Sunday, June 16, 2013

A Single-Stage Heterodyne VFO

A heterodyne VFO is normally constructed using a minimum of three distinct circuit blocks, namely: a VFO, a crystal-controlled oscillator and a mixer. I've managed to pare it down to two stages in the past: a VFO followed by a self-oscillating, crystal-controlled "converter," but I always knew my "minimalist devil" would ultimately be satisfied with no more than one stage.

Past attempts were thwarted due to excessive oscillator interaction. But at last I seem to have stumbled over a reasonably well-behaved "all-in-one" heterodyne VFO circuit. The result employs a VFO configured as a common-collector Colpitts oscillator. Simultaneously, a crystal-controlled Colpitts oscillator works in the common-base mode. The passive oscillator component values selected reflect the need to roughly equalize the two oscillator energies.  

The crystal-controlled oscillator frequency was dictated by the contents of my junkbox; 16.877MHz, in this case. The VFO tunes from 2.877 to 2.777MHz, in order to cover the bottom 100kHz of the 20M band.  

I used a 2N3904 throughout most of the design process. Once the circuit worked to my liking I swapped this modern device for a 1950's-vintage, Germanium surface-barrier transistor. Aside from some component tweaks (R1, especially) the circuit behaved much the same. 

The frequency stability seems quite up to the task in my underground shack, however, I wouldn't think of operating this transmitter under the noonday sun. The Germanium transistors, compounded by the primitive DC biasing technique would likely spell disaster. The VFO isolation is commendable for such a simple circuit.

A tuned RF amplifier, primarily comprised of a second, vintage Germanium transistor, produces an RF output of slightly more than 2mW. The worst spurious frequency output power (at 16.877MHz) measures -26dBc (~5uW). While I intend to add a second RF amplifier stage, I was curious whether or not I could make a contact at 2mW. 

My answer came last evening when I returned K5TF's CQ. He replied straight-away with a 439 report from Atlanta; a distance of 937m/1508km. An avid QRP-op himself, Dick joined my happy-dance with, "WOW 2mW AMAZING." Indeed, it's the lowest power I've used to make a long-haul contact thus far. He missed one exchange from me due to QSB, otherwise, he appeared to enjoy a fairly good copy throughout most of the seven-minute QSO. Dick operates an Elecraft K2 at 5W to a Hexbeam. I had a steadfast - 589 - copy of his signal on my two-transistor regenerative receiver. Enjoying my new-found frequency agility, I subsequently roamed the CW band for several hours, calling the stronger DX stations but without success. 

Still, I was happy with my one QSO. Besides, the transmitter worked well enough to placate my minimalist least for now ;-)

20 June 13: Worked W4TZM in the NAQCC Milliwatt Sprint! (589/559, 1330km @ 2mW) 

    Parts List

R1:  2.2MegOhm
R2:  560 Ohms
R3:  3.3k Ohms
R4:  220 Ohms
R5: 470k Ohms
C1, C2:  1nF (s.m.)
C3: 1.67nF (s.m.)
C5: 120pF (s.m.)
C6: 560pF (s.m.)
C7, C13: 47nF
C9: 0.47uF
C10: 678pF (s.m.)
C11, C12: 15-60pF 
T1: 11 turns / 3 turns, junkbox iron-powder toroidal core
T2: 15 turns / 1 turn, junkbox iron-powder toroidal core
X1: 16.877MHz (junkbox "uP" quartz xtal)
Q1: Philco T1657, Germanium surface-barrier transistor, circa 1959
Q2: Philco 2N502, ditto above    


Tuesday, June 11, 2013

A New Vanguard

Last week I received a surprise package from Jack Ward, curator of the online, Semiconductor Museum. I was astonished to find inside seven examples of the transistors used in the original Vanguard-1 satellite beacons.

Please click here to view Jack's excellent, recent posting concerning these historical devices.

This early run of diffused-base Germanium transistors was made by Western Electric, primarily for use by the US military. For example, in the May, 1959 issue of CQ Magazine, Donald Stoner suggested various substitutes for prospective "Vanguard" circuit reproductions, given the original devices were unavailable to amateurs. If this was the case only fourteen months after the launch of Vanguard 1, I reckoned it would be nearly impossible to find them over 50 years later. As such, it seems an incredible stroke of luck that led Jack to these rare artifacts in the estate of an engineer who'd once been tasked to stress-test them.

Unfortunately, time was not kind to six of the seven transistors that I received. Five of the seven devices had at least one defective P-N junction. The 6th device produced very little HF gain. Fortunately, the 7th device, a GF-45011, is a strong performer. The GF-45011 was originally used in the one-stage, solar-powered, Vanguard-1 beacon transmitter.

I used the one functional GF-45011 to reproduce the original Vanguard 1 transmitter as closely as possible. It's similar to my previous attempt using a substitute surface-barrier transistor. Needless to say, I handled the GF-45011 device with extreme care! Of course the original operated at 108MHz, whereas my circuit works at 14MHz. Moreover, the original solar-powered transmitter produced an RF output power of 5mW, whereas mine produces 38mW. The transmitter, seen below, is comprised of only 11 components. The precious, still-functioning transistor is appropriately gold-plated!

I paired this new transmitter with a 0V1 regenerative receiver (MPF102/2N3904), connected my 41m end-fed wire (at 10m) and began calling CQ on 14.060MHz. 

Ninety minutes passed without so much as a nibble, but a
t 2118z I suddenly heard GW3YDX calling me. 579/519 reports were quickly exchanged. Ron subsequently copied my QTH as well as my transmitter output power. His 5w signal was an easy copy in the two-transistor regenerative receiver...a solid QSO from Vermont, USA to Four Crosses, Wales; albeit, my one and only contact for the day!

The next day W4OP in North Carolina answered my CQ. Dale reported in a follow-up email that my 38mW signal had briefly peaked at 589. I twice heard K5ACO calling me through heavy QRM but we didn't manage to complete the contact. Tnx all the same, OM!

On the following day my call was answered by two Georgian QRP stations: K5TF and W4DU. The contact with K5TF turned into a very pleasant rag chew lasting a quarter of an hour.

I'd like to express my sincere gratitude to Jack Ward for his thoughtful generosity. It was a tremendous thrill to recreate the Vanguard 1 transmitter down to the original - now extraordinarily rare - transistor type. My thanks as well to everyone who listened for my signal in recent days.

Mike, AA1TJ

Thursday, April 18, 2013

Talking to France Via My Electric Razor

I nearly threw away my worn out Braun 5569 electric shaver a few months ago. On second thought, I decided to at least check it for usable components before I disposed of it. It occurred to me a short while afterwards that it might be fun to attempt to convert whatever was inside it into an amateur radio

I got around to opening it up last week. It was mostly built using surface-mount technology (SMT), but at least it used discrete devices vs. a custom-made IC. A waterproof coating made salvage difficult, but careful surgery with a razor blade allowed me to remove all five SMT transistors and half-a-dozen diodes; one of which was a 5V Zener.

The transistors were soldered to bits of copper-clad "carriers" along with flying leads. One transistor didn't survive the transplant operation. The one non-SMT transistor, an NPN power device, refused to oscillate above 1MHz. The remaining three NPN and one PNP transistors appeared to function well enough at 14MHz that I thought I'd have a go at building something for 20m.

My first idea was to build this Japanese 10/6m DSB rig for 20m using a 14.3MHz computer crystal from my junk box.

After the standard amount of tinkering the transmitter appeared to be operational. Two of the tiny Silicon diodes taken from my electric razor matched well enough to produce a decent carrier null in the single-balanced modulator. I used a pair of NPNs in a push-pull PA stage. A forward-biased diode taken from the razor was used to set the PA bias. The RF CW output was ~90mW. An electret mic from an old telephone drove the PNP SMT transistor in the AF amplifier. The VXO provided a tuning range of 8kHz.

Unfortunately, switching the transmitter to the direct-conversion receiver mode produced the expected result. The 100kW, Nashville-based, WWCR, on 13.845MHz swamped the amateur phone signals. Placing a "balancing" pot in line with the the SBM diodes helped considerably, but not enough to completely eliminate the interference. Past experience tells me that at my QTH a narrow, triple-resonator, BPF is needed between the antenna and these simple diode mixers. I decided to pull the DSB rig apart and build instead a 20m CW station.

I built a 0V1, common-base, Colpitts autodyne regenerative receiver using two of the NPN SMT transistors. The Q-multiplied RF resonator easily shrugs off interference from the high power commercial shortwave transmitter. 

The transmitter begins with a variable quartz crystal-controlled oscillator (VXO) made from the third NPN transistor. The final PNP SMT device was used in a common-base, RF power amplifier (PA). The transmitter VXO tunes from 14.055 to 14.061MHz using an xtal cut for the QRP calling frequency. The oscillator free-runs on transmit; only the PA is keyed. The RF output power is 75mW. A 7th order lowpass filter (LPF) holds the harmonics below -45dBc.

The current draw is 1mA on receive and 12mA on transmit using a 9Vdc battery as my power supply.

The bread-boarded transmitter appears in the above photo. The LPF is on the left-hand side, followed by the PA and quartz-crystal oscillator. Two of the transistors taken from the electric razor can be seen in this photo.

Here's a close-up of the two-stage receiver. The regenerative detector is on the left-hand side of the board, followed by one stage of audio amplification. The variable capacitor at the left tunes the receiver from 14.000 to 14.075MHz.

I was pleased to have worked four stations with this setup on April 17, 2013.

W4SX GA 569/339 K2 @5W
KB0PCI MN 569/339 12w
KD4ESO AL 579/559 100w
K5EST MO 569/559 5w

Coming up to the house later in the day I found three Reverse Beacon Network receivers had made captures of my 75mW signal. Oddly enough, all three were located in Europe.

On April 18, I worked three stations; my transmitter still driving an end-fed wire antenna with 75mW of RF power.

N0UR MN 449/229
W4SX GA 579/449
F6DCD France 559/519 K2 @ 5W

F6DCD very kindly called again 35 minutes later to inform me that my signal was still audible near Strasbourg. Denis increased my signal report to 529. Needless to say, I was ecstatic to have "crossed the pond" with the transistors taken from my old electric razor.

On 19 April I received an email from F6DCD

Hello Mike,
I was really pleased to contact you with your 75 mW. I heard you calling on frequency during 1 hour and more. Your sig was really nice on my K2.

Vy 73/72,
Denis, F6DCD

Tuesday, April 9, 2013

"Sputnik-1" Radio Beacon Schematic Found!

The search for the schematic diagram of the world's first orbiting radio beacon transmitter is finally over! 

Отчет о разработке бортовой радиостанции первого советского искусственного спутника Земли /прибор Д-200/. Репринтное издание. - М.: «ИД Медиа Паблишер»,2012.- 122 с., ISBN 978-5-903650-23-1

Published in 2012, this book is a report on the development of the radio beacon transmitter (D-200) that was carried into orbit by Sputnik 1, on October 4, 1957. 

Boris/RU3AX reviewed this book in the April, 2013 edition of the Russian magazine, Radio. His review features the schematic diagram of one of the two radio transmitters carried aboard Sputnik 1. Please click here to open a PDF file of his article.

I would like to thank Oleg/RV3GM and Sergey/UA3ALW for bringing this wonderful news to our attention. 

Wednesday, January 16, 2013

A New Sputnik from the Czech Republic

OK1DPX, has made a wonderful job of his Sputnik transmitter reproduction. Plug-in modules allow for multi-band operation. He even included an on-board vacuum tube multivibrator (beep-beeper). Petr details his work in the current, Bulletin of the OK QRP Club. Please click here to download his article (presented by permission).

Congratulations and well-done, Petr!

Sunday, October 21, 2012

Vacuum Fluorescent QRPp

I received a Tung-Sol 6977 vacuum fluorescent indicator last week from a friend. Wow...this is the smallest vacuum tube envelope that I've yet seen! The above image of an illuminated 6977 was made by Joe Sousa in this informative post. A quick search for early documentation turned up this 1958 article (pp. 5 to 9 ). Naturally, I wondered whether it might be possible to build a radio from this unusual device. 

I used my 6977 to build an 80m regenerative receiver the next day. At first the circuit refused to oscillate, despite having a rather high level of feedback. Temporarily opening the feed-back loop, I noted that it was indeed acting as an RF amplifier. Closing the (positive) feed-back loop again increased the gain, as expected. Obviously, the loop-gain was below unity. Increasing the grid-leak resistor value from 3.9 to 10MegOhms did the trick; adjusting the regeneration control (see schematic) now allowed the receiver to slide smoothly in and out of oscillation. The receiver tunes from 3500 to 3583kHz. I spent the remainder of the evening logging DX stations at the low-end of the 80m CW band. 

The next morning I experimented with an 80m, crystal-controlled, Miller oscillator using my 6977. Working to maximize the RF output, I eventually measured slightly more than 1mW across a 50 Ohm load resistor. Given the 57Vdc anode supply at 320uA, this represents a dismal (~6%) RF output/DC input power efficiency. 

The anode to antenna impedance matching network is to blame. If the efficiency were a more typical value (say, 25 to 30%), I should expect to measure 5mW of RF output power. In this case, the standard rule-of-thumb formula indicates the anode should be presented with a load impedance of (57^2)/(2*0.005) = 325kOhms.

The most suitable output tank inductor in my junkbox was a "Hescho"-type (14.7uH, Qu ~250) ceramic/silver unit that I picked up at the hamfest in Friedricheshafen. The unloaded equivalent parallel resistance of the resulting output tank resonator is (250*2*Pi*3.5^6*14.7^-6) = 80.8kOhms. This is too low to efficiently produce the required impedance transformation. 

The physics of this matching network also implies the tuning will be very sharp. On the other hand, harmonic energy is attenuated quite handily. In fact the second (and worst) harmonic is down by 43dBc, so at least no further spectral filtering is required.

It would be interesting to try a crystal-controlled Colpitts oscillator configuration (raising the filaments up from RF ground via an RFC). Taking the RF output at the filaments ought to ease the required impedance step-down ratio somewhat.  

Vacuum Fluorescent QRPp
Component List

X1: 80m quartz crystal
V:1: 6977/DM160 vacuum fluorescent indicator triode
R1: 100k
R2: 10MegOhms
C1: 20nF
C2: 2.5nF s.m.
C3: 5pF s.m.
C4: 360pF s.m.
C5: 40pF s.m.
C6: 100pF s.m.
C7: 3.9nF
C8: 100uF @ 100V
CV1: 20-335pF air-variable
CV2: 15-420pF air-variable
CV3: 30-360pF air-variable
L1: 14.7uH, high Qu (>250)
L2: 3.3mH
T1: 3.5 to 6uH, slug-tuned "IF can"
T2: audio transformer, 13.2kOhms to 600 Ohms
RY1: DPDT "DIP" relay, 12V coil
D1: 1N4148, 1N4002, etc. 

I decided to build a transmitter/receiver by relay-switching the grid and the anode of a single 6977 triode between the receiver and transmitter circuits. This worked out quite well. As I often do with my regenerative receiver/quartz-crystal transmitter pairs, transmit frequency spotting is accomplished by listening for a tiny "blip" or "ping" in the headphones as one tunes through the series-resonant frequency of the crystal. 

When everything was ready to go, I ran up to the house to post a note on the QRP-L mail reflector with the details of my intended operation. Once on the air it became apparent that it wasn't a good night for milliwatt work; the high QRN was simply masking my signal. 

Nevertheless, I called CQ for several hours. Sometime after I'd resigned myself to the fact that I wouldn't make a contact, there was W1PID calling me! Jim remarked on the QRN, but he also handed me a 449 report. He was a perfect copy on the regenerative set; I called it a 579. What fun! Working a station 112km distant with 1mW through heavy 80m QRN augers for "big DX" come the quiet winter months. 

I made a short video of my Vacuum Fluorescent rig shortly after my QSO with W1PID.

Link: MØAYF, builds regenerative receivers from surplus VFD's. Well done, Des!

Mike, AA1TJ                 


Friday, July 13, 2012

Simplicity's Reward

On July 9, I answered a CQ from KB8BWE using my Vanguard 1 station setup. Charlie came back to me straight-away with a 529 report. Unfortunately, my signal subsequently dropped into the noise. While it didn't qualify as a full-fledged QSO, nevertheless, I was pleased to learn that he was operating with three watts into a three-foot diameter loop antenna. The full details of his station arrived in his follow-up email

"...I was using a 3’ diameter magnetic loop called the “Alexloop Walkham” from here:

The loop itself is a piece of RG213 coax. I had just received it in the mail that morning, and was trying it out on the deck behind my house. It all folds up into a laptop-sized case.

The rig is a TenTec 4040 (Youkits HB-1B) 4-band QRP rig that I got at Dayton this year." 

I have, these past twenty-five years, worked and lived atop Mt. Mansfield; Vermont's highest peak. My task there is to maintain a powerful commercial television transmitter that is coupled to a huge antenna. For all of that, it might seem curious that having my 25mW signal copied by a guy sitting on his deck in Ohio with a 3' loop antenna would be the thing to send a chill up my spine. In fact, my passion for radio minimalism has long since spoiled me for anything else. :-)     

"Simplicity is the final achievement. After one has played a vast quantity of notes and more notes, it is simplicity that emerges as the crowning reward of art."  Frédéric Chopin