When designing a high power communications system, it is important to know that each component will be able to handle the amount of power involved. This is not always the simple matter it should be, due to the fact that manufacturers are not always consistent in their specifications and also because the terms used are not always well understood. As a result there is often some confusion as to whether individual components will be suitable for higher power applications. Getting it wrong can mean anything from disruption or loss of communications to permanent damage to the equipment.

RF power measurements either use average power in a continuous wave (CW), or peak envelope power (PEP) in a variable amplitude transmission. CW measurements are appropriate for constant amplitude transmissions such as FM, PM and FSK, while the PEP measurement is used for AM, SSB and multi-level data modulation schemes. A PEP measurement on a constant amplitude transmission gives the same result as the average power on that transmission.

The amount of RF power any communications system can handle will be limited by the capacity of the individual components (transceiver, coupler, antenna, cables, etc.) to handle RF voltages and dissipate heat. The RF voltages are generally determined by the PEP, while the amount of heat to be dissipated depends on the average power.

Coil Failure

The relationship between PEP and average power depends on the type of modulation. For example, with SSB voice communications there are highs, lows and gaps as people talk and the ratio of PEP to average power can be as great as 100:1; while in a data transmission, the average power can be as high as the PEP, depending on the duty cycle.

Insulator Failure

Voltage (electrical) breakdown can occur when the voltage reaches the point where dielectrics and other insulators break down and begin conducting.

Voltage handling is normally indicated in specifications as PEP (peak envelope power) or peak pulse power handling capacity. PEP is defined as the average power supplied to the antenna/transmission line by the transmitter during one RF cycle at the crest of the modulation envelope under normal operating conditions. This shows what voltage can be handled without RF voltage breakdown occurring.

In constant amplitude systems, thermal breakdown is usually of more significance. Breakdown occurs when the heat build-up exceeds safe limits. In higher power systems, the capacity of insulators to dissipate the heat is of critical importance. With a working temperature of around 250 degrees C, PTFE (Teflon or Fluon) is much better in this regard than polythene which handles around 85 degreesC. Quoted power ratings are based on specified ambient temperatures and it is important to take this into account.

The ability to dissipate heat is expressed in specifications as CW (continuous) or average power. This represents the power level that the transmitter can output continuously.

Peak power handling of transmission lines and antenna components is typically determined by applying test voltage and checking for arcing and leakage. Where there is mismatch creating standing waves on the line, peak power ratings must be derated. This is because the forward and reflected signals combine to produce voltage maximum.

Insulator Arcing salt water spray test

Where open wire lines are concerned, the voltage at which corona starts usually sets the limit. This point is lower with thin wires than with thick ones. Fitting corona rings to spreader insulators can help but an increase in the capacitive load will increase the electrical length of the feeder. Outbreaks of corona can be intermittent or continuous. When the onset of corona increases transmitter mismatch, the resultant losses can extinguish the corona. However, when the mismatch is reduced, the corona can become firmly established.

Some specifications only give PEP ratings so it is difficult to know what the aAverage or CW rating actually is. If data is being transmitted in short bursts with a regular repetition rate, PEP can be compared with average powerAverage or CW by averaging the amount of power in a pulse over the repetition time period.

If the duty cycle of the transmitter is givenknown, it is possible to determine whether a particular data ratepeak power will be achievable or not. The Duty Cycle is the ratio of average power to the peak pulse power and represents the fraction of time a radio is transmitting, or in other words, the percentage of time the power is present.

Continuous Data Transmission - RF Pulse Train

To find the Duty Cycle, you need to divide the pulse width (how long the transmission is) by the period (the time between the start of one pulse and the next one). This gives the Duty Cycle expressed as a decimal. Multiplying by 100 will then express the result as a percentage. So, a transmission with a PEP rating of 100W with and a duty cycle of 0.5 or 50% will mean the pulses are present half of the time (for example 5 second pulses every 10 seconds), and the average power is 50W Average or CW continuous transmissions are possible. A radar with a pulse width of 1.0 microseconds over a period of 1.0,000 milliseconds would have a duty cycle of 0.001 or 0.1% (0.000001 x 1,000 = 0.001).

The RF power is present 1,000th of the time making the peak power 1,000 times greater than the average power. The Duty Cycle can also be expressed logarithmically in dB using the following equation: Duty Cycle (dB) = 10 log(Duty Cycle ratio). This tells you how much less the average power is than the peak power in dB.

In the case of the radar, the average power would be 30dB less than the peak power [10 log(0.001) = -30dB].