Cameron Bricker, January 1999

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.

• How much power can I legally put out my Spread Spectrum Radio?
• How low a Rx signal is acceptable on my RFI-9256 Radio-Modem?
• What do I have to consider when mounting my Antenna?
• Can I mount the Antenna on a metal roof?
• Should I mount the Yagi Antenna vertical or horizontal?
• I have multiple radios, how should I mount the Yagi Antennas to reduce interference?
• Do I need lightning protection?

How much power can I legally put out my Spread Spectrum Radio?

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

Return to top of page

How low a Rx signal is acceptable on my RFI-9256 Radio-Modem?

Down to -103 dBm. Lower than this generates lots of retries.

Return to top of page

What do I have to consider when mounting my Antenna?

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!!!

Return to top of page

Can I mount the Antenna on a metal roof?

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.

Return to top of page

Should I mount the Yagi Antenna vertical or horizontal?

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.

Return to top of page

I have multiple radios, how should I mount the Yagi Antennas to reduce interference?

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.

Return to top of page

Do I need lightning protection?

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.

• If the mast is a galvanised pipe that is concreted into the ground, then this will provide a suitable earth.
• If the mast is attached to a wooden facia then a straight and direct earth cable will need to be connected from the mast to an earth stake.
• If the mast is mounted on a steel roof and the roof is earthed, the roof may provide a suitable earth. Note: Not all steel roofs are earthed. If the roof is not earthed then a straight and direct earth cable will need to be connected from the mast to an earth point stake.

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

Return to top of page 2006-11-09