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Unshielded category 5 twisted pair cable (UTP CAT5) is used as the means of transmission in computer networks using Ethernet 10baseT and 100baseT standards. The cable consists of 4 pairs, its wires have 0.5 mm diameter (the version with cord, instead of single wires has 7 cords, 0.2 mm diameter each, and total diameter equal 0.6 mm). In practice, the cable is commonly used in CCTV applications, as it can be employed both to transmit video signal and power CCTV devices.
Practical tests prove, that despite of different impedance of twisted pair cable and that of CCTV camera (twisted pair has impedance 100 ohm and the camera - 75 ohm), there can't be observed any video quality deterioration (concerning reflections caused by impedance mismatch) even if we use 40 m long cable.
It should be taken into consideration that application of long cable causes voltage drop which may disturb or block camera's operation, therefore it's necessary to make calculations that will give us the maximum distance between power supply and the camera.
One pair can provide current up to 1A, but total current in 4 pairs shouldn't exceed 3.3A; the resistance of one pair (as closed loop) is 0.0572 ohm/feet, that is 0.2 ohm/m.
Example:
CCTV camera powered with U = 12 VDC, acceptable voltage drop dU= 0.1U, current consumption I=150mA.
Maximum acceptable resistance: R[ohm] = dU[V] / I[A] = 8.
In this case, the maximum length of the cable: l[m] = R[ohm] / r[ohm/m] = 40.
It means that the length of twisted pair used as power line for the camera can't exceed 40m.
Twisted pair parameters:
- Impedance Z0 = 100 +/- 15 ohm.
- Wave attenuation - describes loss of signal along the cable: the higher frequency, the higher attenuation; given in dB/m.
- Attenuation of near-end crosstalk (NEXT) - informs what part of the signal comes from the interfering pair to the interfered pair, for the ends of pairs lying on the same end of the cable.
- Attenuation of far-end crosstalk (FEXT) - informs what part of the signal comes from the interfering pair to the interfered pair, for the ends of pairs lying on the opposite ends of the cable.
- Attenuation to crosstalk ratio - characterizes transmitting capabilities of the cable - it informs about the ratio of interferences coming from neighbor pairs to the useful signal.
Illustration of near-end crosstalk - NEXT
Illustration of far-end crosstalk - FEXT
