What is your S-Meter actually displaying!

To check the S-Meter on a HF transceiver against the IARU 6 dB standard, you will need to follow a few steps:

1. Connect a known RF signal source, such as an RF signal generator, to the transceivers' antenna input.
   
2. Set the RF signal generator to output a signal at a frequency and power level that is appropriate for the transceivers' band and mode of operation.
   
3. Use an attenuator to reduce the RF signal level in 6 dB steps. You can use a set of switchable attenuators, a variable attenuator or individual attenuators to achieve this.
   
4. Connect a device that can read either power or voltage, such as a digital voltmeter (DVM), digital multimeter (DMM), power meter (PM), or cathode-ray oscilloscope (CRO), to the output of the attenuator.
   
5. Adjust the attenuator to reduce the RF signal level in 6 dB steps and record the corresponding S-Meter readings on the transceiver.
   
6. Compare the S-Meter readings to the IARU 6 dB per S-Unit/point standard to determine if the S-Meter is tracking correctly.
   
7. If necessary, adjust the S-Meter calibration on the transceiver to match the IARU standard.

It's important to note that the accuracy of the check will depend on the accuracy of the RF signal generator, attenuator and the measuring device. Which mean that to get the most accurate results it would be advisable if ones equipment had been checked against calibrated equipment.

In the past, I have used an Elecraft XG3, which I checked against a NIST calibrated LP-100A. Which in turn helped to characterise a set of switchable attenuators.

Below, you will see some photos depicting an array of attenuators and the XG3 with a step attenuator in action.

The picture on the left is showing the test setup using the Elecraft XG3 and a switchable attenuator to check S-meter tracking on my radio. As long as the test equipment has been checked against a know standard we can make fairly accurate checks using basic test equipment.

And here to the right, an assortment of attenuators.

Below is a pictorial/sketch of the current test setup I use to check the accuracy of a Radios S-meter.

Staring on the left is the signal-generator, followed by two attenuators, one in steps of 1dB and the next in steps of 10dB. The next device is a 6dB splitter which splits the signal from the signal-generator in two reduced signals. One signal going to a calibrated power-meter and the second signal to the test subject i.e. our receiver. We could replace the power-meter with a Voltmeter.

However, if the signal-generator has an accurate signal output display this can be simplified by removing the splitter and the power-meter. Using this test setup it is very easy to make sure that the check of the S-Meter is accurate. As the above picture shows, to compensate for the losses of the test setup, the signal generator is set to provide a -66 dBm signal. This is about 7 dB higher than the -73 dBm level to compensate for the additional losses, 0.7 dB from the interconnections and attenuators, and about 6.3 dB from the splitter.

Equipment used:

• Signal-generator    :HP 8656
• Attenuators         :HP-355C and HP-355D
• Splitter            :VK5HW
• Powermeter          :VK3AQZ RFPM1

NOTE: Whatever signal source or attenuator you use, make sure that its accuracy has been checked against a known reference.

The equipment I've used has been checked and aligned where necessary against NIST-certified instruments including an LP-100A RF power-meter, a Boonton RF power-meter with a 51011-4B sensor, a Fluke 8842A DMM, and a Brymen BM-869s DMM.