Naked Science Forum

Non Life Sciences => Technology => Topic started by: Pollee on 24/07/2018 11:42:32

Title: Is satellite ground station RF radiation measurable on the ground nearby?
Post by: Pollee on 24/07/2018 11:42:32

How do satellite dish ground stations (Skymuster) for NBN transmit and would the RF transmitted be measurable from around the base station itself, i.e. standing within (or living within), a certain distance from the satellite dish. I'm talking about the big satellite ground stations (see attachment) to deliver broadband, not the small home dish. Just wondering what, if any, the health risks would be of living next door to one fo these big satellite dishes?


NBN satellite ground station.JPG (89.65 kB . 699x615 - viewed 9823 times)

P.S. As a first time poster on this forum...forgive me if I haven't posted in the right thread.

Title: Re: Is satellite ground station RF radiation measurable on the ground nearby?
Post by: evan_au on 24/07/2018 12:46:55

The main radio-frequency emissions would be from the uplink from the central ground station (see picture above) towards the satellite.

The parabolic reflector takes microwave energy from the feed horn (at the focus of the parabola) and turns it into a parallel beam aimed at the satellite (and the same in reverse for reception).

It is very important to minimize noise in satellite systems, so the feed horn is designed to collect signals from the parabolic dish without picking up signals from outside the dish. On the transmit side, the feed horn is designed to illuminate the parabolic dish with minimal overspill out the sides.

The net effect is that large satellite dishes can achieve something like 1000000:1 ratio between energy in the beam, and energy lost outside the beam. This is often specified as the antenna gain in dBi, a logarithmic scale comparing the power in the beam to a theoretical antenna that radiates isotropically (the same in every direction).

The main risk would be to birds accidentally flying directly into the beam.

See: https://en.wikipedia.org/wiki/Antenna_gain

Title: Re: Is satellite ground station RF radiation measurable on the ground nearby?
Post by: evan_au on 24/07/2018 13:07:04

After a bit of searching, I found that:
- The Skymuster ground stations have 13.5 meter dishes
- The transmit spectrum is centered on 29 GHz (0.01m wavelength)

A well-designed antenna is diffraction limited. The beamwidth θ may be calculated as:
θ=1.22*λ/D =1.22*0.01/13.5 = 0.0009 radians = 0.05°
Where:
- θ is the angular resolution in radians,
- λ is the wavelength in meters,
- D is the diameter of the dish in meters.

This ensures that almost all the power goes into the beam.
The fences around the installation keep members of the public at a safe distance (for the public and the equipment).

See: https://en.wikipedia.org/wiki/Optical_resolution#Lens_resolution
https://en.wikipedia.org/wiki/Sky_Muster
More photos (click to expand): https://www.nbnco.com.au/corporate-information/media-centre/image-gallery/satellite-ground-stations.html

Title: Re: Is satellite ground station RF radiation measurable on the ground nearby?
Post by: FuzzyUK on 19/08/2019 09:55:40

Quote from: evan_au on 24/07/2018 12:46:55

The net effect is that large satellite dishes can achieve something like 1000000:1 ratio between energy in the beam, and energy lost outside the beam.

You haven't stated the transmit power at the radiator. It's not clear whether the million to one power ratio (60dB) you talk about is the side lobe attenuation or the antenna gain from the dish?

What Pollee (OP) is after knowing whether it safe to live near these installations and my guess is such dishes are nowhere near habitable dwellings and even if they were the side lobe attenuation is liable to be far greater than -60dB.