Feedback - April, 2004 QST
At the risk of a malpractice suit, the Doctor
needs to clarify the question of speaker impedance raised in his December
column (The Doctor is IN, Dec 2003, p 52) and the related Feedback item (Feb
2004, p 54). The original proposition of using a 5 W,
2 W series resistor between the speaker and the transceiver is actually a much
better solution than a 250 W to 3.2 W transformer, which is
wrong. The peak power delivered from a transformer of that impedance ratio
would only be about 150 mW, while the series resistor would deliver about 1 W
to the speaker. Still, nearly half the total power output would be dissipated
by the resistor. The best solution, and one the Doctor stands by (presented in
February's Feedback) is to use an autotransformer, made from a common 70 V line
to voice coil transformer (RadioShack 32-1031) with an open primary. Thanks to
all who wrote in, but especially to Hans Glista, WA1LWS.
In "The Doctor is IN" of Feb 2004, the
television horizontal sweep frequency should be 15,750 Hz, not 15,750 kHz, as
printed. Thanks to Steve Bird, WS7R, for catching it. As an aside, the current
NTSC TV horizontal sweep frequency is 15,734.5 Hz, although it is frequently
referenced as 15,750 Hz (a carry-over from the old monochrome TV days).
In the article by Luiz Lopes, CT1EOJ,
"Designing a Shortened Antenna" (Oct 2003, pp 28-32), there is some question
about the value of the quarter-wave transformer at the bottom of page 32, the
first column. The length given was 27 feet. This is correct for cables having a
velocity factor of 0.83, such as Belden 1426A or 1694A. For cables with a
velocity factor of 0.78 (Belden 8212), it will be 26 feet, and for cables
having a velocity factor of 0.66 (Belden 8215 and 8241), it will be 22 feet.
Page last modified: 03:42 PM, 01 Mar 2004 ET
Page author: qst@arrl.org
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