What is a characteristic of a random-wire HF antenna connected directly to the transmitter?
Why A random wire is an end-fed antenna with no balanced feed line, so the transmitter chassis, power leads and ground system act as the other half (the counterpoise) of the antenna. That means real RF current flows on the equipment itself, which can cause RF burns, distorted audio, and erratic operation of accessories. The cure is a good RF ground, a counterpoise, or a matching unit that isolates the radiator from the station.
Watch out Length is not the issue: random wires work at many lengths, and a wire an odd number of quarter wavelengths long is often a good choice, so the claim that it must exceed one wavelength is false.
End-fed wire = the shack is the other half of the antenna. Expect RF in the station.