Winter 2009 Issue

A Nostalgic 6-Meter
Radiotelephone Station

Do you remember the good old days of AM modulation? If you do, then you will appreciate the fact that there are others who do, too, and are doing something about their nostalgia. Here K8VBL describes what one group in western Michigan is doing and how to build a station in order to join them the next time the band is open in that direction.

By Thomas Turner,  K8VBL

Photo A. The 6-meter radiotelephone station. Left to right: speaker-microphone, transmitter, antenna coupler and SWR meter, receiver, and speaker. (Photos by the author)

Radio amateurs are fortunate to have been allocated the 6-meter band, 50–54 MHz, because “Magic Band” signals in this part of the radio spectrum are propagated by every known mode. In fact, the cause of sporadic-E, one of the most interesting modes of propagation, remains to be discovered. In this period of low sunspot activity, the beginning of Cycle 24, F2 propagation is rare. Sporadic-E is seasonal, centered around the equinoxes (early summer and late December).

To maintain interest in the “Magic Band” during periods of minimal DX propagation and promote experimentation, the West Michigan Six Meter Net was formed. The net meets Tuesday evenings at 9 PM local time on 50.3 MHz SSB. Net control stations in Kalamazoo conduct the net as a round-table, with over 20 stations checking in. At around 9:30 PM the net shifts to 50.4 MHz AM. Many of the AM stations check in using a 1950s-vintage Heathkit “Sixer” transceiver, also known as the “Benton Harbor Lunch Box.” A “WAS” certificate (Worked All Sixers!) is issued to those who use a “Sixer” to contact six other “Sixers.”

How About a Homebrew 6-meter AM Transmitter?

Using a loaner “Sixer” with about 2 watts output into my 3-element Yagi (CQ VHF, Winter 2006 issue), the West Michigan Six Meter Net proved to be a lot of fun and a good way to meet some interesting people. Then I thought, why not build a 6-meter AM rig with a little more power? A look in my junk box revealed all the necessary parts, including a half-bushel of 6L6 tubes from my dear, bygone days of the ’50s repairing juke-box and electric-organ amplifiers. These tubes, dubbed the “poor man’s 807 or 6146” were introduced in the spring of 1936 by RCA (QST, May and June 1936) as the first of the beam power tubes. Radio magazine for April 1937 carried an article “The Bi-Push Exciter” by W. W. (Woody) Smith, W6BCX, Editor. Using two 6L6 tubes in a push-push doubler connection, the 80- through 10-meter exciter soon became very popular. A bi-push is very efficient because it gives a pulse of power every half-cycle to a plate tank circuit tuned to the second harmonic of the grid (input) circuit. Excellent rejection of other harmonics is provided also. The newer 6L6GC tubes have about half as much input and output capacitance as the older type, and function well at 50 MHz. Design parameters selected for the transmitter were: simple oscillator and power amplifier using common tubes, and amplitude modulation using the Heising (see sidebar) principle.

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