Value chain · CLM-1096
In plain terms
In 1977 one rectenna element converted 90 per cent of eight watts of microwaves to DC but only 80 per cent of fifty milliwatts, and the report says why: at low power the diode's own losses dominate. The 85 per cent the programme wanted at low power was not reached.
The claim as recorded
Brown's 1977 receiving-terminal contract set an efficiency objective of 85 per cent at 1 to 100 mW/cm2 at 2450 MHz, which for the element's cell area of about 50 cm2 is 50 mW to 5 W per element, and reported single-element efficiencies of 90.5 +/- 0.5 per cent at 8 W input, 79.5 +/- 0.5 per cent at 50 mW and 45 per cent at 1 mW; the 50 mW result is stated to be "disappointing" against both the objective and prediction, with the cause put on the back-contact resistance of small-junction diodes. Table 3-1 gives each result its diode, breakdown voltage, junction capacitance, load resistance and reflected power: 82.3 to 90.26 per cent for the GaAs-Pt diode 40593 at 1 to 8 W into 80 ohms, 85.8 to 91.3 per cent for the GaAs-W diode 50234CX at the same range, and 34.3 to 81.3 per cent for the 0.80 pF diode 40477 at 0.5 to 100 mW into 1,000 ohms. The power balance behind the figures: diode losses 36 to 7.4 per cent of absorbed power from 100 mW to 8 W, circuit losses 2.62 per cent, the sum within the total probable error of +/-0.75 per cent at the top of the range (90.5 + 7.4 + 2.62 = 100.5 at 8 W; at 100 mW the page's 56 per cent efficiency gives 56 + 36 + 2.62 = 94.6, and the page claims closure only "over an appreciable portion of the range"). The GaAs-W barrier lowered the diode's forward drop by about 0.2 V, "or about 25%".
Evidence behind this: T3-NASA contractor report, not peer reviewed: bench measurements with a stated probable error and a stated power-balance check; the objective stated and the shortfall admitted.
Scope and limits
SINGLE ELEMENTS IN A WAVEGUIDE OR GROUND-PLANE FIXTURE, NOT AN ARRAY, AND "the resulting efficiencies do not represent the best that can be obtained" (normalised, PDF 113). The 91.3 per cent maximum is the GaAs-W diode in the ground-plane fixture at 8 W, four orders of magnitude above the low end of the SPS rectenna's density. Measurements above 10 W were "outside the capability of the standard measuring" equipment (normalised, PDF 113). 1977 diodes.
What it assumes
Efficiency is DC output over microwave power absorbed (incident less reflected), the report's definition; the fixture measurements omit the dipole.
Where it comes from
PDF 113 / printed 103, render read, the summary's "efficiency of 90.5 +/- 0.5% at the 8 watt input level and to 79.5 +/- 0.5% at the 50 milliwatt level" (layer "90. 5" and "79. 5" raw) and "an efficiency of 45% was obtained with an input level of one milliwatt" (render reading; the layer prints "4-5%"); normalised: "DC output levels as high as 17 watts were obtained at efficiencies which were estimated to be as good as at levels below 10 watts" and "It was determined that the total probable error was +/- 0.75%". PDF 112 / printed 102, normalised: "The improvements in efficiency ranged from" a fraction of a percent at 6 to 8 W "to twenty percent at an input level of 50 milliwatts". PDF 59 / printed 49, raw: "The efficiency objective of this technology" development contract was 85% at 1 to 100 mW/cm2 (2450 MHz) (render reading of the rest), and squashed "50 milliwatts to 5.0 watts". PDF 60 / printed 50, raw: "obtained at the 50 milliwatt level is disappointing" (the page: "the new efficiency of 80% obtained at the 50 milliwatt level is disappointing in that it did not reach either the contract objective or the efficiency predicted from previous studies", render reading). PDF 62 / printed 52, Table 3-1 render read: curve 1, configuration A, diode 40593 CPX1G No. 13 GaAs-Pt, Vbr 60 V, 3.6 pF, 1000-8000 mW, 0-1 per cent reflected, 80 ohms, 82.3-90.26 per cent; curve 2, A, 50234CX WC-8 GaAs-W, 59 V, 3.5 pF, 1000-8000 mW, 0-2 per cent, 80 ohms, 85.8-91.3; curve 3, B, 40904 WB No. 3 GaAs-W, 51 V, 0.99 pF, 500 to 2000 mW at 600 to 300 ohms, 86.8, 87.8, 88.1, 86.8; curve 4, B, 40904 WO No. 4, 30 V, 1.01 pF, 100 to 1000 mW, 82.3, 83.9, 85.1, 85.2; curve 5, C, 40477 3PX1 WA-3, 19.5 V, 0.80 pF, 0.5 to 100 mW at 1000 ohms (700 at 100 mW), 34.3, 44.5, 56.6, 64.9, 69.5, 75.6, 80.0, 81.3; configurations A "RXCV element adapted to ground plane fixture", B and C the standard RXCV element with a 240 or 300 ohm quarter-wave transformer, D the standard balanced RXCV element. PDF 49 / printed 39, squashed: "The diode losses are observed to vary from 36% to 7.4% of the absorbed microwave power" and "The circuit losses are assumed to remain constant at 2.62%"; the same page, render read: efficiency "from a low of 56% at 100 milliwatts of incident power to 90.5% for a power input of 8 watts" and the balance "very close to 100% and within the probable error of 0.75% previously established, over an appreciable portion of the range". PDF 116 / printed 106, raw: "or about 25%" (the page: "this reduced the forward voltage drop by about 0.2 volts or about 25%", render reading).
Basis: per cent DC output over microwave power absorbed by one element; watts and milliwatts of input; ohms of load
Related records
- Answers question
- QST-007 not published in this release
Cite this
energyaperture.com/c/clm-1096
CLM-1096, Energy Aperture, as of 1977-08.
Connected records
Other published records linked to this one. A list, not a judgement.
- Also from SRC-558
- CLM-1095The three harmonic numbers the register has carried since the 1978 conference paper come from one rectenna element on a bench at five...CLM-1097By 1976 a rectenna had delivered thirty kilowatts over a mile at 82 per cent, a certified bench system had turned DC into microwaves and...CLM-1098The 1977 plan for a cheap rectenna was two flat planes, the front one doing all the work inside a metal shroud that is also the structure...