Page 245 - Maxwell House

Chapter 5 225

Note that for well-designed dish antennas this factor can be up to 0.8. Putting (5.56) in (5.55)

⁄
we obtain with the overall coefficient = 0.7 and = 27.3 mm while =
1 123 900 or 60.51 dBi. It means that the dish antenna with a generator of 1W RF power can
send to the receiver the same power as an isotropic radiator connected to the transmitter of 1 1
123900W! It, therefore, indicates that the pattern of such a dish should be very narrow like
shown in Figure 5.2.10.
5.2.12 Directivity, Effective Aperture, and HPBW

It is physically clear that the antenna directivity grows as soon as its pattern beamwidth shrinks.
Therefore, we can expect that the simple inverse relationship may exist between these two

parameters. Particularly, for relatively high gain antennas with HPBW < 10 [degrees] = 0.087
[rad]

4 16 16
= ≅ = (5.57)
2 ∙ () 2
Here and is the half-power-beamwidth [radian] in Azimuthal and Elevation

cross section, respectively. The last expression in (5.57) corresponds to antenna with
symmetrical pattern, i.e. = = .

5.2.13 G/T Parameter

is a fundamental RF system design parameter of merit proportional to the ratio between
⁄
the antenna gain G and the noise level measured at the system receiver input in units of noise
temperature T [°K]. In general, that level includes the total noise generated by the antenna itself,
the antenna’s environment and all RF lossy elements as cables, matching network, filter,
amplifier, etc. connected between the antenna and FR receiver. The higher this parameter the
better the sensitivity of system to weak signals. The parameter is typically expressed in
⁄
dB as [dB] = G [dBi] – 10*log . Customary, this parameter should be between 20 -30 in
⁄
the relative units.
5.2.14 Antenna Factor

We must keep in mind that everything starting from our everyday appliances to broadcast
stations, radars, cell phones, even our cars, etc. emit EM waves. They penetrate our body and
can sometimes threaten human health and compromise our environment. RF safety regulations
and standards are typically formulated in terms of the electric field intensity [V/m]. Since the
direct measurements of this intensity in hazard areas is a quite challenging task engineers prefer
to remotely measure the voltage at the output receiving antenna impedance shown in

Figure 5.2.2. Evidently, both values are proportional to each other, and we can introduce a
parameter called antenna factor

= [1/m] (5.58)

It is usually expressed in dB as [dB] = 20log E – 20log V. We present the final expression

1 4 0 0
= � = � [1/m] (5.59)
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