Mobile Internet – it’s just radio

I have had a few questions lately regarding antennas for Mobile Phones / Mobile Broadband. As it happens, the “rules” for getting the most out of your connection are the same for other “line of sight” signals such as VHF radio. That’s because your mobile phone / mobile broadband set is basically exactly that, a radio.
So, as with your VHF set, it really comes down to three factors:

  1. Distance from base station
  2. Effectiveness of the antenna
  3. The connection between the antenna and the set.

Let’s have a look at each of these factors in turn.

Distance

Mobile phone reception is essentially “line of sight”. Because of the curvature of the earth, the higher we position our antenna, the longer the distance that we can maintain our connection. It is not surprising therefore that the best place to install our antenna on the top of the mast. The actual coverage achieved depends on both the height of the base station antenna and the height above the waterline of the antenna on our boat. You can see the effect of antenna height and distance on the diagram below.

Calculating Line of Sight

For those who enjoy a bit of maths, you can actually calculate the theoretical achievable distance.
Distance To Horizon

Yacht A (10m mast) : 11 km (6 NM)
Yacht B (20m mast) : 16 km (8.6 NM)
Tower (100m)           : 36 km (19.3NM)

In the example above, you can see that the achievable distance between Yacht B and the Shore Tower is 16 + 36 = 52 kilometres.

Antenna effectiveness

The quality of an antenna is determined by a number of factors. Antennas are characterised by their “gain”, or efficiency. In effect this is achieved by focusing the antenna’s radiation pattern. A high gain antenna will have a narrow, focused pattern, while a unity gain model will be less so. Of course, a narrow radiation pattern is a disadvantage on a heeling yacht if the resulting “beam” ends up pointing into the sea or toward the heavens!

Antennas are tuned to particular frequency bands, so make sure you select an antenna suitable for the operating frequency. Telstra NextG service runs at 850 Mhz in most places, and therefore needs a matching antenna . Mount the antenna away from metal surfaces and other antennas. On our yacht, Te Moana, the broadband antenna is mounted on a bracket off the top of the mast in order to provide the required space.

Low loss interconnection

Another important aspect of the performance of your system is the quality of the cable connecting the set with the antenna. All cable will cause a signal loss, the longer the cable more critical this becomes. Combined with the need to install the antenna as high as possible, this presents a compromise. The longer the cable distance, the higher the transmission loss, however we want to mount the antenna as high as possble! So only use high quality, low loss coax cable to connect the antenna (the type that’s as thick as your thumb).

A low cost alternative

Of course a well placed antenna and proper high quality cable pre-supposes a fixed mobile broadband modem such as the Ericsson units. For a nifty, low cost, alternative use one of the tiny mobile hot-spot modem units now available (see last month’s article). Put it in a sock and hoist it up to the top of the mast. It will give you antenna height without any cable losses – it works a treat, but not recommended while underway…

Marius

Reference: A good article on Marine Antenna Fundamentals: http://www.boat-project.com/tutorials/vhfant.htm