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To All
As I am an amateur on matters of antennas and
propagation I enlisted the help of Koos Fockens, PA0KDF, who
is
professionally involved. At my request Koos quoted the following
definitions from the Radio Regulations (1994; RR1-25):
6.24 Gain of an Antenna:
The ratio,
usually expressed in decibels, of power required at the input of a loss-free
reference antenna to the power
supplied to the input of the given antenna to
produce, in a given direction, the same field strength or the same
power
flux-density at the same distance. When not specified otherwise, the
gain refers to the direction of maximum
radiation. The gain may be considered
for a specified polarization.
Depending on the choice of the reference
antenna a distinction is made between:
a) absolute or isotropic gain (Gi), when the
reference antenna is an isotropic antenna isolated in space;
b) gain relative to a half-wave dipole (Gd),
when the reference antenna is a half-wave dipole isolated in space
whose
equatorial plane contains the given direction;
c) gain relative to a short vertical antenna
(Gv), when the reference antenna is a linear conductor, much shorter than
one
quarter of the wavelength, normal to the surface of a perfect conducting
plane which contains the given direction.
6.25 Equivalent Isotropic Radiated Power
(e.i.r.p.):
The product of the power supplied to the antenna and the antenna
gain in a given direction relative to an isotropic
antenna (absolute or
isotropic gain).
6.26 Effective Radiated Power
(e.r.p.):
The product of the power supplied to the antenna and its gain
relative to a half-wave dipole in a given direction.
6.27 Effective Monopole Radiated Power
(e.m.r.p.) (in a given
direction):
The product of the power supplied to
the antenna and its gain relative to a short vertical antenna in a given
direction.
Now on the subject of measuring field strength
using CCIR Recommendation 368-7. I quote from the document;
"The propagation curves in this
Recommendation are calculated for the following assumptions:
- they refer to a smooth homogeneous
spherical Earth;
- in the troposphere, the index decreases
exponentially with height, as described in Recommendation 369;
- both the transmitting and the receiving
antennas are at ground level;
- the radiating element is a short vertical
monopole. Assuming such a vertical antenna to be on the surface of a
perfectly
conducting plane Earth, and excited so as to radiate 1 kW, the
field strength at a distance of 1 km would be 300 mV/m; this
corresponds to a
cymomotive force of 300 V (see Recommendation 525);
- the curves are drawn for distances
measured around the curved surface of the Earth;
- the curves give the value of the vertical
field-strength component of the radiation field, i.e. that which can
be
effectively measured in the far-field of the antenna.
So what we measure in the far field is the field
strength caused by the power that must be fed to a short vertical
monopole
over the type of earth used for the graphs.
Koos, PA0KDF points out that your real antenna
may be radiating more power because the measured field strength is that of
the
ground wave on the surface of the earth. But the antenna
may be radiating in other directions as well, for instance upwards
(which is
of use when the sky wave helps you to make a QSO over a great distance) or
downwards into the (non-
perfect) earth, power that is lost for propagation.
The total power radiated by the antenna is the
product of the antenna current squared and the radiation resistance (I
called
this earlier the power "dissipated" in the radiation
resistance). Because not all power radiated propagates along
the earth
surface as ground wave it is not possible to find the power radiated as ground
wave by measuring the antenna current
and after squaring multiplying it by
the radiation resistance as found for instance from an antenna simulation
computer program.
For the same reason the radiation resistance cannot be
found by working backwards from the measured field strength and the
antenna
current.
When instead of the power fed to a vertical
monopole, as used in Recommendation 368-7, you want to express the power
as
e.r.p., you have to replace the monopole on its earth by a vertical,
loss-free half-wave dipole, and find the power that
has to be fed to that
dipole that produces the same field strength as the monopole. Now here we have a
problem: for this exercise we need to know the gain of the monopole over the
dipole. In case of a monopole over perfect earth this gain is 1.83 times (not
1.83
dB!). I stated earlier that this factor should be used for converting
the power as found by field strength measurement to
e.r.p. But I take that
back.Your and my monopole are not over perfect earth. So what is the gain? And
so what is the
e.r.p.? I don't know the answer ....(And how does the Radio
Agency in the UK find the e.r.p.?)
Maybe it is better to avoid using e.r.p. at all.
What you could use is
e.i.r.p., see the definition given above.
As stated before instead of using the CCIR
curves the following equation can be used near the transmitter but in the far
field:
P = 0.0111 * (E * d)^2 in which
P in watt
E in mV/m
d in km.
^2 means
squared
* means multiplication
The equation is valid in case of propagation
over perfect earth. But the CCIR curves show that water or real earth may for
our purpose be considered as perfect earth up to at least 3.5 km from the
transmitter; the recommended minimum distance for measuring field strength in
the far field. Only in case of very dry ground the CCIR curves should be
applied.
You can state your income in for instance pound
sterling or dollar. The figures are different but the exercise does not
make
you any the richer or poorer.
The same applies to the different ways of
expressing power found by field strength measurement. It does not make
your
signal stronger or weaker.
The financial people have found a solution for
the problem of varying exchange rates: the euro.
Should we do the same and express the power
found by field strength measurement in eurowatt?
73, Dick, PA0SE
JO22GD
D.W.
Rollema
V.d. Marckstraat 5
2352 RA Leiderdorp
The Netherlands
Tel.
+31 71 589 27 34
E-mail: [email protected]
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