In the beginning of Ham FM radio involvement in the 1950s, the only equipment available was from manufacturers such as Bendix (yes, the brake people), Aerotron (yes, the same people who brought Gonset to Hams), Federal Signal (which made rail-road equipment), and others such as Link, Carphone, and RCA. You might also have heard of Motorola and General Electric.
The equipment was commercial and required conversion to Ham frequencies. Most radios were only wired for single-frequency operation. You might get a working radio for free, but two crystals were required for each frequency (transmit and receive) and they cost $5 to $15 each. By informal agreement, all two-meter rigs (that is, converted high band equipment) operated on 146.94 simplex. There were no repeaters at first. All communications was station to station. Rigs and antennas were optimized for that one frequency.
Eventually, repeaters were invented. Actually, they were put into use on Ham frequencies, just like they were for commercial users. Physics and experimentation showed that repeater receivers at sites where the transmitter and receiver were at the same location suffered less desensitization if the transmit and receive frequencies were far apart. Ham repeaters used various separations, but that meant a tuned antenna or tuned receiver front end had to be adjusted quite differently from the tuned antenna for a transmitter.
What to do to ensure peak performance? Practically, it was discovered that a split of 600 kHz was about as far apart as you could separate a transmitter and receiver in frequency to enable a single antenna tuning network to work well for both functions. Of course, the antenna would be tuned for a midpoint frequency, and that opened a bit more of the band for other repeater frequencies to be within tuning range.
But it wasn't always that way! The concept of 600 kHz splits was not intuitive. It required repeater owners be convinced they should conform to this radical idea. That is, repeater owners and operators had to be convinced they should spend money to buy more crystals and tune antennas just for conformity to a concept that meant nothing to them locally, especially when the repeater was coming in just fine, thank you.
Now, aside from the practicalities of allowing as many repeaters in the available frequency allocation, this was the tightest frequency split that would work. As repeaters' ranges extended, it became necessary for repeater users and owners to adopt a wider set of principles. (There were overlapping super-repeaters everywhere. You could use a walkie-talkie in down-town Boston and talk to Hams in Albany!)
The big picture became more important than local concerns and needs. A national norm was needed, developed, and adopted. That norm was 600 kHz. A practical, local example of cooperation: In New England, we had three major repeaters, each of which gave fine coverage that overlapped in Boston and its north-western suburbs.
The Concord, NH repeater had an output on 146.94 and an input of 146.31: a 31-94 repeater. Waltham, MA repeater's output was 146.64, input 146.34: it was set on 34-64. (It had less than a 600 kHz split because its receiver and transmitter antennas were separate and far apart.)
Mt. Greylock, in North Adams, MA output was 146.91, input 146.04: set for 04-91. As you see, none used 600 kHz splits. Each repeater trustee was reluctant to change because of their expense and the expense of their users to buy new crystals. But reason prevailed. One fateful day around 1973, the repeaters realigned to 600 kHz splits. Users in each area voluntarily swapped crystals (some met in parking lots to make the exchanges) and few met with any expense. Concord gave its 31 input to Graylock: Graylock was now 31-91. Waltham gave its 34 input to Concord; Concord was now 34-94. Graylock gave its 04 input to Waltham: Waltham was now 04-64.
Agreement and consistency!
Today, 600 kHz splits are the norm nationwide and that frequency split is pre-programmed in all our radios. Now you know why.
Eric Falkof, K1NUN
