND ADJUSTMENT

After tlić circuit has been assembled. connect tlie battery to tlie unit and set an FM radio to a ftequency free of broadcasLs at about 88 or 90 megahertz. Solder a stiff piece of wire about 21 cm long direeth to the collector of Q2 and let the wire stand as \ ertical as possible as a ternporaiy antenna Next tune eariable eapaeitor C5 until tlie ambient room noise is lieaai throuah tlie

V

radio. Best results are achieved if the radio is monitored through hcadphones to prevent audible leedback. There might be ses eral eapaeitor positions which will transniit on the sclected frcquency but typically. one will be much clearer and louder tlian the otliers. When this eapaeitor position is found. slightly stretching or compressing the windings of LI will changc the freÅ‚juency

Km Wlrdest Transmittcr - 3