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SETI League EME (Moonbounce) Station

Measuring Power Output

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A crucial figure is the actual output power being delivered by the amplifier. The whole purpose of the beacon is to transmit a signal to the moon and provide verification and calibration of earth-based receivers. The instrument used for power calibration is the Rohde and Schwarz NAP power - reflection meter. This is similar to the common SWR meter in that it reads forward and reflected power. It differs in that it is controllable via IEEE-488, has much greater precision, and is capable of a lot of internal calculation. Power output measurements are made for each cycle of the beacon, and forward and reverse power is reported. A significant change in these numbers or their ratio is indicative of a possible problem.

While a solid measurement is valuable information, multiple measurements can lead to some confusion. The "standard" way of measuring power is the well-known Bird 43 wattmeter, with appropriate slug. Another, analytical way is to feed the signal into a dummy load/attenuator and measure the output of the attentuator with a spectrum analyzer or power meter. Having done all four, I can tell you with precision the four different power output levels of the beacon! Fortunately, they don't all disagree that much. The NAP power meter is the only instrument I have to continuously monitor power, and despite the fact that the frequency isn't quite within the range of the calibrated sensor, it's at least linear. So, until it gets warmer outside and I (or someone!) climbs the tower with a precision instrument, I am content to go by the following approximation:

200 W read by the NAP at the output of the amplifier translates to 100 W at the antenna connector.

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