A similar-looking (Sony) antenna (12.5cm x 10.5cm) was measured on a 1kHz bridge as 24 microhenry.
The bridge "Q" was very low as expected at 1kHz (well below 1), but gave a good null for its inductance value.
A second test added a parallel capacitor of 560 pf. and measured with an oscilloscope's 10X attenuator probe. A sharp resonance at 1400 kHz resulted...that jives with a 24 uH inductance. Self-resonant frequency was above 5 MHz.
It seems to be a simple coil of wire, with electrical inductance of 24uH.
Seems to be made up of PVC-insulated wire in a single-layer winding of 8 turns, close-wound spanning nearly 1cm.
It is possible that the antenna inductance of 24uH is used with a resonating parallel capacitor inside the radio to tune the AM broadcast band.
Most coil calculators assume a round coil form rather than this "square-ish" form. I had formerly included results from Bob Lombardi's AIRWNDL.EXE coil calculator program. Bob didn't describe his equation - it assumed a round coil form.
A rectangular-loop coil calculator yielded an inductance of about 28uH given the following parameters:
- number of turns: 8
- width 123mm
- height 103mm
- wire diameter 0.8mm
- relative permeability 1
Be aware that the actual coil form is less than a strict rectangle, so measured inductance of 24 uH is still reasonable.