Ok lets start with Frequency. Why?

Frequency is a factor to multiply power by to find the energy, required to transmit or dissipated in a receptor (receiver)

Network Rail's frequency is around 900Mhz like Orange and T-Mobile, a third that of a microwave oven (2450Mhz).

Power and energy are not the same. Radiation may be quoted as having the same power in terms of voltage or amplitude, but if the frequency were twice as high then it would contain twice the energy. So the example of the comparison of the mobile telephone frequency and the microwave shows you that three times more energy is dissipated in the microwave oven, which is a benefit of a higher frequency. This is also why x-rays and gamma-rays are dangerous even at low power, as due their high frequency then dissipate a enormous amounts of energy.

OK back to the antenna in hand.

The antenna controls the power output. Imagine an antenna the shape of a globe, like the sun. It radiates it's energy theoretically equally in every direction. It would therefore be said to have a gain of 1 which is a 0dB (decibel - 10 to the power of 0). Multiply by 1 and the number stays the same. So a gain of 1 or a gain of 0dB is no gain at all, but its not a loss, which is important.

As most antenna are concerned with transmitting over the surface of the Earth, they are constructed to direct the radiation more horizontally than vertically. They focus the beam, like a reflector in a torch or car headlight.

So lets look at the output of 1 watt

With a globe antenna the 1W is distributed over a specific area, but if that was focused into a beam then the resulting antenna would be said to provide a Gain. This gain is a multiple of that which would have been available had the antenna been a globe, well actually the standard is a dipole a stick like structure that radiates a doughnut shaped aura..

The basic terrestrial antenna, a dipole minimises vertical radiation and will have a gain of some 2.15dB over the theoretical global effect. This is often set as standard and called dBD. The theoretical, based on the idea of a sphere is called dBi.

Antenna output standard measurements use a dipole and 0dBW means an output of 1 watt. Theoretically only half a watt is needed to provide the input as the gain is 2.15dB from the theoretical perfect omin-directional antenna.

The antenna used by Network Rail has a output of 55dBm the maximum input is 46dBm provided by equipment in the base cabinet.

dBm, So what's the m.

It represents milliwatts.

The dB is the mathematically function used so that large numbers that would have to be multiplied can just be added. (It's logarithmic function to the base of 10, if that's any better)

For example I have already stated than 0dB = a gain of 1, or no gain at all.

If we are to talk of power, as in watts then we can use dBW, 0dBW = 1 watt. dBmW or more commonly dBm equates to a milliwatt or one thousandth of a watt.

Due to the mathematical function of dB, 30dBm is the same as 0dBw. or
0dBm = 1mW; 10dBm = 10mW; 20dBm = 100mW and 30dBm = 1000mW = 1w

The 3 in the 30 reads as the third power of ten ie 1000. So to divide by 1000 subtract 30 dB. To convert dBm to dBw subtract 30 from the figure and replace the m by a w. Simples :)

Ok! if you are lost, call me or take a break. Else:

The amplifier in the antenna cabinet that Network Rail place at the foot of the mast is rated at 46dBm or 16dBw. 16dBw is just under 40 watts.

If you are not keen on maths use a spreadsheet with the following formula: y=10^(x/10-3) to convert dBm to watts or y=10^(x/10) to convert dbW to watts.

• In the first column put the numbers you want, maybe 0 to 100dBm in steps of 5.
• In the column to the right enter the formula replacing the x with the cell to the left. i.e. =10^(A1/10-3) to find the dBm in watts.

The latest info is that the antenna at Calstock will have a maximum output or 55dBm or 25dBw

Thus the gain of the antenna is 9dB (the antenna output minus the amplifier watts)

If everything is perfectly efficient and all working at maximum power then the max output in the main beam is equivalent to 316 watts.

You can see the benefits of the directional antenna is a reduction in energy/power used to get a high signal strength in a specific direction.

So it's not too good to be standing with your brain or other sensitive parts resting on the antenna, but is it as scary as it seems. Well no! The maximum power than can come from the antenna is 40 watts as that is the maximum the amplifier can produce, but you wouldn't want your head next to a 40 watt light bulb nor I imagine a 1 watt bulb.

Consider the actual amount of power, not energy, that we really could come close to feeling. If the surface area of the antenna is the same as your hand and you place your hand on the antenna and the efficiency is 100% then whatever the antenna gain you will get 40 watts dissipating in your hand. This is a 'Near Field energy' The hand would literally conduct the energy away. When a reasonable distance exists there would only be directional radiation which reduces at the square of distance.

If your hand has an area of 100sqcm and you position it 1m away from an omni-directional point of radiance, your hand will only occupy 1/1257 of the radiated area. So if all the radiation is omni-directed then you will receive 0.03watt or 30mw or 15dBm. However the antenna may have a 9dB gain so you would get almost 9 times more power to your hand if you position it in the main beam at 1 metre else you will get even less power to your paw.

Now there are many loses to the amplifier output and then it is not likely that 40 watts will be produced unless the signal is at max, like someone screaming down the line, and voice is intermittent and only used when there is need to communicate.

So the power out of the amplifier is likely to less than 20 watts, interrupted and very infrequent.

The amount of radiation anyone is likely to be exposed to is very low. In fact much lower than you would get from sleeping with your partner. A body gives off 100w continually mostly infra red which contains proportionally more energy due to being a much higher frequency.

The radiation received from smoking one cigarette a day, largely due to the polonium in tobacco, is over 500 times more than each of us receives in a year from all the nuclear fallout from nuclear weapons and Chernobyle.

To put the danger into perspective with other types of more energetic radiation see the following table as regards X-rays, Radon, cigarettes etc.

So why make such a big issue of the radiation? Because people have fear of the unknown as they don't feel in control. They get tense and ill. This is the reason for objecting to the mast. It's proposed location 'causes' concern, which in turn 'makes' people ill. Strictly this is not true but I will discuss that elsewhere under a new topic entitled 'What's the problem?'

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