Testing was done using four meter masts, MCC Minis and 19dB flat-panel antennae.
Several locations were checked for suitability for Telemetry Hubs. Flat areas with unimpeded line of site to other Telemetry Hub locations are ideal.
|Test Position 1
|Test Position 2
The masts used during the test were 4m high. Links between hub 1, windmill and the shed would benefit from taller masts.
Test sites were chosen as they were relatively easily accessible. The site is fairly open, there are not many trees, hill or other features that may cause issues with signal transmission.
|Location A||Location B||Distance between sites (m)||Free Space signal strength
|Measured Signal Strength
|Measured Safety Margin
|Attenuation due to site conditions (dB)|
Depending on the distance between transmitter and receiver and the frequency being transmitted, the transmission path can be considered to be equivalent to that of free space, (ie: unaffected by the physical environment), if all obstacles, including the surface of the earth, are outside a cigar shaped envelope with the minimum radius at the midpoint being the First Fresnel Radius. If this radius is impeded by the smooth shape of the earth, (eg: a gentle hill), such that the line between the antennas was tangent to the crest of the hill, the signal path attenuation caused by the hill could be as much as 60dB. If the obstruction was a relatively discrete object such as a line of trees or a wall, the attenuation could be as little as about 15dB.
The table below shows the radio signal attenuation in free space for a range of distances. This is equivalent to the results for an ideal unimpeded communications path on site.
|Distance (m)||Frequency (GHz)||First Fresnel Zone Radius (m)||Free Space Attenuation (dB)|
Si = Pt + Gt – Ct + Sr + Gr – Cr – Lfs – Pl
Si = Signal level margin above the noise floor at the receiver (dB)
Pt = Transmitter power (dBm) (= 20dBm approx.)
Gt = Transmitter antenna gain (dB) (=19dB for standard flat panel antenna)
Ct = Transmitter antenna coax cable loss (dB) (= 0.66dB approx. for 3m cable)
Sr = Receiver sensitivity (dBm) (=-85dBm approx. Note that 85 is added rather than subtracted)
Gr = Receiver antenna gain (Gr) (= 19dB for standard flat panel antenna)
Cr = Receiver antenna coax cable loss (dB) (= 0.66dB approx. for 3m cable)
Lfs = Free space signal attenuation. (dB) (= 111dB for 3600m)
Pl = Path losses due to site conditions. (dB) (Between 0dB and 60dB depending on hills and obstructions in communication path)
The above equation does not allow for degradation of the signal path due to wet weather, interference, etc. A safety margin of about 20dB needs to be allowed for reasonably reliable communication.