20m QRPp

 Last week I cobbled together a 20m CW QRPp station using two CK5875 subminiature pentodes made by Raytheon. This line of tubes was designed in the late 1940's for use in portable radio equipment. For example, the well-known AN/PRC-6 walkie-talkie used twelve of these types of subminiature tubes.

This video, produced by the MIT Science Museum, shows Raytheon workers assembling subminiature tubes having the same envelope (begins 10:33) at their Massachusetts plant. Doubtless, my tubes (date-coded 1964) came from this same assembly line.

My crystal-controlled, 20m transmitter uses a single, triode-connected CK5875. A 57Vdc anode-supply results in an RF output power of 40mW. The receiver uses an identical tube in a 0-V-0 regenerative detector.

The setup was judged to be airworthy last Thursday evening. I began calling CQ the following morning on 14.060MHz.

Unfortunately, an accumulation of freezing rain that morning had de-tuned my antenna such that only 25mW was initially radiated. The icing situation gradually improved towards midday; I wish I could say the same for the band conditions. Still, by 1600z my radiated output power had risen to 35mW. That's when I heard KE4YHY answering my call. Don handed me a 559 report from Alabama; a distance of 1600km. Despite the need for several repeats on my end, we went on to enjoy a 12 minute QSO.The band never did improve in the time remaining before I had to QRT. It's a shame as DL3PB had been listening several hours for my signal (thanks Peter, we'll bag it next week!).

29 Nov. 2010: The band was in rough shape again today. However, my second or third CQ with the CK5875 twins brought a reply from K9IS in Wisconsin (1259km distant). My output power was 40mW. In a follow-up email Steve wrote

"Your 40 mw signal was easy to copy the first time around, but QSB made rough copy after that. I believe you said you were using a 1 tube Xmiter and I never did copy the rcvr. Anyway, it is fun to work someone running real low power. Hope to hear you again soon."

Steve and I worked several times last Spring on 20m. I was running 350mW from a 3A5 twin-triode configured as a "Frank Jones," crystal-controlled, push-pull oscillator. A second 3A5 was fashioned into a regenerodyne receiver. Steve used a variety of rigs with output powers ranging from 5w down to 300mW.

After filling half of a page in my log with DX calls last Spring I reduced my output power from 350 to 57mW. Of course the contacts didn't come as easily, and yet I worked N7EF/QRPp running 1w out in Seattle and W7CNL/QRP in Boise, along with five other stations. And then DH1BBO returned my call one morning with his Elecraft K1. I was especially tickled to hear Olaf report his output RF power as 300mW. I'm not sure which impressed me more; working across to Germany with 57mW or hearing a 300mW German station on a receiver made from a single, 3A5 vacuum tube! 

This contact with DH1BBO later inspired me to try my luck on The Bell Ringer with the RF output power throttled down to 10mW. On the first day I (twice!) worked F5NBX for 350,000 miles-per-watt. This was followed by a contact with FM5LD. On the next day I worked  IZ0PEC, for 410,790 miles-per-watt.

My eventual aim is to build a QRPp station using subminiature vacuum tubes that's small enough to carry on hiking trips. I intend to power it entirely from a hand-cranked alternator while sending CW with a mouth-key.

In closing, I'd like to recount a couple of wonderful feats of QRPp DX. Some ten years ago, Jim, AL7FS, completed a two-way, 20m, QRPp QSO with Mike, ZL1MH. At that time Jim was burning up the ionosphere with 4.5mW. Mike responded with 100mW. 

Around 1997, Yoshi, W6/JJ1SLW, pulled off a QSO with JA6SHL located on the Japanese mainland. Yoshi's 40m Pixie had an RF output power of 20mW!

The Matlock-Collins Clock

In the mid-1970's a fascinating catalog arrived in the post from Caldwell Industries of Luling, Texas. While Caldwell catered primarily to home-shop machinists, the owner, John Matlock, was also an amateur horologist. A unique pendulum clock of his design caught my eye in particular. It used the (then-new) 555 timer integrated circuit in order to radically simplify the design of a "free pendulum" chronometer.

The, so called, "free pendulum" time-keepers had a distinguished history. For example, Frank Hope-Jones developed his "Synchronome" clocks in the late 19th and early 20th century. By 1921, W. H. Shortt had greatly improved the Synchronome; primarily by relieving the pendulum from its burden of pulling around the count-wheel. Mr. Shortt's clock set the standard of time-keeping accuracy for the two decades prior to the advent of quartz-crystal chronometer.

Some of you may recall having seen the Matlock-Collins Clock advertised in various DIYer magazines in the mid-1970s. Here is a scan of the Matlock-Collins Clock kit, as offered in the 1976 issue of Caldwell's wonderful, Craftmanship Catalog.

                   
This is a simplified version of Mr. Shortt's "free pendulum" clock. Of course, given that the pendulum is impulsed periodically it's not absolutely free, but that's another matter.

The electronic timer replaces the mechanical count-wheel that's used in the Synchronome clocks. In other words, the pendulum of the Matlock-Collins Clock doesn't have to drag around a little count-wheel in order to synchronize the tripping of the gravity arm. The idea is to let the gravity arm reset-impulse also reset the 555 timer. This timer (electronically) waits roughly one minute before dropping the gravity arm again to impulse the pendulum. But it's the pendulum itself that regulates the overall timekeeping of the clock. As simple as it is, I've never gotten around to building this clock. 

A few years ago I picked up an apparently unused, hermetically sealed tuning fork assembly at a local hamfest. The tuning fork was invented in 1711 by John Shore (a trumpeter in Handel's orchestra). By the mid-1950's the maker of my fork, Philamon Laboratories, had advanced the technology to a high degree of perfection.

I came across this tuning fork again a few weeks ago while searching through my junk box for another component. Seeing it prompted me to build a tuning fork drive circuit that's roughly based on the scheme used in the Matlock-Collins Clock. It turned out to be a very simple and enjoyable project.

My tuning fork-based emulation of the Matlock-Collins design begins with a zero-crossing voltage comparator that's wired to one of the two electromagnetic coils housed in the sealed resonator. The comparator has a very high input impedance. The coil terminal labeled "1" on my tuning fork has a DC resistance of 855 Ohms, while pin "2" has a resistance of 2000 Ohms. I use the higher resistance winding as my "sense" coil. 

The comparator outputs a 1600Hz square-wave; the falling edge of which triggers a 555 timer that's configured as a monostable multivibrator. The timer output promptly goes high and ignores any further trigger pulses until nearly 20mS has elapsed. At this time the timer output falls back to zero and the circuit awaits the arrival of the next falling-edged zero-crossing. Allowing the highly accurate tuning fork signal to periodically reset the timer drastically reduces the long-term accuracy required of the 555 timer circuit.

The timer is followed by a D-type flip-flop that's also configured as a multivibrator. Its purpose is to set the tuning fork drive-pulse to the optimal width; Msgr. Fourier's mathematical poetry in action!

The 555 timer in the original Matlock-Collins Clock operates at just under 60 seconds in order to produce a pendulum to drive-pulse frequency ratio of 30 to 1. The 555 timer in my tuning fork time-base is set to just shy of 20mS to affect a frequency ratio of 32 to 1. Thus, the drive pulses are regulated to 50Hz. The 2N7000 transistor insures that absent the drive pulse, no load is placed across the tuning fork drive coil. As with the original Matlock-Collins design, the brief drive impulses are the only external interference to the "free vibration" of the fork tines. Properly buffered, the output signal might be used to drive either a 50Hz synchronous clock motor or an electronic clock display. 

As a practical chronometer time-base my tuning fork has one potentially fatal defect. The fixed resonator frequency is not easily adjusted. A simpler feedback oscillator arrangement offers the possibility of fine-tuning the oscillator frequency by varying the phase of the drive signal in the feedback loop. It might be possible to gain some control over the  oscillator frequency by introducing a time-delay in my Matlock-Collins drive pulse. Simply varying the pulse-width of the drive might produce some slight change in the oscillator frequency as well. Although my tuning fork is currently operating at 1600Hz so far as my bench frequency counter is concerned, that measurement is far too approximate for time-keeping purposes. It remains to be seen how closely my Philamon tuning fork has been factory-adjusted to 1600Hz. 

Aside from this worry, my Matlock-Collins tuning fork emulation appears to be working quite well. Adjusting the 555 timer potentiometer results in well-behaved discrete jumps in the frequency division ratio. I've watched the circuit work stably at an 80 to 1 ratio. Of course, as the division ratio increases so the accuracy required of the 555 timer circuit is increased. However, the operation is quite robust at the normal division ratio of 32 to 1.

By the way, if the signal output were taken at the resonator one could think of this circuit as a shock-excitation type frequency multiplier (similar to the frequency multiplier used in my Bell Ringer radio transceiver, for example).

Here's a sketch of my circuit.

The signal at the sense coil is shown on top, the drive pulses are at the bottom.

Here's my bread-boarded circuit along with the tuning fork.

 

3:48

I realize it's a touchy subject, so I won't ask you to watch this video from beginning to end; just ten or twenty seconds, beginning at 3:48. 

I think the author did well to quote Bill Moyers

On the heath Lear asks Gloucester: 'How do you see the world?'

And Gloucester, who is blind, answers: 'I see it feelingly.'

I see it feelingly.

By the way, I don't agree with the blanket characterization made at the beginning and end of the video. Rather, I would simply take out of context the last line from a poem written by John Wilmont, The Earl of Rochester

"Man differs more from man, than man from beast."



The Upside of Irrationality

Albert Camus defined an intellectual as someone whose mind watches over itself. To this end, Dan Ariely's findings are worth becoming acquainted with. 

The Upside of Irrationality; Lecture

The Upside of Irrationality; Robert Siegel interviews Dan Ariely

Predictably Irrational; NYT Book Review
    

Fadistas

Fado is Portuguese tradition that arose among the sailors and the working class. It has evolved into a passionate longing; a melancholic lament that's remarkably easy to fall in love with.

Tereza Salguerio, Ao Longe o Mar, O Pastor

Cristina Branco, Gaivota, Cantigas as Serannas