A brief reminder of the link budget (deFrijs' equation; here written in a convenient
form where all quantities are given in dB!!):
- P(received) = P(transmitter) + G(transm.) - L(free space) - L(atmos.)
+ G(receiver)
- P(received) is the power received by the ground station (usually in dBm =
dB above 1mW)
- P(transmitter) is the power of the transmitter (again given in dBm !!! Note
that 0 dBm = -30 dBW)
- G(transm.) is the gain of the transmitting antenna (for the CubeSats we may
take 0dB, as the antenna is but a dipole or a monopole whip)
- Note that EIRP = P(transmitter) + G(transm.) is the Effective Isotropic
Radiated Power
- L(free space) is the Free Space Loss: it is computed from the distance
d and the wavelength λ
L_dB = 20 log10(λ/(4πd))
If we measure the distance in km, and give the frequency (in MHz) instead
of the wavelength, we can use this handy formula:
L_dB = 20 log10(0.3/(4πf_MHz d_km))
This 'loss' simply accounts for the fact that at a distance d the
energy emitted from the transmitter is distributed over a sphere with
the radius d.
- L(atmos.) is the sum of any attenuation by the ionosphere and the
terrestial atmosphere ... for our purposes, it suffices to use an estimate
of 1 dB
- G(receiver) is the gain of the ground station's antenna. Our antenna on
432 MHz has a gain of 19.3 dBi.
- Note that we shall measure P(received) at the antenna terminal of
the station's receiver, but that between this point and the antenna's
feed point there is a masthead preamplifier which boosts the signal
by 20 dB and also the cables from the roof to the station room, which
cause a slight attenuation of the signal (by 2.5 dB). Thus, when we
compute the expected power at the receiver input, we can thus add
+17.5 dB to the antenna gain G(receiver).
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last update: April 2013 J.Köppen