K5LAD - 50+ Years of Ham Radio Memories

Volume XIX

Equipment Frequency Accuracy and the

100 Kc Crystal Calibrator

 When I look at a modern-day ham radio transceiver, I’m still amazed at the digital frequency display on the dial.   It can show you, right down to within one Hz., the exact frequency you are listening on or will be transmitting on.  Not only that, but if I come back an hour later………. or tomorrow…………. or next year and place the display on that same frequency, I know that I’m still on exactly the same place in the radio spectrum.  This is true because the frequency is generated electronically and as long as the laws of physics don’t change, that electronic circuit will cause that frequency from last year to be exactly the same as that frequency today.

Not so back in the earlier days of radio.  All frequency determination for receivers and transmitters was done mechanically, typically with either variable condensers or variable inductors.  The dial for determining where you had them set could best be described as “in the ball park” although a few companies in the mid-century were making equipment that could be a good bit closer.  Still, those expensive radios were not as close as the more inexpensive radios of today.

 “Bandspread” was an often-used term with the receivers of those days.  The more bandspread your receiver had, the further apart could be the individual marks displaying various frequencies.  A ham-band only receiver usually had much larger bandspread but if you wanted to listen to the shortwave bands, that was usually not possible.  An all-band receiver usually started at the AM broadcast band at 530 Kc. and could receive all the frequencies up to 30 Mc.  Note that I’m using the designations of Kc (kilocycles) and Mc (megacycles) instead of KHz (kilohertz) and MHz (megahertz).  Back during those earlier times, the cycles had not been changed to hertz yet (hertz only means – cycles per second) so I use the designations from back when I used these receivers.

To make tuning and “resetability” possible on an all-band receiver, the manufacturer placed a second dial with ham-band calibration printed on the second dial.  The two dials could have probably been called “coarse” and “fine.”  To set up an all-band receiver to use on the 20 meter ham band, the “coarse” dial would be placed with the dial mark on a specific spot, usually on the high end of the band.  The spot on this dial for the 20 meter band may only be a quarter of an inch wide to show 14.0 to 14.35 because that dial had to show complete tuning from 530 Kc to 30 Mc.

The second dial was then used to tune the entire band that covered a full 180-degree span.  The stations within that band could be more easily selected since tuning was much less critical and the user could get a good idea on what frequency they were listening.  Frequency determination was certainly not perfect because the accuracy of the “fine” or bandspread dial was a function of the accuracy of placing the “coarse” dial to the exactly correct spot.  If the coarse dial was off, then fine dial was off.

To help make resetability a bit better, if your receiver had not come with one you could add a 100 Kc. crystal calibrator.  This was a small circuit, often built on a small box chassis, which had usually a single tube and a 100 Kc. crystal.  It was electrically attached to the receiver’s antenna input and when powered up it would place a signal every 100 kilocycles from 530 Kc to 30 Mc.  The crystal was netted onto the exactly frequency by listening to WWV on any of their frequencies and zeroing the 100 Kc crystal to them. This let you place your bandspread dial on, say, 7100 Kc., turn on the calibrator, and tune the coarse dial to the closest signal around the 40 meter band dial spot.  At that point, you knew exactly where was 7100 Kc.

If the bandspread variable condenser was linear, you could be fairly sure that when the dial said 7125 that it was fairly close to 7125.  Later calibrators added a divide by 4 circuit and then the “exact frequency” showed up every 25 Kc, which helped to make it even closer and more accurate.  Still, those earlier receivers were not as accurate as modern day digital dial radios.

Some older manufacturers like Hammarlund, Hallicrafters, and National would come out with a new model receiver in two versions (with different model numbers).  They had the basic same design and specifications but one would have two dials and be called an all-band model whereas the single dial unit would be ham-band only.   You bought the one that fit your needs:   all-band – able to use for broadcast band and shortwave usage, ham-band – good bandspread and frequency readout.  Now-a-days, we can easily have both and when the dial shows the frequency readout to great accuracy, we know we are “right on” that frequency.

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