The equation for calculating the capacitance to resonate with this inductor is: This is made by winding 30 turns of number 24 wire on an Amidon FT-50-61 ferrite core. The inductance used in the tuned circuit in the modulator is 49.7 mH. There are two tuned circuits in the transmitter for which the capacitance must be calculated based on the AM band frequency that’s chosen. The power supply contains three voltage regulators to provide the three needed voltages: 24 volts 12 volts and 5 volts. An onboard 1,000 Hz oscillator is provided for testing and a switch is provided to change the oscillator to the audio input of the transmitter. This antenna is much more efficient than a straight wire three meters long. The antenna is tuned to the broadcast frequency by a capacitor in parallel with it. The antenna is a 1/4 inch copper pipe bent into a circle one meter in diameter.
The buffer amplifier is fitted with a stick-on heatsink. This is followed by a BUF634 buffer amplifier. The modulator is a modified Gilbert cell with a load tuned to the broadcast frequency. Because crystals for AM frequencies are somewhat hard to find, a crystal of four times the desired AM band frequency (a shortwave crystal) is used.Ī dual D flip-flop is used to divide the frequency by four. A broadcast frequency is chosen that’s not used by any local or high power station. The transmitter has several unique features. A schematic diagram of the transmitter is given in Figure 3 (and included in the downloads) and a Parts List is provided.įIGURE 3. The oscillator of the transmitter uses one gate of a digital hex inverter as the amplifier. One unique application is to use two AM transmitters of different frequencies with a stereo source (such as a CD player deck) of vintage style music to broadcast to two separate antique radios providing what could be called antique stereo. The input to the transmitters is an RCA jack.Ī cable with RCA plugs on one end and a 3.5 mm stereo plug on the other end may be necessary for some sources.
#3 transistor fm transmitter with coil on circuit board Bluetooth
The AM transmitter can get a modulation source from a computer, Internet radio, a smartphone (maybe with an audio Bluetooth adapter), an audio CD deck, an audio cassette deck, or a vinyl record turntable with a preamp. Vintage music can also be obtained on audio CDs and old vinyl records sometimes from FM radio on audio cassette tapes. The Sirius XM channel 73 - “40s Junction” - is appropriate for antique radios. Sirius XM provides streaming on the Internet for customers that subscribe to their satellite radio service. Pandora, You Tube, Google Play, Spotify, and are examples of such sources. The Internet is a current source of vintage music and old radio programs. The transmitter is legal to use under FCC part 15 rules because the power is low and the antenna is short.
The transmitter can also be used for any other purpose desired. Such an AM transmitter is the subject of this article.
The solution to this dilemma is to use a low power crystal-controlled AM transmitter to broadcast age appropriate material to the antique radio. There’s nothing comparable to the type of programming that was on the air in the heyday of radio broadcasting. It’s comprised mostly of religious stations, political talk shows, sports talk shows, and gospel and country music. Unfortunately, the content of present day AM radio stations leaves a lot to be desired. Once you have an antique radio, it’s nice to listen to it. They were stripped down to the chassis the chassis was painted silver and the tubes, capacitors, and hookup wire were replaced. Zenith table model radio from the late 1930s, sitting on a speaker box to raise the level of the radio and provide a better speaker for better sound.
Two such radios are shown in Figure 1 and Figure 2.įIGURE 2. One hobby that’s prevalent today is restoring antique radios.
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