Mesh/Antennas

From Sudo Room
Jump to: navigation, search

A lot of people ask about making our own antennas. When you can buy outdoor gear for ~$50 that can create links of hundreds of megabits over distances of multiple miles then trying to create your own antennas stops making a lot of sense in most cases.

There are still some cases where DIY antennas make sense. E.g: High gain omni-directional antennas for 900 MHz are way too expensive!

Frequencies

Wifi gear is easily available for three frequencies:

  • 2.4 GHz
  • 5 GHz
  • 900 MHz (less common)

Polarization and MIMO

Any modern long range wifi unit (e.g. ubiquiti airmax) will use two antennas: One vertically polarized and one horizontally polarized. This increases the available bandwidth by a lot.

Indoor/home units have multiple antennas but those antennas aren't polarized differently and instead take advantage of the fact that the signal from each antenna will take a slightly different path to their destination to overcome interference / destructive reflections on the way. This is referred to as MIMO (multiple input multiple output).

Connectors

RP-SMA

A Reverse Polarity version of normal SMA. This is what is used for most home/indoor routers that have detachable antennas.

Originally adopted to appease the FCC since the FCC was worried about end-users of wifi equipment attaching antennas that would cause the equipment to violate maximum transmission power in a specific direction (e.g. buy a 1 watt maximum allowable power unit and add a directional antenna) but has now become just another standard and the FCC seems to have stopped caring about this.

U.FL

These are tiny snap-on connectors usually used directly on the circuit board inside wifi gear. Used in most laptops to connect the antennas inside the screen to the wifi network adapter and used in some wifi routers to connect the internal or external antennas to the circuit board.

N-type connector

Seen in some professional grade gear and outdoor gear, e.g. the Ubiquiti AirMax Bullet series.

Cable

Almost all wifi gear and modern digital consumer electronics gear uses 50 Ohm coax cable (if cable is used at all). It is advisable to always keep the cable as short as possible as the losses are significant. If you have a choice between extending the length of the coax cable to the antenna and extending the ethernet or usb cable to the wifi gear then always extend the ethernet/usb cable.

RG-58

About 5 mm in diameter. This is probably the most common type of coax cable used for short runs to external antennas. Interestingly this is the same type of cable used in old coaxial ethernet networks, so if you have some of that stuff around don't get rid of it quite yet :) but of course the connectors are different.

RG174

This is one of the standards used for the thin cable often used with U.FL connectors. Not sure how common it is. Several of the cheap antennas from china arrive with this type of cable.