I decided to concentrate first on the problem of producing electromagnetic energy at HF. The issue of spectral quality effectively rules out the likes of arc and spark transmitters. Having considered and subsequently rejected a number of esoteric possibilities, I eventually concluded that a system based on electromagnetic alternators was the most promising. The extraordinary efforts of Ernst Alexanderson, Rudolph Goldschmidt and Marius Latour notwithstanding, systems based on these historical techniques were limited to VLF energy production. To my knowledge, there was one historical exception to this rule.
Building on original work done in 1910 by O. Martienssen, and in the face of the meteoric rise of vacuum tube technology, two German radio pioneers separately undertook to radically increase the upper frequency limit of radio transmitters based on rotary alternators. Karl Schmidt and Dr. Walter Dornig had worked for some years together at Telefunken's transmitter site in Nauen, Germany. By the early 1920's, Herr Schmidt had moved to C. Lorenz A.G. and Dr. Dornig had gone to the Laboratorium für Hochfrequenzmaschinensender.
The circuits they developed and subsequently patented are remarkably similar. In both cases saturable-core magnetic inductors are employed as pulse generators. Driven hard into saturation by a sinusoidal current, the core's magnetic flux rapidly switches polarity; thus creating a voltage impulse. Bandpass filtering is used to recover the desired Fourier component. This method of Stoss-erregung, or frequency multiplication via shock-excitation, is similar to today's step-recovery diode (SRD) frequency multiplier operation.
I'm uncertain whether Dr. Dornig's transmitters were used outside of his laboratory. Remarkably, at least two of Karl Schmidt's medium frequency (MF) transmitters entered into service as AM broadcast transmitters. The more prominent machine was built by Lorenz and broadcast from Munich begining in April of 1926. It transmitted on a frequency of 562kHz. A frequency-stabilized alternator drove a pair of saturable-core frequency multipliers. The first stage multiplied the frequency by a factor of nine; the second by a factor of eleven.
Unfortunately, this transmitter was less than successful. It soon became apparent that a spurious signal was being transmitted along with the carrier. The source of this, so-called, Triller, or Brummetöne was thought to be a vibrational resonance in the alternator housing. The problem had not been resolved by the following March, at which time Karl Schmidt's transmitter was replaced by a vacuum tube model.
Information about the Schmidt-Lorenz broadcast band transmitters may be found here. The transmitter schematic diagram may be seen on page 4, Figure 737 (Abb. 737) of this document.
Herr Schmidt apparently used his shock-excitation technique to produce signal frequencies as high as 1.5MHz in the laboratory; not quite HF, but amazing nevertheless. I reasoned that if this were possible using early 1920's-grade transformer iron, it might be possible to multiply a VLF signal to 3.5MHz with a useful conversion efficiency using modern ferrites. Indeed, I recently discovered that it's possible to do just that!
My present circuit is nearly identical to the one used in Karl Schmidt's broadcast-band transmitter. My first stage multiplies the incoming frequency by a factor of seven. The second stage multiplies by a factor of five. Here's a scope-shot of the 100kHz input and 3.5MHz output signals; both measured across 50 Ohms. The vertical scale is 5V/div for both waveforms.
By Thursday, February the 17th, I had assembled an 80m QRP CW transceiver around this frequency multiplier. My Wein Bridge (R-C) audio-frequency bench signal generator acts as a 100kHz exciter. This signal is amplified by an IRF630 Class-E amplifier. The filtered output produces a 3.06 watt sinusoidal signal at 100kHz. This signal directly drives my Schmidt frequency multiplier-pair, producing an output of 250mW at 3.5MHz. The frequency conversion loss is 10.9dB. Here is a photo of the breadboarded pair of passive frequency multipliers. Please notice the little jars of mineral oil. These are heatsinks for the tiny saturable ferrite cores. It's just like the original 1920's circuit, only miniaturized!
One pair of DPDT relay contacts keys the 3.5MHz signal between the antenna and a 50 Ohm load/attenuator. The other pair switches the antenna between the transmitter and the receiver.
My receiver consists of an input BPF, a packaged double-balanced diode mixer and an impedance step-up transformer directly driving my headphones (no receiver gain). During receive periods the Schmidt-multiplier generated 3.5MHz signal is fed to a resistive attenuator. The attenuator output feeds the oscillator input port of my diode mixer. Unfortunately, this simple setup doesn't allow for receive increment tuning (RIT).
By early afternoon I had fired up the transmitter as a temporary beacon. I ran up to the house to call my friends, Jim, W1PID, and Seabury, AA1MY; both of whom picked up my beacon signal right away. Not long afterward the three of us met on the air. I received a 569 report from Seab, followed by a 599 report from Jim. It may have been 80 years since this circuit was last on the air, and never at HF! It was a wonderful moment.
Running on adrenaline now, I operated my Schmidtschem transceiver until the early morning hours. N1WPU/QRP answered my CQ from the Maine seacoast. Ted handed me a 579 report and I replied with a 559. He was running 5w to a G5RV.
IT9/LY4U turned up with an enormous signal (even using a gainless receiver). I must have called him several dozen times without result. Later Drago, S59A, appeared. I had reason for hope, inasmuch as he nearly copied my callsign correctly two winters ago while I was operating my Reggie at 85mW on 80m. Alas, it wasn't to be this night.
The next morning I found an email message from Chuck, WA1IIE. He said that while I didn't reply to his call, I was 579 just south of Augusta, Maine. I suspect he was at "zero-beat" with my transmit frequency. I'd been appending half of my calls with "UP 700HZ PSE," but it would have been easy to miss, and like any good operator Chuck had replied right on my transmit frequency.
When I checked the Reverse Beacon Network I found that my calls had been captured the night before by eight different receivers; the most distant of which was K4TD in Alabama (1572km). Here's a screen shot of my CQ captures.
While there's plenty of work yet to be done in order to realize my dream, this transceiver marks an important milestone for me. For quite a long time I was uncertain whether or not it would be possible to efficiently up-multiply to 3.5MHz by means of ferrite nonlinearity. The present setup clearly settles that question.
I next hope to add a "9x" stage to my cascade of passive frequency multipliers. At that point the audio frequency signal generator and MOSFET amplifier will be replaced by an electromagnetic alternator. I plan to stabilize the alternator output signal frequency using a 1600Hz electromagnetic tuning fork frequency reference as part of a phase-locked loop (PLL). Although transistors will be initially used in the PLL, I hope to eventully replace them using non-linear magnetic parametric amplifiers. I'll take it one step at a time, but for now I'm going to take a break and enjoy using my Schmidtschem transceiver on the air.