Nothing Like It, Old Technology!

As you studied for your amateur exam you may have heard the term “Hartley Oscillator”. If you obtained your ticket back in the halcyon days of radio, chances are you actually had to drawn the Hartley circuit. Who was the “Hartley” of the Hartley Oscillator fame? Ralph Vinton Lyon Hartley was born on November 30, 1888 in Spruce, Nevada and attended the University of Utah. Hartley was employed at the Research Laboratory of the Western Electric Company. In 1915 he was put in charge of radio receiver development for the Bell System transatlantic radiotelephone tests. For this he developed the Hartley oscillator and also a neutralizing circuit to eliminate triode singing resulting from internal coupling. A patent for the oscillator was filed on June 1, 1915 and awarded on October 26, 1920.

Oscillator Principles
So exactly what is a Hartley oscillator and how is it important to radio? The Hartley, along with the Colpitts circuit, are the two most common types of oscillator circuits. An oscillator is the one of the most important and fundamental building blocks of radio, used in both receivers and transmitters. An old maxim regarding oscillators is “"Amplifiers oscillate and oscillators amplify", while the author is unknown, the statement is quite valid.

Every oscillator has at least one active device (passive device are items like resistors, capacitors, inductors) be it a transistor or a vacuum tube. In the Hartley the active device serves as an amplifier. When the circuit is first energized, random noise is generated within the active device and then amplified. This noise is fed back positively through frequency selective circuits to the input, where it is amplified again and so on, rather like looking at a mirrors reflection in another mirror. Not quite perpetual motion, but much more akin to a pendulum or a swing set. Think of an oscillator being the swing, and the active component being the “pusher” who applies force at the top of the swing to keep it moving.

One needs to add a push to the circuit to overcomes inherent losses until ultimately a state of equilibrium is reached where the losses in the circuit are made good by consuming power from the power supply and the frequency of oscillation is determined by the external components, be they inductors and capacitors (L.C.) or a crystal. The amount of positive feedback to sustain oscillation is also determined by external components.

Hartley Oscillator
Let’s talk about the Hartley oscillator for he simple reason it's my favorite and the building block of a homebrew transmitter that recently landed in the shack of W1UJR. The simplified schematic of the Harley circuit follows below.

Figure 1 - Schematic of a Hartley oscillator

Hartley Oscillator as a Transmitter
As every ham who has ever operated a regeneration set knows, oscillators are also transmitters. If you doubt this, turn on your regen set and tune in a station, now tune another receiver to the same frequency, chances are your hear your local oscillator rather than the broadcast station. We can put this feature to good use by coupling the output of the oscillator to an antenna. Now we can create a method to key the oscillator – turn it on and off, we can actually send CW!

At the 2004 fall Hosstraders hamfest in New Hampshire I came across a most interesting homebrew transmitter. Now I am partial any homebrew project, but this unit was especially well built and after some mild haggling with the seller the transmitter followed me home to Maine. For historical reasons, when buying homebrew gear I always like to obtain the history and callsign of the builder. In this case I discovered the wonderful little transmitter was built by Al Wentworth KB1SX of Lexington, MA. I understand that Al became a silent key in April of 2004, so unfortunately I could not speak with him directly. The seller did indicate that he thought the design was based on an old QST article, so I went to work searching my QST library. Sure enough, in the August 1928 issue of QST I found an article entitled the “1929 Transmitter”.

In 1928 the amateur community and the ARRL in particular, were concerned about the poor quality of signals on the bands. Often a subject of scurrilous review by “The Old Man”, actually Hiram Percy Maxim writing under a penname, amateur signals ranged from “chirpy” from poor supply regulation, to “buzzing” from raw AC on the plates. Rarely was the signal report “T9X” given. The ARRL sent out to change this, and during the late ‘20s a number of articles appeared in QST instructing, and in same cases chastising, radio amateurs on signal improvements.

The Hartley circuit used in my little transmitter was actually featured in the 1928 article as the state of the art, at least in 1928. So I was excited to fire up the unit and see how it actually played! A visit to Larry NE1S, a good friend and very experienced homebrewer of 1920s-1930s design transmitters, was in order. Larry rounded up a power supply, homebrew of course, and the little transmitter was wired up. A flick of the switch and the filament glowed brightly in the UX-210 tube. Now the real question, how would it play, would it transmit and perhaps most importantly what would the note sound like?

This Hartley circuit is not crystal controlled; instead the frequency is determined by the capacitance and inductance of the circuit. So by turning the air valuable capacitor, the frequency of the transmitter could be moved quite readily. The old time procedure for tuning a Hartley to resonance is really quite simple. The filaments are allowed to warm up, then the plate voltage is applied on and the key was closed. Now a small test light, consisting of a flashlight bulb and ring of wire, is brought near the large copper wound inductor to confirm that the circuit is oscillating. The power from the inductor of the transmitter will inductively couple to the small inductor of the light bulb, illuminating the bulb to indicate oscillation. The circuit is then slowly turned for maximum brilliance of the bulb, which also indicates maximum power. Carefully Larry turned the circuit and sure enough, the bulb did glow, so the transmitter was working. More tweakage was done to bring the unit into the amateur band of 80 meters. Now we were both curious, what would this lash up sound like on the air?

Larry wired up antenna and turned on a local receiver to listen to the tone of the transmitter. Some additional tweaking produced a nice clear and solid tone. A snappy CQ was sent and after a few attempts a station answered. He was located near Cleveland, Ohio and his call was KC8VZV. KC8VZV, Bob as we were soon to learn, was running a Kenwood solid state rig and was quite puzzled and concerned by the “drifting” and “chirp” of our signal. So much so in fact that he mentioned it during each exchange. Despite Larry’s best efforts to convey we were running a 1920’s vintage Hartley oscillator, and far from xtal control, I’m not quite sure that he understood. A few days after the QSO I sent off an email to Bob advising him of our old technology. We had an interesting email exchange, it seems that Bob was somewhat of an old timer and not only knew about the Hartley circuit, but had in fact built one himself several decades back.

Bob was our only contact that day, but as I work the Hartley into my station, I am sure that there will be many more. Barnie N1XA was kind enough to contribute some vintage parts so I may build up a 1930s design power supply. Once the matching power supply is homebrewed, the Hartley will soon be joined with a 1933 National FB-7 receiver. I intend to use the 1930s vintage station in the old time contests sponsored by the Antique Wireless Association.

So there you have it, the story of the Hartley oscillator and proof that even old technology can be fun and useful today!

73 Bruce W1UJR

              

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