May 1952 QST
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
QST, published December 1915 - present. All copyrights hereby acknowledged.
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 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
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 June 17, 2016