RFC-1 Advanced Operation page 6.9
6.3.6.a Indirect Power—Theory of Operation
This section provides further information on calculating indirect power. If the sequence above is completed and it
operates successfully then it is not necessary to read this section. This information is useful for troubleshooting.
An indirect power channel has a reading from 0000 to 9999. A decimal point is optional and the unit word will be
either “kilowatts” or “percent power”. If the computed value exceeds 9999 or if either the voltage or current channel
exceeds full-scale, the words "upper limit" are spoken. The telemetry value for the indirect power channel is
computed from the following equation:
P = (E * I * Eff * S) / 10,000,000
P = the four-digit power reading that is calculated—disregarding decimal point
E = the normal four-digit voltage reading in volts—disregarding decimal point
I = the normal four-digit current reading in amps—disregarding decimal point
Eff = efficiency of the transmission line or the entire system from 1 to 1023—disregarding the %
S = the user-selected scale multiplier—1, 10 or 100
E and I are the readings from the plate voltage and plate current channels respectively. These values are in volts
and amps with up to four significant digits but no decimal point. By these rules, the value 10.54 would be 1054.
These values are taken from the two channels below the channel that is calculating the power.
The efficiency value, Eff, comes from the transmitter and transmission line for transmitter power output, TPO,
calculations or from the entire system including antenna gain for effective radiated power, ERP. In the latter case, it
is not unusual for the efficiency number to be greater than 100.
For example, suppose the antenna has a gain of 6.5, the feed line has an efficiency of 75% and the transmitter has
an efficiency of 72%. The overall efficiency (input power-to-ERP) is 6.5 x 0.75 x 0.72 = 3.51. Using this example, the
efficiency would be 351% for ERP calculations. However, the efficiency would be only 72% for TPO calculations.
The efficiency value can be adjusted to allow the computed reading to express a percentage of authorized power. In
this case, the efficiency resulting from the above formula should be multiplied by 100 and then divided by the normal
transmitter output power in kilowatts. The goal is for the reading to be "100.0" at 100% of authorized power. In this
case, select the option for "percent power" the programming sequence above.
The scale multiplier S is used to force the final reading into the acceptable range of readings from 0000 to 9999. To
determine this value, perform a trial calculation of the formula above with the expected normal voltage, current and
efficiency. First use S=10 and compute P. Disregard the decimal point and use the four data digits for voltage and
current. If the calculated value for P is outside of the range 0000 to 9999 or extremely close to either end, use S=1
for a smaller P or S=100 for a larger P.
For example, suppose the expected plate voltage is 8.0 kilovolts, the expected plate current is 3.8 amperes and the
efficiency is 72%. Using the above equation:
(8000 x 3800 x 72 x 1) / 10,000,000 = 218.88
The telemetry reading will use only the digits to the left of the decimal point: 0218. This is a small value given that
the maximum reading is 9999. So in this case it would be better to use S=10 to force the value closer to the middle
of the scale. With S=10 the resulting P=2188 which fits nicely in the scale. Setting S=100 results in P=21888 which
is well beyond the maximum of 9999.
If needed, a decimal point is added to the reading in the channel settings.
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