Things show up on my phone especially when I search through You Tube. One of the recent videos to appear on the screen was "Spy Radios" from WWII.
Generally speaking, we might see these as pretty crude in comparison to what we have today. Often these radios were simple two tube regenerative receivers (6SK7) and a one tube transmitter (6V6). One of the more famous of these spy radios is the Paraset shown below.
Developed by the British SAS, it is officially known as the Whaddon MK VII. Often the array of items above were packed into what looks like a beat-up leather suitcase so it could be easily concealed and transported. The name "Paraset" came from the fact these were air dropped with the spies behind enemy lines. Power could be supplied from the mains or a 6VDC car battery using a Vibrator power supply.
Surprisingly it is a two-band rig covering the 80 and 40M bands and with a 6V6 transmitting tube (Crystal Controlled) you could get 5 or 6 watts from the Paraset. Keep in mind the British had an extensive radio receiving network located in Southern England -- so 5 or 6 watts was always heard. Oh, the Morse Key was built into the front panel.
One of the videos showed a man walking down the street wearing a winter long coat with a loaf of fresh bread under his arm. This was not the Paraset, but the Receiver and Transmitter were under his coat and the power supply was in the loaf of bread.
I specifically mention the Paraset because even today it is frequently replicated. Once I even built the transmitter portion of the Paraset and the output tuning has two light bulbs to aid you in tuning a random piece of wire used for the antenna.
During the Cold War refined versions of the Paraset concept were produced for use behind the Iron Curtain most notably by the CIA. You know the same crazy guys who came up with exploding bubble gum.
Today, the Receiver could be a DCR using the Seed Xiao RP2040 and a Si5351 for accurate frequency generation. For the transmitter (VFO control with offset using CLK1) three transistors would give you 5 watts. It could even have a built-in antenna tuner. All of this would fit inside the loaf of bread including a LiPo battery. I would also see creating a coded message that is stored in the Seed Xiao memory and sent as a code burst. Only 2 seconds on the air is hard to direction find.
If I am not mistaken, friend N2CQR while stationed at the Embassy in the UK had a chance to operate one of several Paraset radios owned by Ian, G3ROO.
Going back to the loaf of bread mentioned earlier, this was important as urban legend has it the OSS (forerunner of the CIA) developed an explosive that looked like flour and could be even baked into a loaf of bread and was edible.
Regrettably today I am not certain we have many hams or engineers that could design or build a Paraset from scratch. The under 50 crowd is looking for a kit on Amazon or eBay and would want the Gerber files so the boards could be bought 5 for $5 from China. Possibly they could write the Software for the Seed Xiao RP2040, but the actual fabrication and building of the whole rig in a small form factor is a hill too high or bridge too far. Did I mention that they would want a complete schematic and everything software on a GitHub.
73's
Pete N6QW