Some other sites list these wire coils as being 17cm, or a quarter wavelength at 434MHz.
The article here has some advice on this style of antenna. Basically, with the one wire, the device ends up being sensitive to you standing next to it (body capacitance). The addition of an augmented ground plane, should stabilize the antenna a bit, and "define" a characteristic impedance.
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Not that this matters in the grand scheme of things. I'm sure a "legal" "license free" band is just abuzz with RF noise. There will be no predicting what happens next (Amplitude Shift Keying).
Paul