Morse beacon keyer for a cluster of transmitters

This article describes another variant of the Simple Morse beacon keyer project. This one is is a simple implementation with six isolated KEY outputs that can be allocated to KEY or /KEY output signals.

There is benefit in using the same keying to all beacon transmitters on site as hearing one beacon provides knowledge of the keying pattern on others that might be weaker.

Key features:

Simple Morse beacon keyer design implemented on an ATTiny45/85;
capability for composite high and low speed Morse identification message to enable decoding under very weak signal conditions;
10-20dB better decoding sensitivity provides early warning of improving propagation;
field upgradeable firmware and message content with onboard connector using the popular and inexpensive Arduino RS232-TTL adapter;
six independent EMC filtered open collector outputs to drive a cluster of beacon transmitters;
outputs can be allocated to a mix of KEY and /KEY outputs;
12VDC power supply requires 20mA;
low cost, simple, robust and reliable;
optional capacity to select from bank of 8 messages based on alarm conditions or other contact closures.

Fig 1:

Fig 1 shows the implementation of the keyer on a small piece of Veroboard, and the inexpensive (A$7) Arduino programming adapter for the bootloader used.

Fig 2:

Fig 2 shows the underside of implementation of the keyer and the programming adapter.

Weak signal capture

Fig 3:

Fig 3 shows an example of Spectrum Lab capture of a very weak 2m beacon signal, inaudible to the ear, and using a 2 minute message cycle that includes "KW" in QRSS3. The example includes both 'direct' signal wit fading and Doppler shifted reflections from aircraft. The fuzzy sections are 12WPM Morse modulation.

Sleuthing the display can often lead to positive identification by exploiting knowledge about the message cycle duration, and sometimes associated aircraft reflections might help with identification whilst the intensity of the 'direct' trace indicates the underlying path conditions.

Schematic

Fig 4:

Fig 4 shows the schematic of the keyer. (There is in fact a filter F being 0.001µF cap to ground from each output at the ULN2003A and a series 100µH inductor to the output connector, but it omitted from the schematic.) This is a quite basic implementation of the Simple Morse beacon keyer, other enhancements are possible.

The DIP switch steers KEY or /KEY to the six output drivers. All switches should be ON except for one which MUST be OFF. The low numbered switches route KEY to the corresponding inverting outputs for an output of /KEY, the remainder route /KEY to the inverteding outputs for an output of KEY.

Field programming

A connector is provided for field replacement of firmware and message content using the popular and inexpensive Arduino RS232-TTL adapter (shown in Fig 1) (utilising ATB - a bootloader for AVR Tiny microcontrollers).

The hardware implements an automatic reset of the MCU when DTR is activated, so the -a switch should be used with the ATBU bootloader utility. ATBU -oR can be used to reset the board after other ATBU operations.

Advantages of the composite 12WPM / QRSS 3 message

The composite message provides several advantages to beacon operator and user:

suited to A1 or F1 modulation modes;
simple field reprogrammable keyer;
free mature featured firmware;
low cost, simple, robust and reliable;
uses existing modulator and transmitter, same transmitter frequency and mode, no licence variations needed;
modest frequency stability requirement;
beacon remains decodeable by ear using 12WPM Morse ID;
beacon QRSS message component can be decoded 10-20dB lower using Spectran or the like, waterfall display visible within seconds of tuning onto the beacon;

Changes