November 1957 Radio & TV News
These articles are scanned and OCRed from old editions of the Radio & Television News magazine. Here is a list of the Radio & Television
News articles I have already posted. All copyrights are hereby acknowledged.
This 60-year-old design for a coaxial line RF monitoring instrument
uses components still readily available since it has no vacuum tubes
and you can still buy the
1N34 germanium diodes that are used as detectors. Only 18 components
(including jacks, meter, case, and switch) are used to indicate
relative power, modulation percentage, and to monitor the signal
modulation. Sampling is done with a high impedance tap on a
through connection so that the impact on characteristic impedance
on whatever coax you are using is negligibly affected.
Coax Line R.F. Monitor
Robert F. Lewis, W8MQU
Output meter for ham shack measures relative power, modulation
percentage, and monitors the modulation.
Views of r.f. monitor. The shielding cover has been removed
in bottom view.
Most amateur radio operators these days, are fairly well supplied
with instruments for measuring the various operating characteristics
of their equipment. Practically any ham can determine, with reasonable
accuracy, his operating frequency or final amplifier power input.
Very few stations, on the other hand, have any facilities at all
for determining the quantity or quality of r.f. output into the
transmission line or antenna system.
In an attempt to help fill this gap, an r.f. output meter was
developed which provides for the monitoring of: 1) relative carrier
output power; (2) amplitude modulation percentage; and 3) aural
monitoring of modulation. In view of the almost universal use of
coaxial output circuits the instrument was designed to be inserted
into a coaxial line without upsetting the characteristics of the
The circuit of the monitor is very simple. No external power
source is required and the total cash outlay for component parts
should not exceed ten or fifteen dollars, depending on the cost
of the microammeter.
Briefly the monitor functions as follows : Resistors R1
and R2 form a voltage divider network across the coaxial
line. That portion of the r.f. line voltage which appears between
the junction of the two resistors and ground is rectified by CR1,
a 1N34 germanium diode. The rectified current passes through an
r.f. filter composed of RFC1 and C1, through
calibrating resistors R3 and R4 and then through
M1 (when S1 is in the "R.F." position). The
audio component of the signal passes through C3 and T1
and is rectified by CR2. The rectified current is indicated
on M1 when S1 is in the "MOD." position. Thus
it is possible to read either the relative r.f. carrier level or
modulation percentage of a signal by merely throwing S1
to one position or the other. Output for aural monitoring is available
at J3. Inter-stage transformer T1 is connected
in a stepdown arrangement to provide a better match between the
low-impedance load and the high-impedance primary circuit.
Schematic and parts list of r.f. monitor.
R1, R2 - See text
R4 - 50,000 ohm pot
- .001 μƒd. disc ceramic capacitor
- .01 μƒd. ceramic capacitor
.1 μƒd., 400 v. capacitor
RFC1 - 100
μhy. r.f. choke
M1 - 0-200 μa. meter
T1 - Interstage trans. 3:1 ratio, connected step-down
(Merit A-2910 or equiv.)
S1 - S.p.d.t. toggle switch
J1, J2 - Coax. connector (Amphenol
J3 - Open-circuit jack
CR2 - 1N34 germanium diode
The resistance values of R1 and R2 are
not given in the parts list as they must be determined for each
individual case. The total network resistance (R1 plus
R2) should be roughly one-hundred times the nominal line
impedance, that is, between 5,000 and 7,500 ohms. It can be readily
seen at this point that the monitor will draw a very insignificant
amount of power from the transmission line, probably not more than
one percent. The ratio of R1 to R2 should
be chosen so that between 5 and 10 volts of unmodulated r.f. will
appear across R2. Much more than this may damage the
germanium diode, CR1, especially with amplitude modulation.
The total power-dissipation rating of R1 plus R2
should be one percent, or more, of the expected transmitter power
output. Both resistors should be of the non-inductive carbon type.
All other component values remain as indicated in the parts list
irrespective of transmitter power rating. It should be noted, however,
that calibrating resistors R3 and R4 were
chosen for use with a 0-200 microampere meter. In the event that
a meter of different range is used, it would be advisable to change
the values of R3 and R4. Thus if M1
were to have a range of 0-100 microamperes, then the values of R3
and R4 should be doubled. The use of a meter of greater
than 1 milliampere range is not recommended.
The construction of the instrument can assume many variations.
However, several points should be observed. First, the unit should
be built in a metal enclosure. The two coaxial connectors should
be mounted close together and their center studs connected by a
heavy wire. Resistors R1 and R2 should be
soldered directly from the coaxial circuit to the nearest available
ground point, preferably to one of the coaxial connector mounting
screws. The resistors should be spaced away from other metal parts
in order to prevent stray capacities which might upset the characteristic
impedance of the line.
Inspection of the photographs will show the mechanical arrangement
of the author's monitor. The case is a standard 3"x4"x5" aluminum
box. On the front panel are mounted the microammeter, S1
and J3. Calibrating resistors R3 and R4
are mounted on the right side of the box, while transformer T1
is atop the chassis on the left. The following components are mounted
on a terminal strip at the back of the case: R1, R2,
CR1 RFC1 and C1. Care should
be taken in the soldering of the crystal diodes to prevent damage
from excessive heat. This can be accomplished by holding the leads
with long-nose pliers while soldering.
Accurate calibration of the monitor for observation of modulation
percentage requires the use of another modulation indicator of known
accuracy or an oscilloscope capable of showing the trapezoidal or
wave-envelope modulation pattern. With the instrument connected
in position in the coaxial line, but before applying power, turn
both R3 and R4 to zero (arm at ground end).
Throw switch S1 to the "MOD." position. Turn on the transmitter
and adjust for 100 percent sine-wave modulation, using an audio
oscillator or some other steady signal source. Turn up R3
until the reading on the meter comes up to a point arbitrarily picked
for 100 percent modulation. Now throw S1 to the "R.F."
position and increase R4 until the indication is the
same as that obtained in the "MOD." position. From this point, the
setting of R4 should be left unchanged. Any future adjustment
necessary to bring the r.f. reading to the reference point should
be done with R3. Due to the nature of speech waveforms,
100 percent voice modulation indications will occur at 60 to 70
percent of the sine-wave reading. Thus, if an audio oscillator gives
a reading of 100 on the meter for full modulation, then average
speech readings should be around 60 or 70.
This little instrument will work with transmitters of any power
and on any frequency. It really comes into its own in v.h.f. applications
where other types of indicators frequently fall down. Perhaps one
of the best features is that the monitor will permit compliance
with FCC regulations regarding the checking of modulation percentage.
Posted September 22, 2014