Audible Transmitter Output Monitor (ATOM) Instruction Manual

Table of Contents


ATOM is a simple tuning aid for blind or low-vision hams who cannot access an internal or external power meter or want an qualitative confirmation of correct output power. For stations that have initially been installed correctly, ATOM will give a recognizable audible alert if something is preventing a transmitter from putting out full power, and causes your CQ's to go unheard. ATOM will catch most common oversights such as not setting VOX correctly, not being in the correct emission mode, not activating an automatic antenna tuner, and also catch more serious problems such as damage to feedlines or antennas when the transmitter has circuitry to limit RF output due to high VSWR.

If you cannot read a power meter or have other quantatative means to know that output is being generated, use ATOM. You'll find that ATOM gives you a very information-rich indication of the RF activity on your feedline, building confidence that all is working as intended.

ATOM is not a VSWR meter. While such meters give you more information, they are also slightly more complicated to operate, and much more expensive. Their capability is more than is needed to answer the question "is my transmitter continuing to work as well as it did when it was initially installed?"

ATOM is connected between the transceiver and feedline (or tuner, if external). It has two SO239 connectors, and either one may be used as the input. When turned on, ATOM sounds a tone whose pitch is related to the transmitted power. Much information can be gleaned from its audible signals, such as how the power varies while an automatic antenna tuner searches for the best match.

ATOM will work well with transceivers having built-in tuners feeding antennas such as G5RVs, or others with VSWRs less than approximately 4:1, or for use with remote antenna tuners connected to any type of antenna. It works between 1 and 54 MHz and with power levels up to around 150 watts.

When used with a remote antenna tuner such as an SGC234 mounted at the feed point of the antenna, ATOM will emit a series of warbling tones while the tuner's program matches the 50-ohm coax to the transmitter/tranceiver. A steady tone is emitted once a good match is found, and subsequent transmissions on the same frequency will yield an immediate steady tone because the tuner will have previously memorized the best settings. ATOM can also be use with external autotuners in the ham shack, but these units usually make enough relay noise while working as to make for less need for ATOM. However ATOM might be enlightening when the tuner is trying to achieve a match close to its limits, if the tuner gives up, and there's no output!

Important warning: Always have ATOM connected to a feedline, whether it be to an antenna, external auto tuner or linear amplifier. If you don't terminate ATOM, an auto tuner in your transceiver will try to tune ATOM's reactive impedance as if it were the output load. This will produce high voltages, even at power levels like 25 watts. Beware!

The circuitry inside ATOM senses the RF voltage on the feed line, and uses it to control a voltage-controlled oscillator (VCO). Unlike a VSWR meter, ATOM is responsive to both forward and reverse power, but still gives a very good idea of what is happening on the feedline. When a transmitter is matched to 50 ohms, it responds to one watt signals with a buzz at approximately 40 Hz. For a matched 100 watt signal, the pitch is around 1KHz, increasing to around 1.5 KHz at 54 MHz. The front end of ATOM is safe for RF voltages up to around 700 volts peak, so its 150W rating for VSWR's less than 4:1 is conservative.

The power switch is "on" when flipped towards the speaker grille. The batteries will discharge after about 500 hours of operation with ATOM turned on but with no RF present, so the switch should be turned off when ATOM is not in use. ATOM should not be used as a side tone monitor since that would shorten the battery life to several hundred hours. In normal use the batteries should last their shelf life, approximately seven years.

How I use ATOM

Changing the Batteries

ATOM uses four triple A batteries. When their voltage drops and they need to be replaced, you will notice that ATOM's audio volume drops precipitously and pitches tend to rise for the same power output. This happens at approximately 50% voltage.

To change the batteries, remove the four flat-head screws in the corners of the box. Replace the four batteries as a set, with the negative side of the batteries against the springs in the battery holder.


This device is really very simple and not much can go wrong with it. But if you think the device is not responding when it should, you can try these things:

Contact Information

If you need to contact the manufacturer of this device, send email to, phone 650-386-6286 or visit


Theory and Word Description of Circuit

ATOM measures the peak RF voltage on a feedline and provides reassurance that an amateur radio transmitter is operating correctly and actually putting out power. The transmitter's feedline is routed through ATOM's housing. ATOM then uses a voltage-controlled oscillator (VCO) U1 to generate an audible pitch proportional to the signal level on the feedline. ATOM's VCO uses an LM555 timer IC, U1, packaged as an 8-pin DIP.

The input and output SO239 coax sockets are grounded to the metal case. The minus side of the 6 volt battery is also grounded, and the positive side is fed through a SPST toggle switch S1 to provide Vcc to the LM555 IC U1 oscillator.

Pin 1 of the VCO U1 is grounded, and pins 4 and 8 are connected to Vcc. Pins 2 and 6 are connected together and to the timing capacitor C1, a 0.1uF dip ceramic capacitor, whose other end is grounded. A 5K 1/4W discharge resistor R2 is connected between pin 6 and the discharge pin 7. Pin 3 connects to a 32-ohm speaker (SPKR1), whose other side is connected to Vcc.

A jumper wire connects the center pins between the SO239 connectors and to the anode of D1 on the circuit board. D1 is the first in a string of eight 1N4148 diodes (D1-D8) used to sample the RF voltage on the feedline. The rectified voltage, which can reach several hundred volts, is fed from the cathode of the last diode D8 through a 100K 1W metal film resistor R1, to U1's pin 7.

In operation, voltages on the feedline are rectified by the diodes and used to charge the timing capacitor C1 by sending current through resistor R1. When the voltage on the capacitor reaches a certain value, the timer U1 triggers, and causes its pin 7 to be grounded, which discharges C1 through R1. When RF is present, this process repeats itself continually. Each time U1 cycles, a pulse is produced on pin 3, which is heard in the speaker. How fast these cycles repeat depends on the voltage on the feedline. They will repeat several tens of times per second for single-digit power-levels, and many hundreds of times per second for powers of 100 watts or more.

Note: For a schematic drawing, access or download the Word version of this manual.


I am indebted to Gary Gordon, K6KV, who guided me through the process of producing ATOM. Gary is a Handiham volunteer and helps handicapped hams living in the Silicon Valley.

Quick Links