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May 1952 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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Have you ever made a power or noise figure
measurement and had to scratch your head over why the reported value made absolutely
no sense? How about measuring a positive gain value through a passive device? I
have experienced this sort of nonsensical phenomenon on more than one occasion,
and to my recollection every time the cause was stray frequencies - often oscillations
- mixed in with the intended signal. This app note from a 1952 issue of the ARRL's
QST magazine addresses the issue in regard to impedance matching networks
that are wideband enough to support RF energy that is outside the band being tuned
- or even unexpected inband signals - with a simple method of avoiding such surprises.
Note on S.W.R. Measurement
In adjusting an antenna coupler to obtain a 1 to 1 standing-wave ratio in
a coax link to the transmitter, it sometimes happens that the best possible adjustment
does not bring the meter reading down to a null. This could be caused by insufficient
matching range in the antenna coupler, or might even be chargeable to a faulty .s.w.r.
bridge. However, there is a good possibility that the trouble is impure r.f.
One of the first things that should be done after building a resistance-type
s.w.r. bridge is to check it with a noninductive resistor of a value equal to the
impedance for which the bridge was designed. If a good null is obtained the bridge
is OK. In making this kind of check it does not matter if more than one frequency
is present in the r.f. applied to the bridge, because the test resistor has little
or no frequency consciousness.
An entirely different situation exists when the bridge is used to check a transmission
line that feeds an antenna, either directly or through an antenna coupler. The load
in this case is definitely selective, and it can be made to look like a desired
value of pure resistance only at one frequency or over a small band, in terms of
percentage. If the r.f. applied to the bridge contains more than one frequency -
for example, harmonics - the s.w.r. at the "off" frequencies usually will be very
high, and a relatively small voltage will give a good-sized meter reading. Since
adjustments to the coupler or antenna matching system have little effect at any
frequency other than the operating frequency, the bridge gives a false indication.
Troubles of this sort are most likely to occur when the output stage in the transmitter
is a frequency multiplier, or is a straight amplifier connected to a frequency-multiplier
driving stage in such a way that either a submultiple or a harmonic of the output
frequency can be amplified and applied to the bridge. A check with a crystal-detector
wave meter at the final tank will show whether a frequency other than the desired
one is present in appreciable amplitude. If so, a tuned trap can be inserted in
the line between the final stage and the bridge to take it out. The wavemeter check
should be made with the amplifier operating at the power level that gives a full-scale
reading on the bridge.
Resistance bridges take so little power that it is sometimes a problem to cut
it down enough. However, with a tetrode final there is a simple solution - disconnect
the screen voltage, ground the screen, de tune the plate circuit, and reduce the
excitation by detuning somewhere in the exciter chain if no other means is available.
Then when the bridge is connected the full-scale initial setting can be approached
by retuning the plate circuit toward resonance and, if necessary, increasing the
excitation. This method usually gives ample control over the output without requiring
a reduction in the amplifier plate voltage, since the plate input will be small
with the screen at zero d.c. voltage. - G. G.
Posted July 1, 2024
(updated from original post
on 6/17/2016)
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