edit: In view of the helpful comments below, I just wanted to show an example of a cheap plastic antenna that does have an inductor inside. In this case, the antenna is shorter and the wavelength longer than the 900 MHz antenna discussed in my question, so the inductor is strictly for loading the short vertical to get it to resonate down at 433MHz. Take home message: not all cheap plastic antennas are just a piece of (straight) wire, some are coiled!
above: annotated screenshot from this video.
On most 2.45GHz WiFi routers the the black vertical antenna is thick at the base (10-12mm) and narrows (4-6mm) near the top. I've been poking around various catalogs and sites and it seems to me that the lower frequency 900 MHz antennas are simply cylindrical in shape with a uniform diameter of about 10-12mm.
Of course what matters is the electrical resonant frequency (center and width) of the antenna, but the shape reveals differences in things like inductive loading schemes for example, and so can reflect differences wavelength with respect to the physical length.
I noticed on the PyCom website they show antennas for the LoPy (LoRa) and SiPy (SigFox) boards as having the tapered shape (that I normally see on 2.45GHz antennas), while the antenna sold separately even though they operate in roughy the same frequency range. They also sell the antenna separately and it is labeled for both.
above: LoPY module from here. Two identical coaxial antenna connectors for external (software-switchable) WiFi antenna, and LoRa antenna are in the top corners.
Since the LoPy has TWO coaxial antenna connectors (top right and top left in image above), one for an optional external 2.45GHz WiFi and one for 800 - 900 MHz LoRa, it is conceivable that one could have both kins of antennas laying around at the same time.
Can the two types (usually) be distinguished by appearance? I left my grid-dip meter at home.
- External antenna.
- RF Cable Assemblies RP-SMA (Female) JK-IPEX MHF U.FL 1.13 100MM
- RP-SMA (Male) Tilt Swivel 1/2 Wave Whip antenna
NOTE: Using the LoRa/Sigfox radio without the external antenna can lead to damage of the device and is therefore not recommended.
The bundle includes:
- 2 LoPy modules
- 1 Expansion Board
- 2 LoRa Antenna kits