Amplifier remote monitor with EMF compliance indication.

One of my recent projects has been building an HF linear amplifier based on some boards purchased from the recent silent key sale.  The amplifier is an A600 version 2.0 by M0HZH, based on two MRF300 LDMOS devices.  The current board version is 2.3 and I have included some of the changes in my version, as well as some other modifications.  The amplifier is capable of 600W CW, I limit it to 300W on SSB to keep things linear.  I have not yet implemented a sampler to provide feedback to the SDR software for predistortion (PureSignal) correction.

For this amplifier I developed my own control and monitoring software to properly sequence the relays, bias and so on, select the appropriate low pass filter and to shut down the amplifier should something go wrong that could risk damaging the device, such as high input power, high reflected power, high voltage, high current, high temperature etc.  An LCD touch display on the amplifier allows the various powers, current, temperature, voltage and so on to be monitored.  If a trip occurs the cause of the event is displayed and the trip can be reset.

The ultimate plan is to put this amplifier (and the Hermes Lite 2 SDR radio) in the garage, close to the antenna feedpoint and further away from noise sources, therefore a method of remote control and monitoring will be required.  A remote tuner will also be needed.  That project is still in the concept stage but is likely to involve stepper motors and variable capacitors rather than switching relays.

The amplifier controller is based on an STM32 black pill board that provides the 12 bit A-D convertor and runs a fast loop to provide rapid trip response.  The Black Pill sends data to an ESP32 that handles the slower stuff such as display updates, monitoring for touch input and so on.  The ESP32 has WiFi as standard and enough memory and processing power to run a web server, and it proved quite simple to create some web pages to allow the amplifier to be controlled and to display the power, temperature and so on.

As I had not used the Python programming language before I decided to write a Python application to provide the remote control and monitoring.  Python is cross platform so the same application should run on Linux and Python, maybe even a Mac - I have only tested on Windows so far.

This is what the display looks like on the PC:

The tabs allow different views of the data - I like this trend view showing the activity over the previous 6 minutes (lots of CQ calls on a dead band).  The green trace is the peak output, white is the average, the red line is the temperature.  The buttons allow the application to connect or disconnect from the amplifier, turn the amp on or off, reset any trip and change antenna.  The operating frequency is displayed, along with the band and selected low pass filter.

Most interesting however is the display of the percentage of the EMF limit that has been used over the last 6 minutes.

When I did my compliance assessment (you have done yours haven't you?)  I found that on the upper bands I might have a problem.  My antenna is an Off Centre Dipole (OFCD) that starts at the top of the roof and heads off down to the end of the garden.  The antenna is 8m high at the lowest point but it is only 4m away from the house next door up the hill.  The hill is quite steep so the antenna is below the top of their loft conversion.

Using the RSGB web based emf calculator, which is very easy to use and recommended, the mode was set to FM, transmit% to 100 and the power adjusted until the compliance distance was 4m using the ICNIRP 2020 limits.

This power is then the average power I can put out over 6 minutes, in my case 135W on the 10m band to remain compliant.  The six minutes comes from the ICNIRP guidelines, which limit the exposure for the general public to 2W/kg averaged over six minutes.  The aim is to keep heating within the head and torso of the body caused by EMF to below 2C.  Interestingly the limit for limbs is 4W/kg averaged over six minutes.

The python code has the power limit for each band stored within it.  The average power transmitted over the last 6 minutes is calculated and displayed as a percentage of the allowance.

If the limit (100%) is approached then one can hand back to the station you are in QSO with and hope they woffle on for a bit, turn the power down, reduce compression, or if you know the neighbour is on holiday or out just keep going!

The trend display shows the effect of various compression settings on average output power quite clearly.

It would be simple to include this EMF limit facility into the power meter I designed some years ago, or for manufacturers to include the feature in their amplifiers.

In summary the Python application provides useful remote monitoring and control of the A600 amplifier, while enabling operation to be within the EMF limits.  This tool should enable operating confidently using higher powers while enabling various strategies to remain compliant.