What kind of structure is this small GSM antenna?
Thanks for including the good photos, and a ruler!
The typical &quot;rubber ducky&quot; antenna is a simple monopole. The center conductor may be straight, but often it is coiled to reduce the physical length of the antenna while maintaining resonance.
There are also &quot;coaxial dipole&quot; antennas which look somewhat like this. The upper element looks like a simple monopole. The lower (i.e. outer) element is usually solid rather than a coil.
But designing an antenna to work on two bands that are in a 2:1 ratio is a challenge. To meet that, I'm guessing the designer started with the upper element as a quarter-wave at the higher frequency. Then the trick: make the lower (outer) element resonate as a half-wave at the lower frequency -- because the bottom end is grounded, not open. In essence you have an asymmetric coaxial dipole with unequal elements, and one end open, the other shorted. I confess to not having seen this design before.
The whole assembly might have been designed with simulation, but they may have simply experimented with a network analyzer until they got decent VSWR and efficiency in the two bands.