SRC-270

Terrestrial Microwave Power Beaming.

Published claims resting on this source

• CLM-1122 About twenty thousand postage-stamp-sized antenna-plus-diode cells, each with an off-the-shelf microwave diode, wired into tiles, then panels a metre square, then four panels side by side: a bit over four square metres in all.
• CLM-1123 A five-metre dish and a 100 kW radar-type pulsed tube a kilometre away, bounced partly off the ground on purpose to get more power onto the receiver, then deliberately blurred so the diodes would not be burnt out: about 550 watts per square metre landing on the array, from a beam that could have delivered three times that.
• CLM-1124 One panel the size of a sheet of paper turned about seven-tenths of the microwave power landing on it into DC, and did best at about three hundred watts per square metre, a little over half the highest level tested.
• CLM-1125 Four square metres of receiver turned 73 per cent of the microwave power arriving at it into 1.65 kW of DC, measured a kilometre from the transmitter; the main doubt is whether the probe antenna used to measure the arriving power sees the ground reflections the same way the receiver does.
• CLM-1126 They deliberately fried each of the four panels with ever-stronger pulses. A bare panel gave up at 672 watts out; clamping its voltage with a Zener let it take about 30 per cent more incoming power before failing, and shorting it outright about 50 per cent more, which is the best any clamp could do.
• CLM-1127 As flown, under two per cent of the microwave power fed to the dish came out of the receiver as electricity, because the beam had to be blurred to protect the diodes. The authors' arithmetic says a properly focused beam onto a receiver seven to twenty-two times larger would get a quarter to nearly half of it through; nobody has built that.
• CLM-1128 A second test used a big radar dish in Massachusetts to light up a strip of LEDs on a tower over a kilometre away, with about a kilowatt flowing during each pulse; the receiver sat well inside the dish's near field, where the beam is uneven, and turned an estimated 60 per cent of the power reaching it into DC.
• CLM-1129 The authors say the 1975 record still stands at twenty times their power, that the custom diodes which set it no longer exist, and that going bigger needs three things nobody has yet built for this job: many small combined transmitters instead of one big tube, a tougher diode, and the electronics to feed a rectenna into a grid.
• CLM-1130 A quick way to size a ground-to-ground beam: at the distance where the dish stops focusing, the cone holding half the power is about twice the dish's radius and the cone holding nine-tenths about four times; anything on the ground poking into the wider cone costs you power, and the usual radio-link clearance rules overstate how high you must build.
• CLM-1131 They ran a radio-propagation model over a map of the actual ground and found that, aimed just right, the beam plus its reflection off the ground would land nearly twice as much power on the receiver as the direct beam alone; a perfect mirror-flat ground would give almost four times, the theoretical best.
• CLM-1132 They then measured it: raising the receiver through five metres above the ground the received power jumped by about 70 per cent, close to the model's 87 per cent; the beam was as narrow as in free space side to side and a little wider up and down; and because the ground reflection varies across the receiver, they had to map the arriving power point by point rather than swing the transmitter past a fixed probe.
• CLM-1133 The exposure limits the paper cites at this frequency are 10 watts per square metre for the public, 50 for workers and 100 on military sites. Their pulsed test averaged below those everywhere except right beside the dish's feed; but had the 100 kW run continuously, the 100 W/m2 zone would have skimmed within three metres of the ground for nearly four kilometres, which is why the receiver itself has to be big enough to catch the beam, and why aircraft overhead are a separate question.

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