The one specification they have never published

1Six things make a radio device legal to sell in Great Britain

If a device deliberately transmits radio and it is placed on the market in Great Britain, the Radio Equipment Regulations 2017 (S.I. 2017/1206) require six things. None of them are obscure. Every radio product you own has been through this.

1. A conformity marking on the equipment or its data plate, and on the packaging. Regulation 44.
2. A declaration of conformity, or a simplified one with a web address for the full text, accompanying every single unit. Regulations 13(3) and 43.
3. Technical documentation drawn up and kept for ten years. Regulations 9 and 11.
4. Instructions stating the frequency band and the maximum radio-frequency power transmitted. Regulation 13(2).
5. The manufacturer’s name and a postal address. Regulation 12.
6. The importer’s name and postal address, where there is one. Regulation 23. The regulations define an importer as a business established in the United Kingdom, so a seller shipping in from outside it is not one.

Five of those have to be with the product or on it. The sixth, the technical documentation, has to exist, be kept for ten years, and be produced if an enforcing authority asks for it. I am working through all six. This article is about the fourth one, and specifically the second half of it.

2Whose job is it to put the figure there?

Before anyone says that is the American manufacturer’s problem, it is not only. The duty to supply the figure is the manufacturer’s wherever they are, and selling direct from outside the United Kingdom does not shed it. Contracting to sell a finished product to a buyer in Great Britain is placing it on the Great Britain market.

And whoever sells it here carries a duty of their own. It is not a vague one, and it is not left to inference. It names the regulation this article is about.

3Six documents, and every page of their website

The current operator’s manual, thirty two pages. The previous revision of that manual. The manufacturer’s brochure. The AML Pro data sheet. The product guide the GB and Ireland seller hands to customers. The twelve page company catalogue. And the manufacturer’s own website, all four hundred and eighty one pages of it.

The same manual prints a maximum output, for the laser

The AML Pro has a Class 3B laser in it, and page three of the current manual carries the laser warning label. It reads:

A maximum output, in milliwatts, printed on the label, because the laser standard asks for it. Turn the page and the same manual describes “a powerful 2.45 GHz radio frequency”. For that one there is no maximum output, on that page or on any other.

They print it for their other locators

It is not as though they are unfamiliar with the idea. The same catalogue that leaves it out for this product prints a transmit power figure for their other locators, on the same pages. If SSI have published it somewhere I have not looked, show me and I will add it to this article the same day.

4Go and look in your case

That is everywhere I can reach from the outside. Regulation 13 is about information that accompanies the equipment, so what matters now is what came in your box, not what is on a website. The figure to look for is a maximum RF power, in milliwatts, in dBm or as an e.i.r.p. Worth checking the rest while you are in there: a declaration of conformity, a conformity marking on the unit and on the case, the manufacturer’s postal address, and the name and address of whoever imported it.

5The number falls out of their own certification file

If the manufacturer will not publish it, the test laboratory measured it. The AML holds one FCC authorisation, granted in March 2012, and the Intertek report behind it is public.

What the laboratory measured

That report does not give a transmit power in watts either. What it gives is the measurement one is derived from. At 2450 MHz, the strongest emission recorded was 111.4 dBµV/m at three metres. Converting that to radiated power is one line of arithmetic any engineer can check. Power equals field strength times distance, squared, divided by thirty. Put the numbers in and you get:

That is a peak radiated figure, worked out from their highest measured field strength using the standard conversion. It is not a transmitter output measurement, because the certification never made one, and it comes from the sample they submitted in 2011, which is the certification they still claim for the product they sell today. You do not have to take my word for any of it. The FCC record is public, and the grant and every exhibit behind it sit at fccid.io/A47-AML. Go and check the line yourself.

It is my arithmetic, not their declaration

Regulation 13(2) does not say the figure should be obtainable by anyone determined enough to find a fourteen year old American test sheet and work out which line matters. It says the manufacturer must supply it with the equipment.

6Forty one milliwatts, at waist height, aimed at twenty feet

Hold the thing the way the manual tells you to. Handle parallel to the ground at waist level, at least eighteen inches out in front of you, so the blade hangs in the air with about half a metre of nothing underneath it.

At 2.45 GHz, published soil data puts the signal’s reach in damp clay at around three centimetres before it drops to a third of its strength. Twenty feet is six metres. Two hundred of those on the way down and two hundred on the way back, each one costing about 8.7 decibels. The round trip comes to somewhere near three and a half thousand decibels of loss. What you have to spend is about a hundred.

Spend that whole budget on soil alone. Ignore the air gap. Ignore the fact that a plastic pipe is a poor reflector. Give it every benefit going.

7Ninety concrete walls, and then back through all ninety

Your broadband router runs at 100 milliwatts at almost exactly the same frequency, and you already know how it copes with one wall. It is why half the country has a repeater on the landing. A concrete wall costs a 2.4 GHz signal somewhere around twenty decibels. Six metres of wet clay costs about eighteen hundred.

A bigger transmitter would not help

The ground is what stops it, not the size of the transmitter, and no power you could fit in a handheld at this frequency would change the answer. That is why the radar that does work runs at a fraction of the frequency, and why nobody builds one to be waved about at waist height either. Ofcom is explicit about that last part.

You keep it on the surface because the moment you lift it, the loudest thing the receiver hears is its own transmitter a few inches away, and the second loudest is the ground directly underneath. The laboratory report for this device records that it “transmits and receives simultaneously”.

8What I am not saying

I am not saying this device is unlawful to sell or to use. Nothing I have found establishes that, and anyone telling you otherwise has gone further than the evidence goes. I am not saying anything about why the figure is absent, because I do not know and neither does anyone else outside that company. It may exist and simply never have been published. Self declaration means there is no way to tell from the outside.

What I am saying is narrow. A figure the law requires, which SSI publish for their own other locators, and which sits one line of arithmetic away inside their own certification file, is not in any document I have been able to find.