Fessenden’s alternators

Now that I’ve described the alternator itself, back to Fessenden’s unique way of using alternators.

In each of these I’m using an unrealistic miniature alternator for convenience. There were some relatively small alternators, but most were huge beasts used as high-power transmitters.

These three circuits weren’t among Fessenden’s patents. They were described and diagrammed in a 1916 review article about his unique work with heterodyning.

At that time Armstrong hadn’t yet developed the real superhet, so there was no comparison. Armstrong developed it while in the Signal Corps in 1918, but didn’t go commercial until he was at RCA in 1923. Here’s a brief review of the real Armstrong superhet:

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In a superheterodyne (superhet) receiver, the original signal is MIXED or modulated with a sine-wave generated by a Local Oscillator. The Local Oscillator frequency is always above the tuned input signal, and the oscillator slides its frequency in parallel with the tuning to keep the DIFFERENCE between the two CONSTANT. For instance, you might be tuning from 500 to 1500 kilocycles, and the oscillator would slide its frequency from 600 to 1600 to maintain a constant DIFFERENCE at 100. The MIXER or modulator outputs the 100 kc difference, which still has the slow wiggles of audio riding on it.

The advantage of the superhet’s complexity is not intuitively obvious. When you have a result that’s always on the same radio frequency, you can filter and process the result in consistent ways, without having to vary the parameters of your processors.

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The first of these Fessenden circuits is a standard transmitter circuit. The second and third are impractical, essentially proofs of concept, not meant as working products. The third one couldn’t work at all without vacuum tubes.

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First, a straightforward simple transmitter using an alternator.

Polistra is at the key.

The alternator runs constantly, and the key simply breaks the circuit to the primary of the induction coil. The secondary steps up the wave to a much higher voltage to feed the antenna.

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Second, a heterodyne receiver that rings a bell when signal comes in on the correct frequency. This is probably closest to the superhet, but still a different basic concept. The centerpiece of this circuit is a unique ringing relay. The loop gets magnetic force from the signal and both alternators. The article seems to say that the loop in the relay expands and contracts to hit the contact and close the circuit to the bell. I’ve shown this action, which seems unlikely. From the way the loop is suspended, it was more likely to rotate around the suspending wire. Still, this action is more interesting!

Without the alternators on, the line from antenna to ground (green wires) is just a tuned circuit, tuning to one radio frequency. There’s no rectification or demodulation, so the inputs to the ring-indicator are too high in frequency to allow movement.

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Third, the electrostatic heterodyne. This circuit is elegant in concept but probably wouldn’t work IRL. The capacitor speaker IS the C in the series LC of the transformer’s primary. The secondary carries the local oscillator frequency from the alternator. So the top end of the speaker gets RF signal from the antenna, and the bottom end gets the higher frequency from the local oscillator.

The resulting mix of RF and increment drives the tone. The “capacitor nature” of the speaker would work at RF, but the diaphragm would only vibrate at the much slower beat freq. So the physicality of the diaphragm IS the detector.

It could work after an RF amplifier, so that the incoming signal would
be strong enough to match the alternator.

Here’s the speaker itself. The diaphragm is one plate of the capacitor, and the other is close behind the diaphragm. Modern electrostatic speakers require a HIGH voltage and a fair amount of power, so they need a significant amount of amplification.