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Ihis reduccs Ihc ciYcctKc rangÄ™ of the transmitter but this might not bc a problem for short-haul transmissions where a smali transmitter form-factor is the goal. A 1/4 \vave antenna was found to excessively load the VCO to point of circuit dcstablization but may be adrpted by adding another stage of buftering allcr the VCO section and driving the antenna this way.

Referring back to figuro 1. tlie signal produced by sound pressure againsl the electret eon den ser microphone element is AC-eoupled through Cl into Q2; Q1 is configured as a class A ainplifier v\ itli a signal gain of about 4 to 1. The amplified signal is coupled into the base of Q2 which ehanges the base-to-emitter junetion eapacitance in the transistor much like that of a \araetor diodÄ™ used in morÄ™ sophistieated electronie tuning cireuits. tire end result being that the base frequencv of the oscillator Ls ehanged proportionately to the incoming signafs amplitudÄ™ and frequency. Ilris efiect is sufficient to eause fiequency modulation to tako place thoreby imbedding the microphone's signal within the RF carrier which is radiated into space via the antenna.

Pow er is supplied by either a standard 9-\’olt alkaline transistor battery or for smali space eonsiderations. a miniaturÄ™ 12-volt alkaline electronie lighter battery. The latter is aetually a unit of eight wateh-eelÅ‚s staeked upon one another and plaeed into a tiny roli of sheet metal which approxinrates tlie size of one "N" celi. Current drain is about 20 milliamps and battery life Ls very good when dre 9-volt transistor battery is used. The electronie lighter battery is good for short transmission cycles and should only be considered w hen space is of tire utmost and aecess to the transmitter is available to replace the spent battery.

CO NSTRUCTION

Despite its simplieity. one must bear in mind that this is a VHF cireuit and as sueh its stability (or lack tlrereof) earr be attributod to how it is constructed. Hacing assentbled many prototypes the author can attest to the benefit of tire so-called "ugly eonstruelion" technic|ue. Tire latter is a temr used by VHF and UHF radio enthusiasts and HAM radio operators which is eharacterized by point-to-poinl winne upon a copper substrale or "ground piane". Its name is atlributed to tire "rats nest" look to tire furished product Despite its ugliness. short leads and the copper substrate ensure tlrat no ground loops or otlrer forms of circuit loss can contribute to the transm itter's instability.

The author has assembled a prototype on a four-by-tlrree centimeter piece of double-sidcd copper-clad epoxy fiberglass circuit board drroueh which a hole had been drilled in each comer to facilitate mounting into some form of cnclosure. Each componcnt is soldered certieally, point-to-poinf with leads as short as possible; ground connections are madÄ™ directly to tire copper ground-plane using a soldering gun or a soldering iron of at least 30 watts. Standofls. onto which components are suspended can be either unils specifically designed for this purpose. or as the autlror has used high \ alue (22 megohm). quarter-watt resistors. This provides some physical stability to the ciicuil and lends a mctlrod by which to create a VCC raiÅ‚ which can feed the circuit The end result is that tire circuit

Km Wlreiess Transmitter - 2