One antenna can broadcast multiple SSIDs, or multiple networks. It is the same concept as a radio antenna that can receive multiple stations.
Most access points today allow you to define multiple network segments, each using a different SSID, and a different encryption.
Using two antennas allows using signal diversity to improve the quality and reliability of the received signals. There are a few ways the receiver can use these two signals, achieving different results based on the complexity of the receiver design:
(as per wikipedia)
- Switching – In a switching receiver, the signal from only one antenna is fed to the receiver for as long as the quality of that signal remains above some prescribed threshold. If and when the signal degrades, another antenna is switched in. Switching is the easiest and least power consuming of the antenna diversity processing techniques but periods of fading and desynchronization may occur while the quality of one antenna degrades and another antenna link is established.
- Selecting – As with switching, selection processing presents only one antenna’s signal to the receiver at any given time. The antenna chosen, however, is based on the best signal-to-noise ratio (SNR) among the received signals. This requires that a pre-measurement take place and that all antennas have established connections (at least during the SNR measurement) leading to a higher power requirement. The actual selection process can take place in between received packets of information. This ensures that a single antenna connection is maintained as much as possible. Switching can then take place on a packet-by-packet basis if necessary.
- Combining – In combining, all antennas maintain established connections at all times. The signals are then combined and presented to the receiver. Depending on the sophistication of the system, the signals can be added directly (equal gain combining) or weighted and added coherently (maximal-ratio combining). Such a system provides the greatest resistance to fading but since all the receive paths must remain energized, it also consumes the most power.
- Dynamic Control – Dynamically controlled receivers are capable of choosing from the above processing schemes for whenever the situation arises. While much more complex, they optimize the power vs. performance trade-off. Transitions between modes and/or antenna connections are signaled by a change in the perceived quality of the link. In situations of low fading, the receiver can employ no diversity and use the signal presented by a single antenna. As conditions degrade, the receiver can then assume the more highly reliable but power-hungry modes described above.
Not sure which way dji implemented the range extender. Regardless, it is not a repeater as you correctly observed, because they are using two different SSIDs.
If they had used it as a true repeater you would have been able to connect to the phantom directly. Further we can learn that the other SSID must be hidden because it does not show up on your device as an option to connect to.
I wonder if this "signal hand-over" is not causing unnecessary delay in the video transmission...
Thanks
