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I was reading 200 Meters and Down but one thing that was unclear to me was that hams in the 1920s were operating on the 200 meter frequency, which is around 1.5 Mhz .... or 1500 kilocycles to use the old phrasing.

What's unclear to me is how they were able to measure frequencies that high. Up to around 20 kHz you could use audio frequencies, but how do you measure Hertz above 20 kilocycles -- especially just using vacuum tubes?

Kevin Reid AG6YO
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RoboKaren
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    Note that the triode (tube) became available only around 1920 itself, and the pentode only around 1926. The first list of amateur stations was published in 1909 – I think the distinction between "radio engineer", "radio consumer", "broadcaster" and "radio amateur" usage might be a bit harder to make than it is today. (My personal beef: Many ham radio circuits still use late 1960's components as if they were "state of the art". The triode was more "state of the art" by 1969, when the LM386 was introduced, then the LM386 is now. Still I read things like "here's this BD140-based circuit. If… – Marcus Müller Jun 15 '18 at 07:18
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    …you want something more advanced and better-sounding, build a good LM386 circuit". When did ham radio become a pool of reinforcement of using outdated technology, after it used to be riding on the edge of technology?) – Marcus Müller Jun 15 '18 at 07:19
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    @Marcus Müller Rant mode: My feeling is that the ham radio branched off from the regular radio flow at some time point, and kept the technological level from the point at which it branched. This then lead us to situation where (to exaggerate a bit) the blind are leading the blind. Namely, hams are training new hams, and with every generation, the knowledge dilutes. What happens then is that those hams write books, and spread the oversimplifications further. Then, they appear in exams and things get worse and worse. – AndrejaKo Jun 15 '18 at 19:39
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    Just look at the so-called "ITU zones", whatever that is, and their number. Then we also have the American antenna length formula and so on and so on. – AndrejaKo Jun 15 '18 at 19:40
  • One could always ask: QRG? – Phil Frost - W8II Jun 19 '18 at 14:47

2 Answers2

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The US National Bureau of Standards (NBS) had a system for basic measurement of frequencies as early as 1911. They used very basic calculations to determine the resonance of an LC circuit. The math was greatly advanced in 1923 in an article in Radio Broadcast magazine entitled ""Reducing the Guesswork in Tuning". The method involved careful measurements of the physical dimensions of the capacitors and inductors that were applied to the equations. There is little doubt that the hams of that period closely followed and took advantage of this development.

During this same early 1920 period, the NBS was already experimenting with quartz oscillators as a frequency standard. They collaborated with other labs to continue their research. By 1929, Bell Labs had delivered four 100 kHz (kc at the time), temperature controlled crystal oscillators (TCXO). The uncertainty of these oscillators were estimated to be 10 ppm. These became the NBS primary frequency standards in this period.

Glenn W9IQ
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  • And for VHF, they used Lecher wires. – Mike Waters Jun 15 '18 at 04:51
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    @MikeWaters That is true - a practical example of transmission line effects. – Glenn W9IQ Jun 15 '18 at 06:59
  • Great answer! I’m still unclear of how even the NBS was able to measure megahertz frequencies with the equipment of the time. Can you clarify? – RoboKaren Jun 15 '18 at 17:54
  • @RoboKaren You can probably find that information on Google. :-) – Mike Waters Jun 15 '18 at 19:33
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    @RoboKaren The perspective is that they estimated the transmit or receiver frequency based on the LC values in the transmitter or receiver. They didn't measure it in the sense of a frequency counter. Once crystal oscillators became mainstream, these could be detected on their harmonics to better determine the actual frequency by zero beating on a receiver. – Glenn W9IQ Jun 17 '18 at 00:02
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[Discovered this via a comment on another answer.]

According to their Wikipedia article:

Lecher lines were used as frequency measuring devices until frequency counters became available after World War 2.

The idea is to short a transmission line after some distance, forming what we now would call a resonant stub. The resonant reflections set up standing waves; the nodes of those waves could then be detected with an RF probe:

The voltage goes to zero at nodes located at multiples of half a wavelength from the end, with maxima called antinodes located midway between the nodes. Therefore, the wavelength λ can be determined by finding the location of two successive nodes (or antinodes) and measuring the distance between them, and multiplying by two.

But in practice, rather than having a fixed distance-to-short and moving an RF current probe, it sounds like usually there was a movable "shorting bar" under which was marked an appropriate scale:

In operation, the U end acts as a coupling link and is held near the transmitter's tank coil, and the shorting bar is slid out along the arms until the transmitter's plate current dips, indicating the first node has been reached. Then the distance from the end of the link to the shorting bar is a half-wavelength.

Apparently Lecher lines were even used to calculate the speed of light (working from known frequencies back…) before the turn of the century, "an important confirmation of James Clerk Maxwell's theory that light was an electromagnetic wave like radio waves"!

By September 1946, a "Frequency measurement at UHF" article in Radio News still listed this technique, as well as others which have already been mentioned in other answers/comments here:

[…] in general practice such great accuracy is not necessary, and frequency measurements at u.h.f. are generally performed by the following methods:

  1. Wavemeters.
  2. Lecher wires.
  3. Heterodyne frequency meters.

IIUC, what the article describes as a wavemeter is essentially a resonant circuit with a conveniently-labelled variable capacitor.

(A PDF of the full issue is currently linked as the first citation on the Lecher lines article, which starts on page 50.)

natevw - AF7TB
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  • Now I'm curious about how users of a Lecher line apparatus estimated the velocity factor of the transmission line. – rclocher3 Jun 19 '18 at 13:41
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    @rclocher3 With air as a dielectric, the VF is over .98, IIRC. Sorry I can't put my hands on that info right now. – Mike Waters Jun 19 '18 at 19:12
  • I used to use a heterodyne frequency meter. Google BC-221R. Very accurate, more so than Lecher lines. – Mike Waters Jun 19 '18 at 19:18
  • @MikeWaters Aha, so for a carefully-constructed open-wire transmission line, the velocity factor is known to a high degree of precision. That explains it, thanks. – rclocher3 Jun 19 '18 at 20:56