You know about coaxial cable (Fig. 1). We all use it in one form or another, and it seems simple enough. But while modern cable products are better than ever, there are some real subtitles in their selection and application.
Connecting dc and low-frequency ac including audio is easy. You just run some wires from point A to point B. The biggest challenge may seem to lie in choosing the right connector (see “Coax Connectors”).Your main concern, though, is resistance over the longer runs as well as high-power or high-voltage signals. Frequency doesn’t usually enter into it. But try that with signals with frequencies over a few hundred kilohertz, and weird things start to happen.
At these frequencies, the inductance and capacitance of the cables begin to come into play. The serial inductance and shunt capacitance form a distributed low-pass filter. The cable begins to store energy and delay the signals applied to it, not to mention attenuate them. The cable becomes a transmission line with very specific characteristics.
A cable doesn’t act like a transmission line until it is more than 0.1 ? long at the frequency of operation. For example, one wavelength at 450 MHz is:
? = 984/fMHz in feet
? = 984/450 = 2.19 ft
0.1 ? = 0.1(2.19) = 0.219 ft or about 2.63 in.
At this frequency, a pair of conductors over 2.63 in. long will have the characteristics of a transmission line.
The basic characteristic of a transmission line is that the cable will act like a complex impedance (R ± jX) to a signal source unless it is terminated in its characteristic impedance (ZO). The characteristic impedance (sometimes called surge impedance) of a transmission line is a function of the inductance (L) and capacitance (C) per foot or other unit of length or:
ZO = v(L/C)
ZO is a pure resistive value. An infinite length of the transmission line will appear to be a resistance equal to ZO to a signal source. Terminating any other length of line with a resistive load equal to ZO will appear to be a resistive load of ZO to a generator.
If the transmission line isn’t terminated in its characteristic impedance, the generator will see a complex impedance that is a function of its length. In addition, an improperly terminated transmission line will produce reflections. Signals not absorbed by the load are reflected back down the line toward the generator producing standing waves.
Standing waves are stationary variations of voltage and current along the line. These standing waves are the sum of the incident or transmitted signal and any reflected signal not absorbed by the load. In a matched line or one properly terminated, the voltage and current along the line is constant. Standing waves are undesirable, as they can cause signal distortion (for pulses), losses, and excessive voltages or current.
Coax cable is an ideal interconnection medium because it is self-shielding. The electromagnetic wave that propagates down the line stays entirely within the cable, except for some leakage where the shield isn’t solid. Solid foil shields do a better job than braid. But there are coax cables with two or more shields to ensure no signal leakage.
Unlike twisted pair, coax signals do not produce nor are they subject to cross talk and other coupling problems. Coax keeps noise and stray signals out and the desired signal in, meaning you can run coax cables directly parallel to one another or with twisted pair without interference.
COAX SPECIFICATIONS
The primary specification of a coax cable is its ZO. The most common value is 50 O, with 75 O also widely used. Most wireless and test applications use 50-O cable. Cable TV and VIdeo uses 75-O cable. Other available impedances are 93 and 125 O, but they aren’t as common. The impedance is set by the physical nature of the cable—specifically, the inner and outer conductor dimensions, their spacing, and the dielectric constant (e) of the insulating medium.
Voltage standing-wave ratio (VSWR) is an important factor in applying coax, but it is not a specification as such. It is usually calculated as:
VSWR = ZO/ZL or ZL/ZO
depending on which proVIdes a value greater than one. ZL is the load resistance. VSWR is actually the ratio of the maximum peak voltage to the minimum voltage along the line. It is related to the reflection coefficient (G), the ratio of the reflected voltage VR to the incident voltage VI:
G = VR/VI
The ideal G is 0. VSWR is calculated using the reflection coefficient: VSWR = (1 + G)/(1 – G) The ideal VSWR is 1, but many applications can tolerate mismatches with VSWR as high as 2 or 3 without excessive power loss. Figure 2 relates VSWR to power lost due to reflection.
The velocity factor (VF) is one more common parameter. It is the ratio of the propagation of the signal in the cable to the speed of light. Also, it is a function of the dielectric constant of the insulating material:
VF = 1/ve
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