It is important for licensed amateurs to
know how much power they are allowed to use and how much power they are actually using. In
the UK the maximum amount of power permitted when transmitting is 400w, but not on all
bands. On some bands and parts of others the maximum amount of power permitted is very
much less. If you do not know how much power you are using, you might be in breach of the
terms and conditions of your license, in which case you will lose your license. You have
been warned.
Power is measured in Watts.
This unit is named after a man called James Watt (1736 - 1819). 1 Watt is equal to 1 Joule
per second. Joules are the units of energy, named after James Prescott Joule who was a
brewer (1818 - 1889).
If you are very old fashioned, you might
measure power in a different kind of unit = horse power. 1HP is the same as 750 watts. I
suppose this is how much power a horse is using when it pulls a cart load of beer from the
brewery to the pub.
Watts:
Power can be defined as "the rate of transferring
energy". Think about the light bulb in your bedside lamp; it might be 25w.
This means that every second 25j of electrical energy are turned into light (that is only
if it is 100% efficient, in fact some of the energy ends up as heat instead of light).
When I am using maximum power on my motorcycle (well over the speed limit) 60,000 watts of
energy are being used every second. Here are some typical values:
| 25w |
the rating of the light bulb in a bedside lamp |
| 100w |
the rating of the light bulb in my study |
| 1Kw |
the power rating of my electric kettle |
| 3Kw |
the power of a 3 bar electric fire |
| 60Kw |
the power of my motorcycle |
| Millions of watts |
the power supplied to a small town |
There are a number of equations which can be used in calculating
power, but the one you need to know about for the Radio Amateurs Exam (P = V x I) is
explained below.
If you understood Ohm's Law you will
find this simple: there is a relationship between power, voltage and current shown in the
equations below. If you know two of the values you can easily work out the third one.
| P = V x I |
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P |
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| V = P / I |
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| I = P / V |
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V |
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I |
The "P over V and I
triangle" should help you to remember these
three equations. If you know the voltage and current and want to calculate the power, you
use the first equation. If you know the power and current and want to calculate the
voltage, you use the second equation. Lastly, if you know the power and voltage and want
to calculate the current, you use the third equation.
You can do some simple calculations for yourself using one of
these equations. You know that your mains voltage is supposed to be 250v (that is a
nominal value). You also know that your bedside lamp uses 25w. Using the third equation
you will see that it uses 100 milliamps. What about your kettle? 1000w and 250v fed into
the third equation gives 4amps.
You could go around your house and look on the back of various
appliances to find their power ratings. There should be a little label on the back of your
TV giving this information.
My rigs will only produce about 50w when I am transmitting by
telephony. I would like to think that all the electrical energy fed into my antennas ends
up as radio waves, but I know that 100% efficiency is just not possible: some of the
energy will end up as heat. The current in my feeders (the co-axial cables feeding the
antenna) should be about 1 amp and the voltage about 50v. (It is not really as simple as
this because the currents in my feeders are r.f. (radio frequency) not ordinary direct
currents.
Caveat: What
happens in my radio antenna when I transmit is not quite so simple: it is not DC current.
When I transmit some points on the antenna have a very high potential difference relative
to earth and other points have a very high current. My radio amateurs licence allows me to
use up to 400 watts on some frequencies. If I decided to purchase a more powerful
transceiver (rig) you would be well advised to leave my antennas alone.
Note: this page was first written for students of the Radio
Amateurs Exam.
Now read my page about the decibel watt (dBW) which is the unit used in the
Amateur Radio Licence (A) or (B) Terms, provisions and Limitations Booklet BR68. If you
have this booklet have a look at the "schedule" in the middle of the booklet (if
you have not got BR68 get one now). You will find figures like 26dBW (400w), 22dBW (160w),
20dBW (100w), 16dBW (40w), and 15dBW (c32w) in column 4 the "Maximum Power level (in
dB relative to one watt) PEP".
Nigel D Purchon G0WKM.
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