The first tracker built.  APRS.  Used in a High Altitude Photography balloon project (link).

Weight wasn't a primary factor and it was the first PCB ever designed.  Modular for sensors and transmitting equipment.

Even used a full-size through-hole 28 pin ATMega328P and a coax cable for an antenna (ground wave).

First "pico"WSPR  tracker built, attempting to re-use some concepts learned during high-altitude photography.

Changed the transmitter to be Si4463 and re-wrote a lot of the software to behave accordingly.

Transitioned to surface mount for everything except serial interface pins.

Used multiple power regulators to allow for dynamic disabling power consuming subsystems.

Antennas became lengths of wire as opposed to coax.

First useful WSPR tracker.  Replaced radio transmitter with Si5351 clock generator.

Tightened up board layout somewhat.

Exposed more pins to diagnose serial, I2C, GPS, etc, and allowed for a cutaway of that section to reduce weight.

Dropped the second power regulator and used a purpose-designed load switch IC for shutting down subsystems.

Battery powered.

Now gone to 0.6mm PCB thickness via JLCPCB.

Temperature sensor via passive temperature-influenced resistor (didn't work well enough in the end).

Battery powered initially, and began integrating solar cells into the system by removing the power regulator and tying the cell output directly to where the regulator VCC out would go.

Most modern re-design.

New smaller GPS.  Smaller ATmega328P,   Most parts that can be lead-less now lead-less (eg load switches). 

Temperature sensor now via I2C and replaces temperature-sensitive resistor.

Removable leads now on the side because routing to the end became impossible.

Solar only, no option for power regulator or batteries.