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Power Ratings
Peak and average power ratings are both required to fully describe the
capabilities of a transmission line.
Peak Power Rating
Peak-power rating of a transmission line is limited by the voltage
breakdown between the inner conductor and the outer conductor. Voltage
breakdown is independent of frequency, but varies with the density of the
pressuring gas. Peak power ratings are typically stated for the following
conditions:
- VSWR = 1.0:1
- Zero Modulation
- Dry air at atmospheric pressure (0 lb/in² or 0 kPa)
Pressurization and/or the use of high-density gases with high dielectric
strength can be used to increase both average and peak-power ratings.
For a given transmission line pressure, the increase in peak power rating is much more significant than the increase in average power rating. Pressurizing a transmission line to 10 psi with dry air increases the peak power rating by a factor of 1.9 but only increases the average power rating by a factor of 1.14.
Typical peak-power ratings of common line sizes using a safety factor of 2.
| 7/8" |
1-5/8" |
3-1/8" |
4-1/6" |
6-1/8" |
| 44 kW |
148 kW |
442 kW |
510 kW |
1500 kW |
Average Power Rating
Average-power ratings of transmission lines are limited by the safe long term
operating temperature of the inner conductor and dielectric supports. Average
power ratings are typically based on the following conditions:
- VSWR = 1.0:1
- Zero Modulation
- Ambient temperature 40°C(104°F)
For AM or FM applications, the average power rating must be de-rated for VSWR.
Television transmission ratings are based on sync peak video power. Typically
a value of 83% of the television transmitters nominal rating is used.
Average Power Tables
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Derating = |
Average Power Rating
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Derating Factor |
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Derating Factor = |
(VSWR)² + 1
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2 (VSWR) |
Attenuation
Attenuation of a transmission line varies with frequency, temperature and load
VSWR. Typical attenuation curves are based on:
- Ambient temperature of 20°C (68°F)
- VSWR = 1.0:1
To correct for actual attenuation based on operating conditions, the temperature
of the inner conductor or applied power must be compensated for. If operating
at less than the maximum power rating for the transmission line, the inner
conductor temperature can be determined by finding the temperature rise between
the outer conductor and ambient temperature.
If the transmission line is operated at it's maximum power rating, then the inner
conductor is assumed to be at 100°C. Determine the correction factor and multiply
the standard condition attenuation values by the correction factor. The new
attenuation value will be based on the actual operating conditions.
VSWR
VSWR loss is generally insignificant for normal operating conditions. For long
transmission line runs, the proper line section length must be used to avoid
VSWR build up due to discontinuities at the flange joints. Select the proper
line length for the operating channel or frequency from
Determining Line Section
Length Table.
Critical Frequency of VSWR Build Up
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n - integer |
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L - Line length in feet |
VSWR/Return Loss Conversion
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RL = 20 Log |
(VSWR + 1)
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(VSWR - 1) |
Pressurization
Transmission line must be sealed and charged with a dry gas. Pressurization
protects against moisture and increases the peak power capacity. If moisture
is present in a transmission line, there will be an increase of loss and a
decrease in the maximum operating voltage. It is essential to purge newly
installed or repaired transmission line before power is applied to the system.
The required line pressure and type of gas will depend upon desired operating
conditions. Pressurization above 30 lb./in.² is not recommended.
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