In this AM Broadcast Transmitter Micro Power circuit, a 74HC14 hex Schmitt activate inverter is acclimated as a aboveboard beachcomber oscillator to drive a baby arresting transistor in a chic C amplifier configuration. The oscillator abundance can be either anchored by a clear or fabricated adjustable (VFO) with a capacitor/resistor combination. A 100pF capacitor is acclimated in abode of the clear for VFO operation. Amplitude accentuation is able with a additional transistor that controls the DC voltage to the achievement stage. The modulator date is biased so that bisected the accumulation voltage or 6 volts is activated to the achievement date with no modulation. The achievement date is acquainted and akin to the antenna with a accepted capricious 30-365 pF capacitor.
About 20 milliamps of accepted will breeze in the antenna advance (at frequencies abreast the top of the band) back the achievement date is optimally acquainted to the oscillator frequency. A baby 'grain of wheat' lamp is acclimated to announce antenna accepted and optimum settings. The 140 uH inductor was fabricated application a 2 inch breadth of 7/8 inch (OD) PVC aqueduct anguish with 120 turns of #28 chestnut wire. Best achievement is acquired abreast the aerial end of the advertisement bandage (1.6 MHz) back the antenna breadth is alone a actual baby atom of a wavelength. Input ability to the amplifier is beneath than 100 milliwatts and antenna breadth is 3 meters or beneath which complies with FCC rules. Achievement ability is about in the 40 microwatt ambit and the arresting can be heard about 80 feet. Radiated ability achievement can be approximated by alive out the antenna radiation attrition and adding by the antenna accepted squared. The radiation attrition for a dipole antenna beneath than 1/4 amicableness is
R = 80*[(pi)^2]*[(Length/wavelength)^2]*(a agency depending on the anatomy of the accepted distribution) The agency depending on the accepted administration turns out to be [(average accepted forth the rod)/(feed current)]^2 for abbreviate rods, which is 1/4 for a linearly-tapered accepted administration falling to aught at the ends. Even if the rods are capped with plates, this agency cannot be beyond than 1. Substituting ethics for a 9.8 bottom dipole at a abundance of 1.6 MHz we get R= 790*.000354*.25 = .07 Ohms. And the attrition will be alone bisected as abundant for a monopole or 0.035 Ohms. Radiated ability at 20 milliamps works out to about I^2 * R = 14 microwatts.
