Saturday, February 26, 2011

System Karl Schmidt 80m Results

I had an exciting week on the 80m CW band with my Karl Schmidt 80m QRP transceiver. The final tally was 38 QSOs with 31 Stations in 13 states and 2 provinces. My best DX contact was with K5KW in Oklahoma (2116km). 

You may hear a recording of my signal taken from the receiver of VA2DLJ (153km) by clicking here. A recording made by W1PID (112km) can be heard here. NV1B kindly sent me this recording (196km).

The lowest powered station that I worked was KB1LZH at 35mW. Peter was participating in the Nashoba Valley Amateur Radio Club "Battery Challenge" contest. By the time he worked me his battery was on its last legs and his output power was down to 35mW. His rig consisted of an unregulated Colpitts VFO followed by a buffer stage. When Peter began sending I would hear his signal with a high-pitched beat-note. However, the battery voltage would promptly begin to droop, causing his signal to rapidly slide toward the infrasonic end of the audible spectrum.

What's more, he later told me that the entire 80m CW band could be swept by a 20 degree rotation of the tuning dial. Peter fine-tuned his operating frequency by waving his hand over the VFO circuit! As he was sending I could clearly hear him fighting to hold the signal frequency steady. Just as bad, my primitive setup didn't allow me to chase his signal.

My friend, DL3PB wrote

"A free running oscillator supplied from a dying battery with sophisticated fine-tuning on one hand - a 35-times magnetic multiplier driven from an RC-bridge oscillator along with a gain-less receiver on the other - Gosh, and I thought I was nuts?! Fantastic QSO, better than any DX, thanks for sharing that!"

Indeed, I loved every minute of it! KB1LZH was kind enough to send me this photograph of his 35mW transmitter.


In the October 1923 edition of Elektrotechnische Zeitschrift (ETZ), Karl Schmidt detailed an experimental CW transmitter that he built and operated at the C. Lorenz A.G. laboratory in Berlin.

An alternator drove a one-stage, magnetic frequency multiplier.The transmitter operated on a frequency of 333kHz with an RF output power of 1.5kW. His article included a sketch of the "sloper" antenna that he used, along with a promising signal report from Amsterdam.

Herr Schmidt concluded his article with news that large numbers of these types of transmitters were under construction at Lorenz. These were said to have output powers ranging from 1 to 500kW and a top frequency of 500kHz. In fact, only a handful of these transmitters were ever put on the air. Within the space of a few years this imaginative technology had been all but forgotten.          

My Station Log

AA1MY, 569/569, 157km
W1PID, 569/599, 112km, 2W/10W
N1WPU, 559/579, 314km, 5W to G5RV
WB2QMY, 559/559, 408km, 100W to G5RV
N1RX, 559/579, 91km, 1W                    

NU4I, 449/329, 831km
KQ1P, 229/579, 333km, 1W

VE3GTC, 549/418, 238km, 5W
W1ESC, 569/579, 170km
KB1LZH, 229/559, 200km, 35mW

VE3RRD, 459/449, 554km, 5w to vertical loop
WA1IIE, 569/579, 238km, 5W
K1SEZ, 569/539, 293km, 200W
AA1XV, 339/429, 331km, 5W

VA3FB, 569/579, 276km, 100W to inverted vee
W1IE, 559/539, 941km

WA2OQJ, 439/229, 375km, 5w to a dipole
W2LJ, 229/339, 410km
VE1DY, 229/449, 726km
W3TS, 569/559, 528km
W4FOA, 229/329, 1486km
KB8TT, 339/339, 758km
N7UN, 569/579, 362km
K5KW, 559/549, 2116km,  Best DX

VE3DJX, 569/569, 274km, 5w
WA3SLN, 569/579, 460km
K3HX, 229/559, 709km, 4w
WA1HFF, 559/589, 226km, 5w to a dipole
WA9ETW, 339/519, 899km, 100w to end fed wire @ 15'
WR9H, 449/229, 1327km
W2SH, 569/589, 405km,  5w

SWL Reports Received

NV1B, 559, 196km
N0AR, 229, 1608km
KF4TAP, 334, 1778km (Yukon, MO)

Saturday, February 19, 2011

80m QRP Via Historical German Radio Technology

I've dreamed and schemed, off and on, for the last twenty five years over the prospect of constructing a complete, high-frequency (HF) amateur radio station without the use of vacuum tubes or semiconductors. While nearly every radio fit this description in the early days of wireless technology, they were generally limited to operation at very low frequency (VLF).

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.

The "Exciter"

Thursday, February 3, 2011

An SCR Audio Frequency Amplifier

I came across a book some years ago in which the author briefly mentioned that it was possible to employ an SCR (silicon-controlled rectifier) as a low frequency linear amplifier. I've never heard of this anywhere else, but the question of whether or it was possible has remained in the back of my mind ever since.

Well, today I decided to investigate. After running down a few blind alleys I eventually hit upon a working circuit.

The gate (input) impedance is an inverse function of the anode current. The 10k Ohm resistor in the anode supply line sets the gate input impedance to 50 Ohms. Not having a suitable wide-band matching transformer on hand, I decided to try a narrow-band match using a capacitive divider. The combination of L2 and C4/C5 resonate at 725Hz. The impedance is stepped-down from 99.5k Ohms to match my 600 Ohm headphones. The loaded Q is higher than I would have liked, but aside from that it produces an efficient match. 

The circuit produces just over 21db of power gain with low distortion. I quickly connected this amplifier to the simple receiver that I constructed yesterday

One of the first stations that I heard was W2LB on his 1929-style breadboard transmitter. He built it using a #27 triode for the Bruce Kelly Memorial 1929 AWA QSO Party. Larry has posted some photos of this transmitter on his web page. Hearing this great little rig was the icing on the top of a very FB day in the shack.

Three stages of SCR amplification ought to drive a small loudspeaker handily; at least on 80m. For now, I'm happy with the single stage. It's going to be fun telling my contacts that all my receiver gain is provided by a 1962-vintage silicon-controlled rectifier!

This evening (2/7/11) I ran a simple circuit model through a "Spice" simulator. The SCR was modeled using a 2N3906/2N3904 transistor pair. It was necessary to increase the anode-to-supply resistance to 70k Ohms in order to present a 50 Ohm input impedance. Aside from the fact that my vintage 2N2323 SCR produced somewhat more gain, the model behaved much the same as my breadboard circuit. The circuit model is shown below, along with the quiescent DC node voltages.
Notice that transistor Q1 is in saturation. This essentially leaves Q2 acting as a common-base amplifier. However, in this case the collector and emitter terminals are reverse-connected. This, along with the forward-biased diode shunting the amplifier input port accounts for the reduced gain as compared with a conventional common-base amplifier. 

Here's a plot of the amplifier input impedance from 100Hz to 100kHz.

In the image below, a 725Hz input signal is plotted at the top and the amplified signal appearing across RL is plotted beneath. The signals are in-phase; just as one would expect from a common-base amplifier.

Minimal Signal Fun on 80m

I spent an hour yesterday afternoon building a simple, gain-less, direct-conversion receiver. I got the idea for this particular setup from my pal, DL3PB. The receiver consists of a factory-made, double balanced diode mixer connected to my headphones via an impedance step-up transformer.

My beat frequency oscillator (BFO) uses a single 7400 NAND TTL IC package. Two stages are working as a 3.58MHz ceramic resonator VXO. The other two gates act as buffers. This provides a 10dBm square-wave drive signal to the mixer with a frequency range of 3506 to 3590kHz. The BFO circuit is essentially my Vermont Snowflake transmitter; minus the low-pass output filter and keying circuit.

The receiver is connected to my half-wavelength, end-fed, "L" antenna via a laboratory grade step-attenuator box. 

Everything was ready to go just as W1AW's code practice transmission started-up (at a distance of 262km). I had a fair copy (with some drop-outs due to QSB) with 43dB of attenuation switched-in. During a particularly strong peak I was able to copy several words with 48dB of attenuation. Provided W1AW is running 1kW, it appears that I might have copied their signal with no local attenuation had they been transmitting with an output power of 50mW to as low as 16mW.

Tuning down the band, some European stations were now audible. G3VMW was running 300w. I was able to copy him with 10dB switched in. Also copied through the 10dB pad was IK2UWA. In all, I copied six to ten European stations; YT1AA, EA8CK and IK2CKR among them.

I next heard WB8SIW on his Johnson Viking 1, followed by WB3T on his Heathkit HW-9. John, W3WWP, came through the 10dB pad nicely with his 5w and attic antenna. K1ON, up near the Canadian border, was so strong that I had "armchair" copy with the phones lying on the bench. I had no problem copying him through 45dB of attenuation.

The catch of the evening was K1TMJ's beautiful, 1937 Utah Jr. transmitter heard through the 20dB attenuator. Jim and I chatted over the telephone this morning. He says that his Utah Jr. is putting out 9 or 10w. Taking my 20dB pad into account, it seems I could have copied him with as little as 90 to 100mW.