Q: What kind of structure is this small GSM antenna? Two of these types of antennas comes with the ADALM-PLUTO SDR evaluation kit.


According to this page, they should be JCG401 antennas for the 900 and 1800 GSM bands, but the datasheet does not tell anything about what kind of antenna structure this is.
It looks like a monopole connected to the center conductor, but with some kind of helical structure around it. The helical is connected to the shield(ground?). Wouldn't it interfere with the radiation from the center conductor?
Or is this some kind of dipole structure?

A: Thanks for including the good photos, and a ruler!
The typical "rubber ducky" 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 "coaxial dipole" 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.

A: The centertap conductor is actually a dielectric such as the gigahertz frequency don't create much of a potential drop in de helical monopole. What i don't know is what kinds of dielectric does it have the bare conductor. Given case the dielectric is very high, the center pole is actually reflecting electrons at the gigahertz frequency.
