Both peak and average power ratings are required to fully describe the capabilities of a transmission line. In most applications, the peak power rating limits the low frequency or pulse energy, and the average power rating limits high-frequency applications, as shown in Fig. 4.4.
Peak power ratings are usually stated for the following conditions:
VSWR = 1.0
Zero modulation
One atmosphere of absolute dry air pressure at sea level. The peak power rating of a selected cable must be greater than the following expression, in addition to satisfying the average-power-handling criteria:
E pk > Pt × (1 + M)^2 × VSWR (4.6)
where E pk is the cable peak power rating in kilowatts, Pt is the transmitter power in kilowatts, M is the amplitude modulation percentage expressed decimally (100%=1.0), and VSWR is the voltage standing
wave ratio.
From this equation, it can be seen that 100% amplitude modulation will increase the peak power in the transmission line by a factor of 4. Furthermore, the peak power in the transmission line increases directly with VSWR.
The peak power rating is limited by the voltage breakdown potential between the inner and outer conductors of the line. The breakdown point is independent of frequency. It varies, however, with the line pressure (for an air-dielectric cable) and the type of pressurizing gas.
The average power rating of a transmission line is limited by the safe, long-termoperating temperature of the inner conductor and the dielectric. Excessive temperatures on the inner conductor will cause the dielectric material to soften, leading to mechanical instability inside the line.
The primary purpose of pressurization of an air-dielectric cable is to prevent the ingress of moisture. Moisture, if allowed to accumulate in the line, can increase attenuation and reduce the breakdown voltage between the inner and outer conductors.
Pressurization with high-density gases can be used to increase both the average power and the peak power ratings of a transmission line. For a given line pressure, the increased power rating is more significant for peak power than for average power. High-density gases used for such applications include Freon 116 and sulfur hexafluoride.
An adequate safety factor is necessary for peak and average power ratings. Most transmission lines are tested at two or more times their rated peak power before shipment to the customer. This safety factor is intended as a provision for transmitter transients, lightning-induced effects, and high-voltage excursions resulting from unforeseen operating conditions.
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