November 1957 Radio & TV News
Wax nostalgic about and learn from the history of early
electronics. See articles from
Radio & Television News, published 1919-1959. All copyrights hereby
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 line.
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
R3, R4 - 50,000
C1, C4 - .001 μƒd. disc ceramic capacitor
C2 - .01 μƒd. ceramic capacitor
C3 - .1 μƒd.,
400 v. capacitor
RFC1 - 100 μhy. r.f. choke
M1 - 0-200
T1 - Interstage trans. 3:1 ratio, connected step-down (Merit
A-2910 or equiv.)
S1 - S.p.d.t. toggle switch
- Coax. connector (Amphenol 83-1-R)
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
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
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