Antenna testing

How to test the OLPC XO antennas.

Why test them?

• they may have been damaged by impact, misuse, or electrostatic discharge,
• to classify laptops for repair,
• to verify a repair,
• to survey a site,
• to find parts of a site that have low coverage.

Linux, GNOME or Sugar

• connect to an access point,
• use Browse, Firefox or Epiphany to visit a web page,
• while transferring data, display the signal and noise values:

iwconfig eth0

Open Firmware

Open Firmware contains an antenna testing facility. It requires a minimum version:

• XO-1, to be advised
• XO-1.5, to be advised
• XO-1.75, requires Q4D19 or later.

Testing against an access point

You will need:

• an open wireless access point, with or without an IP network behind it,
• an OLPC XO, the test target.

Start the XO and get the Ok prompt, then type:

essid NETWORK
test-antenna

Where NETWORK is wireless access point name.

An updating display of signal strength is given. It looks roughly like this:

Associate with: X
keys: (0,1,2,l,r,d,a,s,q)
now rssi -33 snr 57 nf -90 avg rssi -33 snr 57 nf -90 rx d tx d

During the test the following values are displayed ten times a second:

• now rssi - the latest beacon received signal strength indicator, in dB, calculated as the signal to noise ratio plus the noise floor,
• now snr - the latest beacon signal to noise ratio, in dB,
• now nf - the latest receiver noise floor, in dBm,
• avg rssi - the averaged received signal strength indicator,
• avg snr - the averaged signal to noise ratio, in dB,
• avg nf - the averaged noise floor, in dBm,

The rx, tx or ant values show the antenna allocation. The wireless card generally uses both antennas.

Watch the avg rssi value. Move the laptop in relation to the access point, raise or lower the antennas, enclose them with your hand, and look at the result. Press enter to keep a result on screen. You should see:

• rssi will increase as the laptop moves closer to the access point,
• rssi will change slightly between raised or lowered antennas.
• rssi will decrease as the antennas are enclosed by a hand,

During the test there are keyboard keys with special meaning:

• key s to perform a network scan,
• key a to deassociate and reassociate with access point,
• key q to quit.

The test can be used to indicate the performance of antennas, transmission coax, termination, socket, antenna switch, and receiver. But there are sources of unreliability in the test:

• the radio noise environment,
• the precise position of the antenna or laptop, such as in a null,

These sources of unreliability make comparisons difficult, but if you see substantially different values greater than 20 dB between the two antennas, then one may be damaged.

The test also does not test transmit by the laptop, only receive.

If you identify an antenna as damaged, schedule the laptop for a detailed repair. Look carefully at the coax cable leading from the wireless card to the axle of the antenna. If the cable is twisted, or broken, then replace the antenna. Check that the connector is latched into the socket on the wireless card. The top surface of the connector should be parallel to the card. The antenna itself is not serviceable.

Antenna configuration

The wireless card may be instructed to change the antenna preference using the 0, 1, 2 and 3 keys. The effect varies by laptop model:

• XO-1, the command is accepted, "1" is the right-hand antenna, and "2" is the left-hand antenna,
• XO-1.5, some modules, the command is refused ('"Result = 1),
• XO-1.75, the command is accepted,

However, covering one antenna and watching the now rssi values shows that the wireless card may still switch between antennas.

Setting up a test laptop

You might do this if an access point is unavailable. You will need:

• an OLPC XO of known provenance, the test host,
• an OLPC XO, the test target.

Start the test host XO and get the Ok prompt, then type:

ok select /wlan:force
ok d# 11 " X" adhoc-start
ok test-antenna

X is the network name. You may choose any name according to your site convention.

The values are not updated until another node joins the network.

Start the test target XO and get to the Ok prompt, then type:

ok essid X
ok test-antenna

Both sets of values should begin to update.

During the test, certain messages may be emitted, and these have special meanings:

• Event: Link Sensed - the other laptop is connected ... or more exactly, the number of nodes in the network has increased from one to more than one, occurs within two seconds,
• Event: HWAC - adhoc BCN lost - the other laptop is disconnected ... or more exactly, the number of nodes in the network has fallen to one, occurs within about ten seconds.

The values displayed by each laptop relate to the receive path of that laptop. If there are more than two laptops associated with an adhoc network, the values shown will include receive performance of transmissions from both laptops.

References

Link Budget

The link budget is the total gains and losses of a radio link. The link budget must be sufficient for the OLPC XO to use a wireless internet.

For an OLPC XO using an access point, this is the sum of:

• for the access point:
  • the transmit power,
  • the loss in the transmission line (the coax and connectors between the transmitter to the antenna, normally very short),
  • the gain of the antenna,
• for the environment:
  • the loss due to distance,
  • the loss due to obstructions,
  • the loss due to noise, (from other radio transmitters),
• for the laptop:
  • the gain of the antenna,
  • the loss in the transmission line (the coax and connectors between the antenna to the receiver),
  • the sensitivity of the receiver.

For two OLPC XO using an adhoc network, the link budget is the sum of:

• for the first laptop:
  • the transmit power of the wireless card,
  • the loss in the transmission line (the coax and connectors between the transmitter to the antenna),
  • the gain of the antenna,
• for the environment:
  • the loss due to distance,
  • the loss due to obstructions,
  • the loss due to noise, (from other radio transmitters),
• for the second laptop:
  • the gain of the antenna,
  • the loss in the transmission line (the coax and connectors between the antenna to the receiver),
  • the sensitivity of the receiver.

See

• http://en.wikipedia.org/wiki/Link_budget