Friday, November 18, 2011

My First QRPp Contact

My "Elmer" KØYTI (now AAØA) handed me a stack of 73 Magazine back-issues back in 1971. I was fourteen years-old at the time and already becoming bored with my store-bought radio equipment. I responded to an advertisement in one of the 73 Magazine issues (shown below) by sending off an order for $3 worth of MPF102 JFETs. I chose the MPF102 because it was the least expensive device offered!

      
Having received my field-effect transistors I set about trying to build a crystal-controlled oscillator for the 40m band. I mounted the components on several bakelite tie-strips, which were themselves screwed down onto a wooden base.

After several days of trial and error I finally heard the oscillator's signal on my shortwave receiver. I used my only piece of test equipment - a VOM that I had purchased from a nearby Radio Shack outlet store - to measure the total DC input power to the oscillator. With a fresh pair of 9Vdc "transistor radio" batteries in series, I calcuated the input DC power was just over 50mW.    

Soon afterwards I had the idea of coupling this oscillator to my 40m dipole antenna. Having no idea how to calculate the optimum number of coupling turns at the tank inductor, I tested various windings by measuring the relative output at the antenna with my VOM, set to read "volts," in series with a germanium diode.

Having placed my straight-key in the circuit I began calling CQ. I didn't know if I could be heard outside my neighborhood, much less outside of my city. 

I was still at it some days later with not so much as a nibble. I must have sent out five-hundred calls to no avail when, at last, I heard a station returning my call sign. It was W3EGL/Ø in Rochester, Minnesota; a distance of 388miles or 624km. We easily exchanged reports. Obviously, I was ecstatic over the contact.

Alas, both his QSL card and my old log book are long gone, so I can not tell you what the signal reports were. However, I recall from his QSL that he'd been using a pair of military surplus "Command" radios..."BC" something or other. I remember thinking it was pretty neat that he'd heard my tiny signal on such old and basic equipment. 

I called for some time, off and on, in the following weeks, but this was to be my only QSO made with a handful of milliwatts in those days. What with school, dating and jobs, I eventually left amateur radio for what turned out to be a period of over twenty-five years.

I returned to the hobby in 2008 with the aim of making "homebrew milliwatting" my primary focus. In early 2010 I rebuilt the MPF102 one-stage transmitter from memory. Using the same drain supply voltage as before, I noticed the output power varied considerably from one MPF102 to another (the Idss and Vp values lie in a broad band of values from one device to the next), but it generally fell within the range of 10 to 20mW. 

However, by 2010 I had developed an interest in early semiconductors. Accordingly, I replaced the MPF102 with one of the earliest modern silicon JFET's.

The first commerical field effect transistor appeared in France in 1958. This was the Technitron; brainchild of Stanislaus Teszner. Unfortunately, this germanium alloy device suffered from very low gain and high reverse-leakage.

Crystalonics of Cambridge, Massachusetts produced the first commercially available silicon JFET in 1960. Although the reverse-leakage was vastly lower than in the Technitron, and the transconductance was an order of magnitude higher, it was a poor perfomer in comparison with modern silicon JFETs. In fact I have four of these devices in my collection. Only one of the four - type C632 - is barely capable of producing sustained oscillation at 4MHz. The listed transconductance for the three type C631 devices in my possession is only 125uMhos with a pinch-off of 30V and up to 50pF of interelectrode capacitance!

It was the advent of Jean Hoerni's planar technology in 1959 that paved the way for modern, high-performance silicon JFET's. The first of these to be produced was a line of P-channel devices made by Texas Instruments, beginning in 1962: the 2N2386, 2N2497, 2N2498, 2N2499 and 2N2500.

I replaced the MPF102 in the QRPp transmitter of my youth with a 2N2499. My sample bears a date-code of September 1962; a very early device indeed. Given the May 1966 Texas Instruments price guide lists the 2N2499 for $12.90 (equivalent to $86 in 2010), I shudder to think what it must have cost in 1962 (the first JFET project to appear in QST Magazine debuted in 1966).   

The RF power output was actually a bit higher using the 2N2499; at 28mW vs. 10 to 20mW with the MPF102s. I paired this transmitter with a regenerative receiver built from an early 2N2386 JFET (regenerative detector), direct-coupled at the source resistor to a 1957-vintage 2N107 PNP germanium transistor (AF amplifier stage).

On 6 May 2010, K1GOW in Providence, RI answered my CQ on 40m. Joe was running 3 watts to an end-fed wire only 15 feet off the ground. The distance between us was 167miles or 270km. The reports were 569/439. 

On the next day I worked KA2PQY in Milmay, NJ (338mi/544km) with 569/339 reports.

On 10 May I worked two stations: N2AYI in Carney's Point, NJ (332mi/535km) with 579/539 reports, and, KA2KGP in Forestville, NY (340mi/547km) with 569/339 reports. BTW, Tom's QRZ page notes that he is a deaf CW operator.  

2 comments:

  1. What an amazing story, and inspiring, too! Thank you for sharing it.

    I need to deepen my own understanding of these circuits and their components.

    73,

    Todd, NØIP

    ReplyDelete
    Replies
    1. Hi Todd,

      Thank you for the nice comment! I'm afraid the tremendous fun I've had with these ultra-simple radios has spoiled me forever. :-)

      Kind regards,
      Mike, AA1TJ

      Delete