Which mobile for you?

If you’re not already aware, Telecom plans to switch off its early-generation 025 network in March next year. Many boaties, especially commercial operators, have purchased old Telecom carphone units for their boats. Norman Holtzhausen explains why.

The old phones are big and clunky, but have a three watt output and a large, external aerial. They have an excellent range, reaching claimed distances of up to 90km (or 50 nautical miles) from the nearest base station. By contrast, today’s standard mobile handset has an output of just half a watt, and a workable range that’s usually less than 10nm.

Concerned boaties have formed an interest group to pressure Telecom into keeping the 025 network alive. Based on recent events surrounding Telecom and the telecommunications commission, I would not rate their chances too highly. So if you want good mobile coverage at sea, what are the alternatives?

The two contenders are the GSM system used by Vodafone, and the CDMA (and derivatives) system used by Telecom’s 027 network. Don’t worry about 3G for now - that mostly affects the amount of data the network can carry, which is largely irrelevant for voice calls.

Which of the two networks will give you a better range? The answer’s far from simple, and equally dependent on the mobile handset as on the network operator. Still, there are four main points to consider:

Handset Power

The power output of the device, measured in watts. Most mobile handsets have a power output of just half a watt – puny compared with the three watts produced by the old carphones.

Since radio waves decrease in strength according to the inverse square law, at double the range the signal is only one quarter the strength. Or, to get twice the range requires four times the power output. So the higher the output the greater the range.

Unfortunately, three watt phones for either GSM or CDMA do not exist – but it is possible to buy a two or three watt booster unit for your phone. The inbuilt aerial is unscrewed and the booster cable attached. A new external aerial is connected to the booster.

Due to some quirks in the local licensing regulations, these units cannot be sourced from a New Zealand supplier. Powertec Telecommunications (www.powertec.com.au) in Australia is probably the first port of call to consider. Note that you MUST get the right type of booster, or it simply won’t work (see sidebar for frequency details).

Aerial

Raw power is only part of the story: the next item affecting the range is the aerial, and a suitably tuned and mounted aerial will probably increase range more than boosting the power. The important characteristic of an aerial is its amount of gain (measured in db). Note that an aerial is a passive device – it does NOT give your mobile any more power.

What it can do is to focus the power of your device more efficiently. A high-gain aerial sends more signal out horizontally (and less straight up in the air), so it’s a better choice for a fixed mount on a boat.

Most mobile phones are fitted with low-gain aerials, since the radio signal in a city bounces off buildings and can come from any direction. This also means the phone works equally well no matter how you hold it.

The effects of a high-gain aerial are dramatic. A 5db aerial can increase the range of your phone by as much as 40 percent, a 9db up to 60 percent more range. Another way to increase the efficiency is to mount the aerial on a metal plate (called a groundplane) at least 40cm in diameter (This size is just larger than one wavelength of the radio signal being emitted.)

Since radio waves are reflected by the metal, no energy is wasted by being directed down into the water. Incidentally, forget about those little stick-on aerial boosters – they just don’t work.

So the first investment to make for your boat is an external aerial for your mobile. These are available from the same place you bought your phone, or from third parties such as Powertec and Tennatron (www.tennatron.co.nz)

Depending on the model of your phone it may be convenient to buy a car kit (for your boat) that includes an aerial. You can clip the phone into its cradle and run it off the boat’s power and use the aerial. At the end of the day you put the phone in your pocket, avoiding the need for a separate “boat phone”.

Height

The third factor affecting phone range depends on your location, or more specifically, how high above the water you are and the height of the nearest network cell site. Out on the water there are no structures for the signal to bounce off, so cell phones need “line-of-sight”. Due to the earth’s curvature, the higher you are the further they can “see” and hence the greater possible range of the phone.

A cell tower is typically around 10m high, which at sea level only gives it a visibility of around 6.5nm, assuming you are at sea level yourself. If you are standing on a boat and the phone is, say, 1.5m above sea level, you’ll increase the range by 2.5nm, or 9nm in total.

Most cell towers are typically put on top of buildings or hills. However, the relationship between height and distance is not linear – at a 50m height the horizon is still only 15nm away. So five times the height gives less than twice the distance. (For a quick calculator to work out the distance to the horizon, go to www.boatsafe.com/tools/horizon.htm and enter your height above sea level.)

The best thing, then, is to position your aerial as high as possible. On a game fishing boat this could be as high as 6m off sea level, adding over 5nm to the total distance. So assuming a tower situated 50m above sea level, and an aerial height of 6m, the maximum range is around 20nm. If you don’t have a fitted aerial then your best option is to stand on the roof – not recommended in rough weather!

Network

The last issue to consider is the range provided by the networks themselves. At first glance the numbers seem to favour Telecom’s CDMA system. GSM has a fixed maximum cell range of either 35km (18.5nm) or 70km (37nm), depending on the technology installed at the tower, while CDMA has no theoretical maximum range. (The reason for these differing ranges is technical and we need not concern ourselves with it here).

However the CDMA technology’s range depends on the traffic loading on the cell. A fully-loaded CDMA cell, such as in a busy urban centre, may have an effective range of less than 1.5km, whereas a GSM cell’s range is unaffected by loading. This loading may vary by season, especially in popular holiday destinations, and hence range may vary quite dramatically from one day to the next.

So CDMA has a greater theoretical range but may have a poorer effective range. Have a look at the coverage maps on both networks’ web sites. Bear in mind these maps are “best-case coverage based” using an industrial eight watt handset and industrial-grade aerial, so your range will not be anything like shown on the map. Note that only some areas are covered by one or the other of the networks.

Work out what sort of range is possible (based on the height of your closest cell tower and your own height above the water). Some investigation may be required to find this out. Decide how far your typical trip takes you off the coast.

Finally – if everything works out the same, check which network gives you the best deal.

Frequencies

Vodafone’s GSM mobile network operates in the 900Mhz and 1800Mhz frequency ranges while Telecom’s CDMA network is in the 870-890Mhz range (sometimes confusingly called the 800Mhz spectrum). By comparison, the marine VHF radio channels are in the 156 to 157Mhz range.

Mobile phones, by virtue of their digital technology, are duplex (i.e. you can talk at the same time as your caller) unlike a VHF radio which is simplex (i.e. you can talk or listen but not both at the same time).

Mobile phone technology also divides each frequency among multiple users, so multiple calls occur at the same time. On VHF, each channel can be used by only one person at a time.

GSM achieves this multiple caller use through a system called TDMA (Time Division Multiple Access), which is similar to that used by the older Telecom 025 network) while Telecom’s 027 network uses CDMA (Code Division Multiple Access).

Be careful when importing boosters from the USA since some of their networks use a different frequency. Your phone may be a “tri-band’ model that works overseas, but the booster must be the correct band. For CDMA buy a unit that is compatible with the Sprint CDMA network, and for GSM ensure you get a 900Mhz unit.

It’s sometimes argued that the frequency used by a radio has a bearing on its range due to the effect of atmospheric interference. In general, the lower the frequency the longer the wavelength, and the less it is affected by pollution, etc.

The wavelength of a VHF radio is 1.8m while the GSM band’s wavelength is around 33cm, so neither system is unduly affected by atmospheric interference. The long wavelength of VHF is why a groundplane is seldom installed for a VHF aerial – it would need to be about 2m in diameter.

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