Recent experiments with simple receive loop antennas led me to take another look at a Sony loop I bought some years back when visiting Akihabara. It's an accessory I'd seen in the instruction manual with my radio but it was never available, and is now discontinued I believe.
I've never been terribly happy with it probably because it has fixed tuning spots designed for short wave broadcast bands rather than ham bands.
It's an elegant design, the loop itself is flexible and folds down very small to fit in a travel bag. The "controller" takes two AA batteries. The controller box has a roll of cable inside perfect for sticking the loop on a hotel window while listening in bed.
When I first looked inside I was amazed at the complexity in there, a circuit was needed to figure out what's going on.
It's funny searching the web for service manuals these days, there are a bunch of sites offering to sell you PDFs but if you search cleverly using Google you can search for the product code and document type PDF and often turn up the service manual as I did here.
While the circuit is quite readable, the text is in Japanese but Google Translate does a pretty good job.
It's quite a sophisticated design.
The loop is a 48cm diameter that should be 1.94uH, this is tuned with four varactor diodes with a DC control voltage from 4V - 16V to tune a designed 3.95Mhz to 20.4Mhz.
There is an RF preamplifier stage right at the loop.
The control box is powered from 2 AA batteries and uses a voltage multiplier to step way up in order to reverse bias the varactors enough.
Some Sony radios send DC out the antenna socket and in this case this voltage is used to turn the device on when the radio is turned on. All very clever except that I'd prefer to have a tuning knob and higher Q (the loop actually has a 100K resistor across it to deliberately lower the Q).
I tried making my own control box that supplied 0-9V from a potentiometer for continuous tuning but alas the resonant frequency only moves from 3,600kHz to 5,400kHz. So clearly the high voltage is required.
Back to the drawing board, I'll keep working on making my manually tuned loop a bit more permanent.