Satellite Comms and Antennas

Satellite Communications

Satellites provide a valuable link during disasters since it requires no local terrestrial infrastructure beyond where you are setting up.  Cell phones require cell towers within a few miles to be working and not overloaded.  Wireline services require a connection through the disaster to where you are.  Satellite systems do require a power source.  Depending on the size, it can be a vehicle’s 12 volt power outlet, a portable generator or a vehicle mounted generator.

A satellite is in an orbit around the Earth.  There are many different ways to position a satellite in orbit depending on the need.  A common orbit for communication satellites is a geostationary orbit 22,236 miles above the Earth.  Precision is needed when working with satellites at that distance.  One degree off and the satellite will be missed by 388 miles.  That is like aiming to land in Washington DC and really ending up in Detroit or Boston.

Antennas

The antenna used makes a big difference.  Let’s start by looking at a two-way radio antenna.  Most handheld two-way radios have an omni-directional antenna.  That means it doesn’t favor any specific direction so orientation doesn’t matter as much.  This gain in flexibility is matched with a loss in “punch” or sending power.  Imagine a basic light bulb in a lamp with no shade.  It spreads light everywhere.  That’s how an omni-directional antenna works.

What if you want that light to be focused to only project sideways?  Like an all around white light on a boat.  The bulb and the lens are constructed to direct the light in a specific pattern.  This is similar to a high gain antenna.  The main punch of the radio signal is increased perpendicular to the antenna by reducing the energy projected parallel out the top and bottom of the antenna.

Now you want to project light in a single focused direction such as a spot light or flashlight.  The bulb is constructed with reflectors and other features to direct the light.  The same is true with radio antenna.  A directional antenna is also called a beam or Yagi-Uda antenna.  Most of your neighborhood roof mount TV antennas take this form.  A series of metal rods direct the radio waves to a higher focus then the use of a single rod.

Wait, a TV antenna?  I thought those were receiving only?  The neat thing about antennas is that they will receive with the same characteristics as they transmit.  A highly directional antenna is more sensitive and will better pick up a signal from its pointed direction then an omni-directional antenna.  However, if the same signal came from a different direction then where the directional antenna is pointed, then the omni-directional antenna will receive it better.

So why don’t we always use directional antennas?  Think back to two-way radio repeaters.  The repeater’s antenna an example of when you want to broadcast the signal widely.  Directional antennas are good for communications between two known locations.  Onmi-directional antennas are good when you don’t know where the other location is, or it keeps changing and moving the antenna continually is impractical.

Going back to our analogy of light to describe radio waves, now imagine that you need a highly focused light.  A laser pointer; it is designed to send out a highly focused beam of light that can be seen for long distances.  The radio version of this is the satellite dish.  The transmitter bounces the signal off a parabolic reflector which theoretically sends all the energy in the same direction in a narrow beam.  These very narrow focus antennas are called “very small aperture terminal” (VSAT).

This series of examples is just discussing the shape of the antenna relative to the direction and focus of the radio waves.  It is possible to use practically any frequency with any shape of antenna so long as the antenna is properly tuned.  Different radio bands are naturally more efficient for communication modes when combined with certain types of antennas.

The important thing to remember here is just because you can do something doesn’t mean you want to do it.  This is where you need to rely on your radio technicians to design the most effective system using the right frequencies and modes to get the message to the final destination.

 

Additional readings