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September 1950 QST
 Table
of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
from
QST, published December 1915 - present (visit ARRL
for info). All copyrights hereby acknowledged.
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In classic National Company style, this full-page
advertisement took the form of a mini tutorial / anecdote about using equipment and devices
they sold - both for Hams and for design engineers. September 1950's topic, which appeared
in QST magazine, was the use of noise limiters in CW (Morse code) transmitter / receiver
sets. The noise limiters functioned as squelch controls and were adjustable for both positive
and negative excursions. According to John J. Nagle (K4KJ), writing on the QSL.net website,
"HRO" stood for "Helluva
Rush Order," which was a rearrangement of the originally proposed "Hell Of a Rush" that came
about because of a hurried modification to an existing product.
National Company Advertisement
{Number one hundred ninety-seven of a series}
 It is interesting to note the increasing interest lately
in the use of noise limiters for C. W. use. Of course, the value of these simple little devices
for phone operation has been well established for some time. The usefulness of of such a device
was brought home quite forcibly to the writer while operating 80 meter C. W. (believe it or
not!) at W1WB/1 during the recent field day. The transmitter was a surplus job that left the
receiver running while the transmitter was keyed. The head-phones clamped over our ears with
huge rubber earflaps that must have been designed for ears much larger. Every time the key
was operated, a terrific clatter rattled through our head like a rivet hammer in the subway.
We tried removing the phones during transmission but with one hand holding the key on a knee
and the other hand operating it, there was too much delay getting rid of the key and getting
hold of the phones again. And you know how much time a C. W. man will allow you for such things
during Field Day operation! In this instance, a noise limiter would have been worth while
just to save wear and tear on the ear drums.
While a single-ended noise limiter with a fixed adjustment is satisfactory for phone operation,
it is far from adequate for good noise-limiting with C. W. signals. At National, we recognized
this fact years ago and that is the reason that the noise limiter used in the HRO-7 and HRO-50
has for many years been of the double-ended type with an adjustable rather than a fixed threshold.
Phone limiters usually are adjusted to clip the positive modulation swing at about forty
per cent with one diode. Further, this adjustment can be made automatic so that this clipping
level can be maintained as the signal strength varies. A second diode is not needed to limit
the negative modulation swing as this is taken care of by the detector diode itself.
When receiving C. W. signals, conditions are considerably different. The operating level
is now usually set not by the signal but by the strength of the injected BFO carrier! If a
signal is being received that is many times weaker than the BFO injection, noise peaks can
now go to several hundred per cent of the signal voltage in both positive and negative directions.
The limiter which worked so well on phone is now almost useless. It is now necessary to clip
both sides of the audio signal delivered from the second detector and to adjust the clipping
level to match the amount of audio recovered. Only then can good C. W. limiting be obtained.
The above is just what the HRO limiter was designed to do years ago. It also helps to keep
key clicks from wrecking your ear drums and can be used as a C. W. AVC as mentioned in a recent
QST article. If you C. W. men have not been using your limiter for these purposes, you haven't
been getting the most out of your HRO.
Cal Hadlock, W1CTW
Here are all the National Company advertisements I have:
- National's Company:
Merry Christmas and Happy New Year, January 1941
- National Company:
Noise Limiters, September 1950
- National Company: Modulators, April
1942
- National Company: Switches,
May 1939
- National Company: TMS Condenser, March 1939
- National Company: TVI, October
1953
- National Company:
Amateur Radio Activity, December 1952
- National Company: HRO-50
Receiver, May 1951
- National Company:
Noise Limiters, September 1950
- National
Company: Modulation, October 1947
- National Company:
Merry Christmas and Happy New Year, January 1942
- National Company:
Merry Christmas and Happy New Year, January 1941
- National Company: Switches,
May 1939
- National Company: AC Power Strip,
April 1939
- National Company: TMS Condenser,
March 1939
- National Company Catalog: NC80X
/ NC81X Receiver Advertisement, September 1937
- National Company:
IF Circuits, September 1935
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