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Questions and Answers

I like reading old magazines. I came across a Q&A column in one and found some useful info, so I thought I'd start collecting and sharing them here. I am presenting them in chronological order. They are copied here verbatim, though I have used modern abbreviations and conventions (AM/FM instead of a.m. & f.m., KHz for KC, etc.).

Originally this was going to only be Q&As, but there weren't that many and I found other things peppered about; so it's expanded a bit. These are things I found informative, interesting, useful, or simply amusing. I did not write any of these myself and I will not vouch for their accuracy, completeness, utility or safety. Use them at your own risk.

Many of the magazines can be found here: AmericanRadioHistory.com. Look for Early Journals and Site Features on the top menu bar for the old stuff.


Selecting an Antenna Wire

In selecting wire for antennas it is too often the practice to simply go into a radio store and ask for so many feet of antenna wire, taking pretty much whatever the dealer offers. Where the wire is to be used for an ordinary "L" type antenna for broadcast reception there is little harm in this procedure because the exact length of the antenna is not critical, nor the strain on the wire great.

Where the antenna is to be used for transmission, or for a self-resonant receiving antenna greater care is needed. Here the antenna length is critical and usually the antenna must be stretched taut under a considerable amount of tension. Moreover, if a tree is used as a support the wire may be under tremendous strain during storms. For such uses, solid wire is usually employed and is generally available in hard-drawn and soft-drawn forms.

Hard-drawn wire is recommended for such critical service. The reason is that it lias close to twice the strength of the soft-drawn variety; and what is more important, soft drawn wire under strain will stretch as much as 25 per cent as against less than 1 per cent for hard-drawn wire of the same size and subjected to the same strain. Actual tests made on No. 12 wire showed the soft wire broke at 150 pounds pull whereas the hard-drawn broke at almost exactly 300 pounds. Just before breaking, the soft wire had stretched from 5' to 5', 11", whereas when subjected to the same strain of 150 pounds the hard-drawn wire showed no appreciable stretch. At just short of 300 pounds pull the hard-drawn wire showed elongation of approximately inch.

To avoid sag, and for the antenna length to remain fixed, use hard-drawn wire. If the strain is likely to be greater than 300 pounds use copper-clad steel wire which is still stronger.

Radio News, May 1938, p. 30

Sharing an Antenna

S.R.S., Bangor, Maine: is it good practice to operate two receivers from the same outside antenna? One is a short-wave set and the other I use for just local broadband reception.

Answer: Yes, it can be done. Connect .002μF condenser at antenna post of each set, and then to same antenna.

Radio News, May 1938, p. 36

Adding an "R" Meter

J.T.Q., Santa Barbara, Calif: Is there any way i can add an "R" meter to my all-wave superheterodyne receiver? I have an 0-10 milliampere meter. Would this be suitable?

Answer: If your receiver includes automatic volume control it is a simple matter to add a signal-strength meter. The AVC system causes the grid-bias and therefore the plate current of the controlled tubes to vary in accordance with the strength of the signal. A strong local signal will result in a very high grild-bas and low plate-current whereas a weak signal will permit almost normal plate current to flow. By placing a milliammeter in the B-plus lead to the IF amplifier these variations in plate current provide a relative measure of signal strength. The widest range of meter variation is obtained with a meter which will just show full scale defelction with normal plate current, which is the plate current when no signal is tuned in.

The best way to accomplish this is to insert the meter in the B-plus supply to two tubes if it is an 0-10mA meter because the two tubes will draw more than 10mA. The meter is then shunted with a 100-ohm rheostat and the rheostat adjusted until the meter reads full scale on no signal. Weak signals will then retard the meter slightly while local signals may drive it back almost to zero, depending on the type of tubes used and other factors.

You can calibrate the meter in terms of the "R" scale by comparison with a receiver having a commercial "R" meter, or you can do it approximately by noting the meter readings on what you would consider an "R-9" signal, an "R-8" signal, etc. and then adopting these as standard. This is not exact, of course, but then neither is the "R" scale exact, its interpretation varying widely with individuals.

Radio News, July 1938, p. 63

Pre-Selectors vs. Converters

F.O., Cleveland, Ohio: What is the difference between a pre-selector and a converter; both units to be used ahead of a receiver?

Answer: A converter consists of an oscillator and mixer similar to those in super-heterodyne receivers. its purpose is usually to extend the range of a standard receiver. In using the converter ahead of a receiver, the receiver is tuned to some frequency within its range and left that. It serves in effect as the intermediate amplifier for the combination, the converter providing the tuning. A pre-selector is a straight tuned radio-frequency amplifier, the purpose of which is to provide greater sensitivity and better image selectivity. Both it and the receiver must be tuned.

Radio News, July 1938, p. 63

Changing the Crystal Frequency

If you with to raise the frequency of your crystal, the procedure is not so difficult as you might imagine. Ordinary kitchen cleanser such as Old Dutch or Bon-Ami powder will act as an excellent grinding material. Place a small amount of the powder on a piece of plate glass and use just enough water to make a thin paste. Then simply polish down the crystal by rotating it in the paste solution, pressing down firmly in order to obtain an equal amount of pressure over the entire surface of the crystal. Do not attempt to grind too much, but after a minute or so, wash the crystal and test it in the oscillator.

To lower the crystal frequency, apply India drawing ink to both surfaces with a small brush or mark them evenly with a lead pencil. Frequency response can usually be lowered from 10 to 30 KHz. The lead pencil method has the advantage that one can merely erase a few kilocycles when this is desired.

Radio News, November 1938, p. 39 (Radio Gadgets)

Death Ray

F.C.H., Miami, Fla.: I've been hearing a lot about this so-called "death-ray" which is supposed to kill bugs and parasites on grain and wonder if this could be used on trees in our orchard? Can you give me any information on this subject?

Answer: The "death-ray" is simply a rather powerful short-wave transmitter, 250 watts or more, operating at some frequency between 3 and 30 megacycles. When grain is passed through a strong high-frequency field, germs and bugs are killed but the grain is unaffected. This same method is also used experimentally for purifying water. it would not be practical to attempt to treat an orchard in this manner because each bough and twig would have to pass through this electrical field. We understand that considerable experimental work on such apparatus was done at Rutgers University.

Radio News, November 1938, p. 46

©opyright by James Ollinger. All Rights Reserved.