Radio astronomy has published a cost for wireless power transfer. It is a separation
\ndistance of about five kilometres, and it was computed for inductive chargers delivering
\nmore than thirty watts below 30 MHz. For a space-to-Earth beam there is no number at all,
\nand this article works out roughly how large the missing one is.
\nThe source is the Summer 2026 issue of CRAF News, the newsletter of the European Science
\nFoundation's Expert Committee on Radio Astronomy Frequencies, published in September 2026.
\nIts article "Wireless Power Transfer and Radio Astronomy: an emerging compatibility
\nchallenge", by Fabio Giovanardi of the Arcetri Astrophysical Observatory at INAF in
\nFlorence, is the first place this project has seen the radio astronomy community put a
\nquantified compatibility assessment of wireless power next to a statement about space-based
\nsolar power.
\nIt is worth being exact about what that is and is not. CRAF is a scientific expert committee.
\nIt does not set the ITU agenda, it does not write the Radio Regulations, and a newsletter
\narticle is not a work item. Nothing in it changes a pinned edition. What it does do is put a
\nmethod and a distance on the record, and the interesting part is where that method stops.
\nThe assessment behind the article is the Addendum to ECC Report 333, which the newsletter
\ndescribes as covering higher-power, non-beam inductive wireless power systems operating in
\nspecific bands below 30 MHz and delivering more than 30 W. The bands at risk are given as
\nthe radio astronomy allocations at 13.36 to 13.41 MHz and 25.55 to 25.67 MHz, and the
\nprimary case study is LOFAR, which ASTRON's own telescope page puts at more than 70,000
\nantennae across Europe, with 38 stations in the Netherlands, six in Germany, three in
\nPoland, and one each in France, Ireland, Latvia, Sweden and the United Kingdom.
\nThe compatibility analysis takes a single-interferer, ground-wave approach under
\nRecommendation ITU-R SM.2028, applies the protection criteria of Recommendation ITU-R
\nRA.769, and draws emission assumptions from ETSI EN 303 417, CISPR 11 and CISPR 14. The
\nresult, in the newsletter's own words, is that "a separation distance of around 5 km from
\nthe radio observatory would be necessary". Because the devices are portable, the article
\nsays enforcing such a zone is a regulatory problem rather than a planning one.
\nThe newsletter also sets out why the terrestrial problem is getting worse rather than
\nbetter. Electric vehicle charging, covered by ECC Report 289 and its own addendum, is put at
\ntransferred power of up to 11 kW for light duty and up to 500 kW for heavy duty. CRAF's
\npoint about these is not the single-unit figure but the aggregate: a distributed fleet of
\nchargers raises the noise floor in a way that is harder to mitigate than one high-power
\ntransmitter, and low-frequency signals propagate well. Against that, the one consolation is
\nthat vehicle chargers are fixed installations, so a separation distance is at least
\nenforceable. The portable devices in the 30 W class are the ones nobody can put a fence
\naround.
\nThen there is half a page on space-based solar power. CRAF's position is that the
\ninterference problem does not stop at ground level, and that, quoting the Summer 2026
\nnewsletter, "SBSP transmissions would be visible over continental scales and could affect
\nmultiple radio astronomy observatories simultaneously". There is no distance, no power
\ndensity and no band in that paragraph. It is a flag, not a study, and the article says so by
\ncalling for the question to be taken up within ITU frameworks rather than reporting an
\nanswer.
\nThis is the part that matters for anyone planning a receiving site. The five kilometres
\ncannot be carried across.
\nRecommendation ITU-R SM.2028 is listed on the ITU's own editions page as SM.2028-0 (09/2012),
\nIn force, under the title "Protection distance calculation between inductive systems and
\nradiocommunication services using frequencies below 30 MHz". Inductive systems. Below 30 MHz.
\nA ground-wave propagation model is the right tool there and the wrong tool for a microwave
\nbeam arriving from orbit at a steep elevation angle.
\nRecommendation ITU-R RA.769 is more pointed. The in-force edition is RA.769-2 (05/2003),
\nlisted by the ITU as incorporated by reference in the Radio Regulations, which gives its
\ntables real regulatory weight. Section 2 of its Annex 1 opens by saying that the threshold
\nlevels in Tables 1 and 2 "are applicable to terrestrial sources of interfering signals". It
\nthen handles geostationary satellites as a special case, and recommends that for
\ntransmitters in the geostationary orbit the tabulated levels be adjusted by a further
\n15 dB, because a telescope pointed within about five degrees of such a transmitter sees it
\nthrough a much higher side lobe.
\nRA.769-2 also anticipated the problem in 2003. Among its considerings is the observation
\nthat "some transmissions from spacecraft can introduce problems of interference to radio
\nastronomy and that these cannot be avoided by choice of site". Siting is the entire
\nmitigation available for terrestrial wireless power. It is the one mitigation that does not
\nwork against a satellite.
\nHere is the arithmetic, labelled as an estimate because it is mine, derived from two
\ndocumented figures rather than taken from any study.
\nTable 1 of RA.769-2 gives, for continuum observations centred on 4 995 MHz over a 10 MHz
\nbandwidth, a detrimental threshold power flux density of −171 dB(W/m²). That is the 4 990 to
\n5 000 MHz radio astronomy band, and it is the nearest protected continuum entry in the table
\nto the band space solar power concepts most often name. The same table gives −201 dB(W/m²)
\nat 13.385 MHz and −199 dB(W/m²) at 25.610 MHz, which are the low-frequency bands CRAF is
\nworried about. Both figures assume a 2 000 s integration, a 0 dBi side lobe, and a single
\nantenna.
\nTake an upper bound on what a ground spot would be allowed to carry. ICNIRP's 2020
\nradiofrequency guidelines set a whole-body reference level of 10 W/m² above 2 GHz, and
\nIEEE C95.1-2019 is the companion standard a United States site would be assessed against.
\nA beam constrained to 10 W/m² at the spot sits about 181 dB above the RA.769 threshold for
\nthe adjacent protected band. Apply RA.769's own 15 dB geostationary adjustment and the
\nfigure is about 196 dB.
\nThat is not a claim that a beam would cause 181 dB of interference. The beam is not in the
\nradio astronomy band, so what the number describes is the out-of-band suppression, in the
\ntelescope's side lobes, that a space-to-Earth system would have to demonstrate in a
\nneighbouring protected allocation. It is a rough upper bound and it ignores the bandwidth
\nconversion between the table's 10 MHz reference and a real spurious emission mask. The
\nreason to state it anyway is that nothing in the literature this project can reach states
\nanything at all, and 181 dB is a different order of engineering problem from the 5 km
\nexclusion zone CRAF has costed. Note also that 5.8 GHz, the band most frequently discussed
\nfor this, is not regulatorily available for space-to-Earth power transmission; there is no
\nallocation and the ITU's own Report SM.2392-2 (06/2026) is a report, which recommends
\nnothing and binds nothing.
\nThe terrestrial precedent is worth reading closely, because it is the one that will be
\nreached for.
\nECC Recommendation (25)01, "Emission limit requirements for Wireless Power Transmission
\n(WPT)", approved 14 February 2025, sets in its Table 4 an unwanted emission limit in the
\nspurious domain of −20 dBµA/m per 10 kHz at 10 m across 5 to 30 MHz, for non-beam generic
\nWPT. That is the figure CRAF cites, and reading the recommendation first-party confirms it.
\nTwo other lines in the same document matter more. Its considering k) records "some study
\nresults in ECC Report 333 found limits to protect certain radio communication services that
\nare lower than those in this Recommendation". The regulator knew the studies asked for
\ntighter limits and set looser ones. And its recommends 6 states "that WPT applications
\npermitted under this ECC Recommendation operate on a non-interference, non-protected basis".
\nFor an operator that is the sentence to notice. Non-interference, non-protected means the
\nwireless power system carries the whole risk: it must not interfere, and it has no claim if
\nsomething interferes with it. If that framing is inherited by beamed systems, a ground
\nreceiving site near a protected passive allocation is not a planning risk that can be bought
\noff. It is an unbounded liability with no standing.
\nNothing yet, and that is the honest answer. No edition in this project's registry moved this
\nweek. But it adds a column to the question of where a receiving site can go, and the column
\nis not one that appears in a land search. Distance to a radio astronomy facility is
\ncurrently nobody's site-selection criterion for beamed power. On the terrestrial evidence it
\nis already worth five kilometres for a thirty-watt charger, enforced through a device
\nstandard rather than a licence, and the only body that has written the satellite case down
\nhas written it down as a concern rather than a calculation.
\nThe useful posture is to treat the distance as unknown and to say so, rather than to assume
\nthat an allocation at 5.8 GHz would make the adjacent 4 990 to 5 000 MHz band somebody
\nelse's problem.
\nA published compatibility study for beamed space-to-Earth power against RA.769, by anyone,
\nusing a propagation model valid above 30 MHz. If that study comes back with a required
\nsuppression well below the 181 dB estimated here, because a real spurious mask and a real
\nantenna pattern are far better than an upper bound on main-beam density implies, then the
\nframing above is alarmist and the correct answer is that filtering solves it.
\nEqually, a first-party ITU-R work item that takes the space case under an existing agenda
\nitem would change the shape of this. CRAF's article routes the question to WRC-31
\npreliminary agenda item 2.2 under Resolution 910 (WRC-23), which this project has already
\nexamined. If instead a WRC-27 item absorbed it, the timeline moves forward by four years and
\nthe argument that nobody is working on it fails.
\nOne limitation is stated rather than hidden. The Addendum to ECC Report 333 itself was not
\nread for this article: docdb.cept.org failed TLS certificate verification on every attempt
\nfrom this run, so the Addendum's scope, its five-kilometre result and its ECC Report 289
\ncompanion for electric vehicle charging are reported as the CRAF newsletter states them, and
\nhave not been independently verified against CEPT's own text. ECC Recommendation (25)01 was
\nread first-party, through a different host, in its 14 February 2025 edition; a later amended
\nedition is indexed and was not reachable.
\n