2019-05-26 Low Power AM Transmitter

I Can’t Find One

I’ve seen several AM broadcast band transmitters on websites and on YouTube, but all of them require a 1 MHz oscillator module, which is difficult to obtain. I see modules for every other frequency not in the 550 to 1600 kHz AM band, but it seems all of the ones (and crystals too) for the AM band are sold, and not available. The modules put out a square wave, so they will transmit on multiples of the module’s frequency. Also, the crystals and modules are not adjustable, they are just a single frequency. So I want to build an AM transmitter that will allow me to choose the frequency it is on. In the metro area where I live, there are so many AM stations that it is difficult to find a frequency that is available.

I Finally Found One I Like

I found this ‘build a beginner’s AM transmitter‘ project on YouTube, so I decided I would build it and modify it to get it to put out a bit more power.

With the 27k resistor severely limiting current, it can be heard within a few feet of the 3 meter antenna wire, but not in the next room. The way it is it could be used to align a radio if the radio is close to the antenna wire.

The schematic at the end of the video called for a wire wound around a toilet paper roll to give 12 microhenrys, but I used a half inch toroid with 13 turns, giving 12 uH. The added orange winding is a few turns for driving the antenna. The schematic called for two sets of three 1000 pF (102) ceramic capacitors in parallel, I replaced them with two 3300 pF (332) capacitors. These gave a frequency of 1204 kHz.

I used a 2N3904 for the transistor. The 27k between the +9VDC and the circuit restricted the circuit to very low current so I reduced that resistor value until it seemed to be loudest. I measured the resistance and it was 5.1k. I used a 0.047 uF instead of the 104 ceramic capacitor on the audio input, for better high frequency audio response. Even so, the input should be driven by a device’s speaker or earphone output.

Photo shows how small this is. The dark green wire in left is modulation input. The short orange wire from the toroid is for the 3 meter antenna wire.

Update May 28 – Paul Burnside
I replaced the 27k with the 10k trimpot and a 2.2k resistor in series. The higher current increased the output, and I adjusted it to see where it seemed to be optimum. It measured 5.1k. There was no change in the frequency, 1204 kHz. The transistor was a 2N3904, so the optimum point may be different depending on the transistor. So far it has very stable frequency, not the FM I’ve experienced in the past.

Parts Values For Other Frequencies

It looks like this is a variant of a Vackar Oscillator. I’ve built this kind of oscillator before and it can be very stable. It is a modification of a Colpitts Oscillator. You can find out more by searching for Vackar oscillator.

I calculated that for a frequency of 600 kHz, the coil should be 30 microhenrys and the three capacitors in parallel should be replaced by a “472” 4700 pF ceramic capacitor. If you use the 12 uH coil with a 4700 capacitor, the frequency should be 948 kHz. For the 12 uH coil with 2200 pF capacitors the frequency should be 1385 kHz. For 1600 kHz, the coil should be 9 microhenrys and the three capacitors in parallel should be replaced by a “222” 2200 pF ceramic capacitor.

Other values will give frequencies in between. Two variable capacitors could be used, but the values would have to be 500 pF or more for each, and the usual variable capacitor is expensive, as much as several tens of dollars US. It’s just easier to pick combinations of ceramic capacitors for the three in parallel until the right frequency is reached. Just start with low capacitors and add until the frequency comes down to what you want. Also, if the coil is on a toilet paper roll or plastic pill bottle, it’s best to wind the coil with taps every few turns. Just use an alligator clip to clip on to one of the taps to change the frequency.

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