For coax, the VF is usually in the 0.6 to 0.9 range. Table 1 shows the most common dielectrics used in coax and the velocity factors. The VF affects the length of a wavelength of a cable. One wavelength is:
? = 984/fMHz
One wavelength of coax is:
? = 984(VF)/fMHz
Capacitance per foot is another common parameter. It too depends on the dielectric constant. The typical range is from about 6 to 31 pF per foot. Note that the lower the dielectric constant, the lower the capacitance per foot and the lower the decibel (dB) loss in the cable.
One especially important specification in high-power applications, the maximum voltage rating, is usually given as the RMS value of the maximum voltage rating. It ranges from 1000 to 15,000 V. Be sure to know the maximum peak value (1.414 × RMS) of the signal to be transmitted to ensure you are within the safety range.
If you know that there is a mismatch involved from cable ZO to load, then to determine the approximate effective value of voltage involved, multiply the actual input voltage by the square root of the expected VSWR. Incidentally, some coax also carries dc. The maximum dc voltage that can be applied is about three times the ac voltage maximum.
Time delay is an inherent characteristic of any transmission line, as it takes a finite amount of time for the signal to propagate through all that inductance and capacitance. That time delay (tR) shifts pulses and produces phase shifts in sine waves. It is a function of the dielectric constant:
tR = 1.016 ve ns/foot
One of the most critical specifications of coax is its attenuation, which is usually stated in dB power loss per foot. Specifications are sometimes stated as dB/100 feet or as dB/100 meters. Of course, that value increases with the frequency of operation.
When high frequencies and long lengths are involved, the cable represents a major loss of power. For example, a common RG- 58A/U cable has a typical attenuation of 5.3 dB per foot at 100 MHz. That is a loss of –0.53 dB/foot. If you put 100 W into this cable, you will get out only 29.5 W. That is a massive loss of 70.5 W in the cable itself. Attenuation is critical. For a given application, your job is to select the cable that will have the lowest possible loss—and keep the cable as short as possible.
A trend in wireless today is to locate the transmitter and/or receiver at the top of the antenna tower to avoid high transmission line losses. To achieve a desired output power, the designer has to produce a more expensive higher-power transmitter to compensate for cable loss. Tower-top electronics has gotten easier with smaller and lighter components, but it is still an issue in the cellular business where the need to climb towers for maintenance and repair and wind loading are still big problems.
A coax cable is a long low-pass filter whose cutoff frequency decreases with length (Fig. 3). But you can use coax well up into the gigahertz region. This is where waveguide is normally used. Yet for short runs of cable, coax is a reasonable design solution. Just watch the attenuation figures and select the lowest-loss coax you can find. Lengths of coax from a few inches to a few feet are practical at frequencies up to about 50 GHz. Generally, the larger the diameter of the cable, the lower its attenuation—but also the lower the operating frequency.
SELECTING A CABLE
There are thousands of different cable sizes and types. The most common ones are designated with the letters RG. The RG standards came out of World War II. RG means radio guide, and the U suffix often attached to the RG designation means universal. The RG standard is no longer used, and different RG numbers will probably have different specifications from manufacturer to manufacturer. Militaryspecification coax cable has an M17 designation. The standard is MIL-DTL-17H. The international standards with the IEC are 60096 and 61196.
A good choice is to stay with the popular and common types of cable, as they are widely available from multiple sources and cost less than some of the specialty cables (Table 2). The primary application will determine the most important specifications. Other important cable specifications include operating temperature range, the outside diameter of the cable, and the weight of the cable in pounds per foot.
Also, consider the environment, such as rain, wind, and ultraviolet exposure, as well as if cable flexing is involved. Coax does not flex well. Examine the manufacturer’s specifications and applications carefully.
HARD LINE
As its name implies, hard line is coax that isn’t flexible like regular coax cable. It’s essentially a pipe within a pipe whose outer conductor can be up to several inches in diameter. Keep in mind that hard line isn’t waveguide, though. It truly is coax cable, but it’s designed for high power and low loss at UHF and low microwave frequencies. It is widely used for radio and TV broadcast antenna feeds and cellular basestations.
Most hard line is made with a solid copper outer shield with a solid copper inner conductor that may also be a small tube. The dielectric insulation between the two may be a foam polyethylene, air, or pressurized gas like nitrogen. The gas keeps the interior of the line dry since moisture may collect and attenuate the signal in most pipes. When air or gas is used, plastic or nylon spacers are used internally to keep the spacing between the conductors stable and consistent.
A good example of the latest type of hard line is the Cellflex series of cables made by Radio Frequency Systems (Fig. 4). They’re available in diameters of 0.5, 0.875, and 1.625 in. The center conductor is a copper tube, and the outer conductor is a corrugated aluminum tube. The corrugation makes the tubing bendable.
The dielectric is polyethylene foam with a VF of 0.90 and a capacitance of 22.9 pF/ ft. The impedance is 50 O. As for specs, the 0.875-in. cable is usable up to 5 GHz. The attenuation at 1 GHz is a low 1.28 dB/100 ft. Power rating is 2.53 kW at that frequency.
Coax has been around for decades. With its continuous improvement over the years, it is still the connecting link of choice for RF and video. Fiber-optic cable may be making continuous inroads of its own, but for now, coax is still king.