[Android Head-Units] Tuatara:
Antennas ... more than you ever needed to know ...

Ok, this is actually a three-part question and multi-part answer. I'll go through the important points first:

1). An antenna's polarity is an entirely different thing from the 'wiring' polarity (it's not like a battery).

2). Connecting the antenna is important to do correctly and without a short-circuit or partial short (cross-talk or induced noise will degrade performance).

3). There is no 'easy' way to connect multiple antennas to a single source (there will be a wavelength and impedance mismatch).

Nearly every Wi-Fi device sold today utilizes an omni-directional, "dipole" antenna which is vertically polarized. This polarization is how the SIGNAL is transmitted, and has nothing to do with the wires connecting it. Imagine waving your hand up and down. Looking from the side, you can see your hand very clearly. Looking from the front, you can see your hand, but not quite as easily. Looking from above, it gets much more difficult to see the movement - especially if you're further away. Wave your hand side-to-side, and from the side it becomes difficult to see, stays the same from the front, and is dramatically improved from above. Same thing goes for WiFi antennas & signals.

Turn your tablet sideways, rotating it to see that movie, and the WiFi signal quality can drop by 3dB to as much as 20dB (every 3dB is a halving of receive power). If you wonder why your 4-bar signal suddenly becomes 1-bar ... turn your device 90' ... it will improve. That's antenna polarity.

Now, for the actual wiring part - there is a centre wire, which is surrounded by a shield in the coax cable which is connecting the antenna. The signal itself is only provided on the centre conductor, and the coax shield (the braided wire part) is the signal ground. The signal ground acts as a shield for the signal being carried inside. This applies to everything from the antenna, to your audio cables, to your headphones. The signal is 'carried' on one wire, and the other wire is a ground to protect the signal from interference (until it gets to the end-point - the speaker, the audio system, or the antenna in this case).

When wiring up your antenna, before taking it off, make a careful note of which side of the connector was connected to the braid/shield, and which side was connected to the core. Then, strip the coax carefully, ensuring that you do not break the braid, or the centre conductor - carefully wrap the braid around the centre conductor - taking care not to damage the plastic covering (insulator) separating it from the braid around it. Twist this together, and carefully pre-solder the braid, and centre conductor so as to make it easy to solder to the android box. Check (and double-check) that you haven't heated the braid too much, and melted it into the insulator. Even a single wire 'coming close' to the centre signal wire can cause degradation (cross-talk, shorting to ground, impedance mismatching, etc.). Do it cleanly, connect it back up in exactly the same way, and it'll work perfectly.

For multiple antennas (i.e. in routers, and other devices), these are either diversity antennas (special connections), or they are multiple antennas (different frequencies - 2.4/5.1Ghz, multi-path correction, beam-forming, etc.). They are not just antennas wired up to be connected in series, or in parallel. An antenna has a specific impedance - for a WiFi antenna it's 50 Ohms. Putting two in parallel would reduce the impedance the transmitter saw to 50/2 = 25 Ohms. It would then think the antenna is shorted, and would reduce the power accordingly - otherwise it would be damaged. Similarly, connecting two in series would change the impedance to 50*2 = 100 Ohms. It would then think the antenna is damaged, and attempt to raise the power, effectively overheating and again damaging the transmitter. Short answer - don't do this. Ever.

While there are splitters (like for your analog TV signal) which work at 75 Ohms and can be used to split the TV signal to multiple devices - each 'split' of the signal will (at best - assuming no losses) reduce the signal level by 3dB (i.e. halving it). You then connect up two antennas again (each running at half power), which results in the net power output being the same. Your only benefit? Communications noise, because you've created a multipath signal (i.e. two places broadcasting the same signal over each other from different points/places - like dropping two stones into a puddle - makes it more difficult to 'hear/see' the real wave from a single point), and have degraded your signal and reception.

Even though this is a simplified (ELI5) explanation, this is the basic principle. If you want better 'power/reception', get an antenna with more stacked dipoles (i.e. a 7dBi antenna instead of a 5dBi antenna). Every 3dB is a doubling of effective power/receive sensitivity - so go that route if you feel you need to.

In my opinion, a 5dBi antenna would be absolutely adequate for the android box. Unless you're into war-driving, and trying to find tons of unsecured access-points around the city (seriously - this still happens in the world today?!!? - secure your access points people!), 5dBi will give you plenty of coverage in your driveway, to get to your home router.

And yes, you can 'cheat' like I did, and use a WiFi antenna on your 3G dongle. Here in NZ, the 3G signal is at 2.1GHz, close to the 2.4GHz wifi spec, and the 2.4GHz WiFi antenna does a pretty good job of boosting the transmit/receive of the tiny little dongle PCB antenna. It won't provide 5dBi, but around 3dBi (due to frequency mismatch) ... but this is still a 'doubling' of the built-in dongle one. To check GSM frequencies in your country go here. Connecting it up is just the same process - but here you need to figure out the ground/shield for your particular dongle to wire up the coax to the external antenna.

Aaaaand I think I've blabbered on long enough. I'm starting to think about getting into teaching ... now that I'm getting old and tired of the commercial rat-race. :cool: