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The maximum permissible cable length between a Meinberg Antenna-/Converter
unit and the GPS-receiver merely depends on the characteristics of the antenna
cable. Only coaxial cable with an impedance of 50 Ω can be used in principle,
because the impedance of the cable has to match the output and input impedance
of the Antenna/Converter unit and the receiver.
Besides the impedance, the most important property of the antenna cable
is the attenuation factor for the highest frequency transmitted on the
cable. In a Meinberg GPS-receiver system this is the intermediate frequency
(IF) of 35,42 MHz, send from the Antenna/Converter unit to the receiver.
An attenuation of 30.4 dB is allowed for the IF by which the maximum
distance between the Antenna/Converter unit and the receiver is prescribed for
a specific attenuation of an antenna cable.
The attenuation factors of coaxial cables are given for freuqencies
of 10 MHz, 50 MHz, 100 MHz or 800 MHz in the data sheets usually. To determin
the attenuation factor for the IF, the following approximation formula
can be used:
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α2 = α1 * √ ( f2 / f1 )
with:
- α1 : attenuation for f1
- α2 : attenuation for f2
Example: the coaxial cable RG-58 C/U has an attenuation factor of 17
dB/100 m for a frequency of 100 MHz. By using the equation above, the attenuation
for the IF can be calculated to:
α2 = 17 dB/100m * √ ( 35,42 MHz / 100
MHz ) = 10,12 dB/100m
If the maximum attenuation of 30.4 dB for the IF is taken into account,
the maximum length of the antenna cable - when using a RG-58 C/U cable
- can be calculated now:
lmax = 30.4 dB / α2 = 300 m
If a coaxial cable of a higher-grade is used, the distance between the
Antenna/Converter unit and the receiver can be increased according to the
lower attenuation factor of the cable without the necessity of an additional
amplifier. If the calculation above is done for the coaxial cable RG-213
/U (attenuation factor: 7 dB/100 m for 100MHz) for example, the maximum
cable length is 700 m.
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