Coax Line R.F. Monitor
November 1957 Radio & TV News

November 1957 Radio & TV News [Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Radio & Television News, published 1919-1959. All copyrights hereby acknowledged.

This 60-something-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. The article originally appeared in a 1957 issue of Radio & TV News magazine.

Output meter for ham shack measures relative power, modulation percentage, and monitors the modulation. 

Robert F. Lewis, W8MQU

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 step-down arrangement to provide a better match between the low-impedance load and the high-impedance primary circuit.

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, R_2, 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 November 4, 2021
(updated from original post on 9/22/2014)