Value chain · CLM-1130
In plain terms
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.
The claim as recorded
For a terrestrial link NRL 2022 rejects Fresnel-zone and mobile-communications clearance rules as "highly inaccurate for power beaming scenarios, producing overly conservative guidance for selecting antenna heights", and instead approximates the beam as "a conic section extending from the transmit aperture to the target location" whose radius, for an aperture "optimized to focus power at a distance" d_TRX, is rho = (8 / pi) x (d_TRX / d_FF) x r_TX x sqrt(ln(1 / (1 - eta))), where d_FF is the far-field distance and eta "is proportion of the transmit power contained within" rho; so at the far-field distance the 50 and 90 per cent cones have radii of 2.1 and 3.9 transmit radii, and terrain must be kept out of the 90 per cent cone, which "even for power beaming over a flat surface" can require significant elevation of transmitter or receiver at a structural cost.
Evidence behind this: T2-peer-reviewed design method: a closed-form beam-radius estimate taken from the authors' own 2021 review [1] and a stated position on rules of thumb; no measurement in this row.
Scope and limits
A DESIGN APPROXIMATION, NOT A MEASUREMENT, AND THE PAPER NEVER EVALUATES (1) FOR ITS OWN LINK - the 3.2 m and 5.8 m are this record's arithmetic. Valid for an aperture focused at d_TRX; the paper's own fallback for other distributions is the Fresnel-region 3 dB and 10 dB widths. Terrestrial geometry (tilt, heights, terrain); the space-to-ground case has d_TRX >> d_FF and is outside Fig. 9's axis.
What it assumes
The "ideal distribution [1]" of the authors' 2021 review (SRC-269, whose beam derivation is not read for this record) underlies (1); d_FF per [20] (Milligan), taken as 2 D^2 / lambda in the recomputation.
Where it comes from
PDF 4 / printed 31: design rules of thumb such as Fresnel zones (its [18]) and the popular approximations for mobile communications ([19], [20]) can, the paper says, be highly inaccurate for power beaming, producing overly conservative guidance for selecting antenna heights (normalised); the transmit beam is approximated as a conic section extending from the transmit aperture to the target location (normalised); for a transmit aperture whose amplitude and phase distribution is optimised to focus power at a distance (normalised), the Gaussian beam approximation of its reference [1] can be used to estimate the beam radius (normalised); equation (1) as transcribed in the claim (zoom-5 crop read); "is the transmit aperture's far field distance [20] and" (R1); "is proportion of the transmit power contained within" (raw); the 50 and 90 per cent beam radii can instead, the paper says, be estimated from the expected Fresnel-region 3 dB and 10 dB beam widths respectively, if the transmit aperture distribution deviates significantly from the ideal distribution of [1] used to calculate (1) (R2); even for power beaming over a flat surface, the paper says, it can be necessary to elevate the transmitter significantly, or to up-tilt the transmitter and elevate the receiver (R1, running onto PDF 5 / printed 32), to keep terrain out of the 90 per cent beam cross section, and for long range links the structural costs could be significant (R1). Recomputed: (8 / pi) x sqrt(ln 2) = 2.12; (8 / pi) x sqrt(ln 10) = 3.86; at d_TRX / d_FF = 0.5, 1.06 and 1.93; Fig. 9 (render reading) ends its 50 and 90 per cent bands near 2.1 and 3.9 at 1.0 and near 1.0 and 1.9 at 0.5. For the paper's own link (r_TX = 2.7 m, d_FF = 2 x 5.4^2 / 0.0309 = 1,887 m, d_TRX = 1,046 m): rho_50 = 3.2 m, rho_90 = 5.8 m.
Basis: beam radius rho in units of transmit aperture radius r_TX; distance d_TRX in units of far-field distance d_FF; fraction eta of transmit power inside rho
Cite this
energyaperture.com/c/clm-1130
CLM-1130, Energy Aperture, as of 2022-01.
Connected records
Other published records linked to this one. A list, not a judgement.
- Also from SRC-270
- CLM-1122About twenty thousand postage-stamp-sized antenna-plus-diode cells, each with an off-the-shelf microwave diode, wired into tiles, then...CLM-1123A 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...CLM-1124One 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...CLM-1125Four 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...CLM-1126They deliberately fried each of the four panels with ever-stronger pulses. A bare panel gave up at 672 watts out; clamping its voltage with...CLM-1127As 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...CLM-1128A 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...CLM-1129The authors say the 1975 record still stands at twenty times their power, that the custom diodes which set it no longer exist, and that...More on the source page.