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

• to be advised

• requires Q4D19 or later

Testing against an access point

You will need:

• an open wireless access point,
• an OLPC XO, the test target.

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

essid X
test-antenna

Where X is the SSID of the network provided by the access point.

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

The numbers shown are:

• now rssi - the latest received signal strength indicator, in dB, calculated as the signal to noise ratio plus the noise floor,
• now snr - the latest signal to noise ratio, in dB,
• now nf - the latest 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 configuration.

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,
• Event: HWAC - adhoc BCN lost - the other laptop is disconnected ... or more exactly, the number of nodes in the network has fallen to one,

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