1924: A Five Watt Sending Set for $25

The first question about a transmitting set that a fellow wants answered, —or maybe it is his dad who wants answered—is, “How much is this thing going to cost?” Well, that all depends. You can buy all of the fixin’s and do-gadjits and make a set that cost a couple of hundred dollars and still have only a 5-watt set, or you can buy only a few things, make the rest of the parts yourself, hook ’em up in a good old circuit, and talk to amateurs several hundred miles away for $25. The latter sounds the best.

Mason, H.F., “A Five Watt Sending Set for $25” QST, Vol 8 No. 2 (Sept 1924), p. 56.
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Central Electronics 10B Exciter Restoration

Just as I got it

This past spring I picked up a Central Electronics 10B exciter. This is a low power radio transmitter that was part of the on the first generation of single-sideband voice on Amateur Radio bands in the 1950’s. You can think of these transmitters as full-carrier double-sideband AM transmitters that use phasing circuits to cancel the carrier and the unwanted sideband. In fact, if you ever written software for an SDR (like one of Tony Parks’ softrock kits) its pretty much the same complex multiplication using a quadrature signal, just with analog parts rather than software. What’s old is new again!

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1920’s-Style Homebrew Mica and Glass-Plate Capacitors

One of those not-so-safe things. This transmitter is full of homemade components.

Building a radio with homemade condensers (that’s the old word for capacitor I’ll be using herein) is an art as old as radio itself, but one that had largely passed away by the mid 1920’s. Constructing a condenser for your spark transmitter was a right of passage in the days before radio licenses. The old spark sets often used tin foil and various materials for dielectric: glass, mica, oil or oil-soaked paper, etc. Air could be used, but it was both mechanically difficult and the relatively low dielectric constant and breakdown voltage was a challenge for the amateur who needed a large capacity at at least a few kilovolts for a small set. Oil dielectric was a messy but viable option, especially for its self-healing properties in case of an arc (and is still used in high-voltage capacitors today). Power loss due to corona discharge from the edges of a the metal plates in air were a problem with the RF voltages involved, and this was often mitigated by rounding corners of the plates and even immersing the whole sandwich of materials in a high-dielectric-constant binder such as shellac or paraffin. Using a solid binder also tended to improve the physical construction. Discarded window glass and 4×5″ photographic plates were popular materials; it helped to be friends with a glazier or photographer.

For typical example of homemade glass-condenser construction for a 1 kW amateur transmitter, see the July 1914 issue of Wireless Age, p. 847, available over at American Radio History. The only materials are shellac, glass, and tin, so quite a bit of of money would be saved compared to purchasing a commercial mica condenser, like the ones here:

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A “Better” Crystal Oscillator for the DL4YHF Frequency Counter

TL/DR: I like DL4YHF’s idea of making a frequency counter out of a microcontroller. I don’t like the Colpitts crystal oscillator that my Chinese clone added to it since it won’t work below 5 MHz very well. I made a better one with a 74HCU04 hex inverter an a couple extra parts.

That 400V bypass cap has delusions of grandeur.

In the last post we saw that the ever-popular “1 Hz to 50 MHz Frequency Counter and Crystal Tester” came up a bit short in the “crystal testing” department when it came to testing crystals in the low HF range (< 7 MHz or so). Now, if you really need a crystal oscillator and frequency counter for crystals in this range, I think there is a better way for the money and time, but if you already have one of these things and want it to work with lower frequency crystals, it might be worth doing something like the following. (H/T: ZL2PD had similar thoughts, though his clone has more power in the oscillator and probably does a little better at low frequencies out-of-the-box.)

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