An active receiving antenna and amplifier, requiring no tuning throughout its range.
To say this antenna is sensitive is an understatement. Mine is receiving a massive S2 MSF signal on 60kHz, all the way from Anthorn in Cumbria, down here in Somerset.

If you are into operating data transmissions via radio, here is a simple interface employing isolation transformers on the audio input/output lines and an optical isolator on the PTT line.
Designed primarily for the Kenwood TS-440 transceiver, but easily adaptable for any make or model of transceiver.