Arbiter Systems 1084A, Operation Manuals

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4.4 Technical Details on GPS, Antennas and Cables 21
4.4 Technical Details on GPS, Antennas and Cables
4.4.1 Antenna Cable
Length and Loss Considerations
Standard Antenna Cable
The standard antenna cable assembly included with the clock is constructed using a 15-meter
(50-foot) length of RG-6 type low-loss coaxial cable, terminated with male Type F connectors.
Optional lengths of RG-6 coax are separately available for longer runs; see Table 4.2, Cable Data
and Accessory Information.
Effects of Cable Parameters
To receive GPS signals and properly operate the clock, the type and length of the cable are impor-
tant. Due to their effect on specific parameters described in the following paragraphs, any changes
to the length and/or type of antenna cable should be made carefully. Damaged cables may also
affect performance.
Cable Delay
The velocity factor and the physical length of the cable determine cable delay. During the initial
factory calibration of the clock, a value for cable delay (based upon the length and type of cable
supplied) is entered into the clock memory. Firmware uses this figure to counteract the effect that
the delay has upon GPS timing accuracy. The value entered for a standard 15-meter cable is 60
nanoseconds. For other cable options, the delay is tabulated below. The formula for calculating
cable delay is:
(4.1) T = λ
1
CKv
+ 1 ns
Where:
T = Cable delay, in nanoseconds;
λ = Cable length, in meters;
C = Speed of light (3
×
10
8
meters per second);
Kv = Nominal velocity of propagation (0.85).
One nanosecond is added to the calculated value to account for the length and velocity factor
of the short connecting cable inside of the clock.
Attenuation
Attenuation depends upon the cable length, and the loss per unit length. The total attenuation
must be limited to 21 dB (maximum) at the GPS L1 frequency of 1575.42 MHz. Loss up to 42 dB
can be accommodated with the separately available 21-dB in-line preamplifier (P/N AS0044700).