This video shows how to build a radio that transmits a simple 1 kHz tone on the AM band, which can be heard in a handheld radio or car radio up to 100 yards away. The radio uses an LC oscillator to drive a push-pull amplifier made from a 2N3906 and 2N3904 transistor. The audio tone is generated by a 555 timer. The radio can be powered by up to 40V, but in this video I'm only using 12V.
I experimented with transmitting on 1030 kHz then increased my frequency to 1620 kHz by changing the capacitors in the LC oscillator that generates the carrier wave. Because the frequency is so low in both cases, the antenna is nowhere near the wavelength, or even quarter wavelength. For antennas shorter than L/4, two major problems occur:
-The radiation resistance is extremely small (i.e. only a very small amount of the current in the antenna actually travels out as radio waves)
-The antenna acts as a capacitor and has huge impedance from capacitave reactance. In the video, i attempt to balance out the capacitave reactance with an adjustable inductor (or "loading coil"), with fairly good results.
The total power radiated was well below 1 mW in both cases. This is probably less power than something like a garage door opener transmits.
Note that radio transmissions in the US/Canada/Europe are regulated, and this device is only for demonstration. If you want to build a larger power transmitter, make sure you've got an amateur license and use one of the frequency bands allocated to amateurs.
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