In the process of installing the Radio Telemetry system for our Victorian network I had numerous questions I needed answers for. The following list of FAQ's stems from the questions I raised, and the answers I received to them. Some of them are general, and others are specific to the Radio-Modems I chose to install.
If you have any other questions on this topic, or would like to add your own FAQ's to this page, please email us.
Legal Output Power: LOP 4 Watts EIRP out of Antenna => +36 dBm out of Antenna
Antenna Gain: AG eg. 15 element Yagi => +16 Dbi
Other Losses (Cable etc.):
OL
Typically 3dB (Long Antenna cables may contribute more)
Radio Output Power: ROP
30dBm (1 Watt)
20 dBm (100 mWatt)
10 dBm (10 mWatt)
0 dBm (1 mWatt)
Examples:
15 element Yagi with Radio
set to 30 dBm.
AOP = 30 dBm - 3 dB + 16 dBi = 43 dBm which is > LOP
15 element Yagi with radio
set to 23 dBm (200 mWatt)
AOP = 23 dBm - 3 dB + 16 dBi = 36 dBm which is = LOP
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Down to -103 dBm. Lower than this generates lots of retries.
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Mount the antenna in a clear space, as far as possible from buildings, metal objects, trees etc. with clear line of site (where possible) to the other station.
The antennae should be mounted at least 3 metres from the radio.
Orient the Antenna accurately in the direction of the other station. If the direction is not known, the received signal strength can be monitored to determine the correct direction.
Mount the antenna as high as possible. Height is everything!!!
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Any metallic object close to the antenna will distort the radiation pattern. The antenna should be mounted at least 3 wavelengths above the roof, more when possible. i.e. For a 915 MHz SS radio > 1 metre, although > 2 metres is recommended.
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Yagi antennas can be mounted horizontally or vertically, as long as both stations have the same polarisation. Setting antennas at opposite polarisation will result in 20 dB or more losses in receive signal strength.
Note: Omni-directional antennas employ vertical polarisation. Yagi's transmitting to an omni-directional antenna should therefore be set vertically polarised i.e. the elements pointing vertically up/down.
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There are basically two options; either mount all Yagis vertically polarised on a single mast or mount them horizontally polarised on separate masts as far away as possible from each other.
The spacing between vertically mounted antennas (antennae isolation) is dependant on frequency, transmitted power and radio sensitivity. Multiple 915MHz spread spectrum radios antennas should have at least 2 metres of vertical separation, more if possible.
Note: 2 metres of vertical separation on vertically polarised antennas is equivalent to more than 50 metres of horizontal separation on vertically polarised antennas.
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If you have an antenna like a Yagi mounted on a mast, attaching an earth cable to the mast is highly recommended. If you want to check up your legal requirement regarding earthing refer to AS 1768-1991. You should also have some sort of lightning protection/surge arrestor before the radio if you want to protect the radio from a lightning strike.
The outside of the Yagi that you clamp to the mast is ground.
A lightning protection device such as a "Broadband HF/VHF/UHF Coaxial Protector" should be placed in line with the antenna cable. The best point is the one that gives the closest and most direct point to ground. These devices are available in bulkhead or flange mount packages, and the case is connected with a straight and direct earth cable to the ground point
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