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Comment by wallaBBB | Hacker News Reader

Not really - antenna is tuned to the maximum allowed emission power per local regulations (e.g. big differences between EU, US and JPN regulations). Certification tests do not include any human handling, since it's really unpredictable and varies. Therefore, tests are just the fob with a (per specifications) positioned Rx antenna (highly controlled environment).

BTW similar effect is observed with the fob being next to the bottle of water. There are peer reviewed studies on these topics, that my RF engineer shared with me when I asked him way back about it, but are heavy on the RF/antenna theory, beyond my understanding.

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I read it in the sense that they would retune or reengineer it if it was discovered that holding it in a hand resulted in poorer performance - ie. they need to factor in the environment it operates in (a hand beside a body) after they design it on paper.

There are 2 limiting factors taken into account during design - regulations and peak current that the battery can supply.

The battery thing (usually a coin cell CR2032 that can supply ~13mA per spec) can be a bit mitigated with a beefy capacitor (though RF packet length and space in-between have to be appropriate for the capacitor recovery, not to brown out the system).

Usually the bigger bottleneck is the regulation, so you tune for best performance in a clean environment. Hand and body during operation are really unpredictable to model in/retune, but luckily usually adds for the better. Anyways the range of the classical RF fobs (@3xx-4xx MHz and 8xx-9xx MHZ bands) is well beyond the real world needs that the 10-20% drop of performance due to certain fob positions will make a difference. If it does, you probably have a low battery.

Next gen fobs (if they hit the mass market) are BLE and there the range performance is more critical. Dunno what will be the approach there.

What can cause problems are ornament and the mechanical key insert. It has to be extensively tested that there is no coupling with those metal parts that are close to the antenna. Problem comes sometimes when they decide to redesign a bit the keyfob without going through the tests internally (e.g. oh it's just a minor tweak of the radius of the metal ring around the fob housing...) and then the certification fails and has to be repeated (not just money, but getting the slot for it...). Some OEM might even be inclined to think they don't need a certification for such a minor change and push it to the market, making the problem worse if there is an issue.

Regarding the ornament things on fobs, there were even instances of them magnetically coupling with the wireless chargers or even immobilizers and that lead to some melting...

Any Antenna designer wants to see the antenna being an elevated ground-plane or dipole mounted clear of obstructions well above the phone.

However he is stuck with a compromise design and knows that there are various effects that a nearby conductive body can have on the antenna's performance.

- Anything nearby will effect the tuning of the antenna. And if the antenna is no longer at resonance, then it's efficiency is greatly reduced.

- Anything nearby will disturb the antenna due to its missing ground plane, which will change both the tuning and the loading (eg impedance).

- And anything nearby will act as Directive or Reflective elements thus greatly effecting the radiation pattern (eg directivity).

In the early days, it was common for phones to have an external whip antenna, and also a socket so an outdoor antenna could be plugged in if required.

But forcing the designed to include the antenna within the body of the phone, (and without an effective ground plane) results in a mess of compromises.

Unfortunately the general public thinks that phone works by "magic", and is somehow exempt from the fundamental need for a radio to have an efficient external antenna.

> antenna is tuned to the maximum allowed emission power per local regulations

Actually it's not. The antenna is tuned to give a peak in output. It's either correctly tuned, or it's off tune.

It's the design of the whole transmitter/battery/antenna which chosen to give a compromise between battery life and max allowed output.

Few transmitters will come close to the max legal power, and if they did, de-tuning the antenna would result in a very inefficient compromise.

If you include a (dummy) human in the test environment you will (with luck) be close to optimum. By not including a human, you can be sure that the results will be worse in actual use.

Not sure what products your experience comes from, but I am talking explicitly about keyfobs. This is generally produced by Tier 1 suppliers, not the OEMs themselves, and not too many players in the field. With the MCU suppliers it shrinks even more (talking about handsfree models, most common in the market, utilizing LF for localization and standard RF bands for Tx).

MCUs are highly specialized (also not publicly available), and you can play with the tuning registers up to several digits without much performance loss. 3xx/4xx MHz ones you can easily go over the regulation limits (IIRC in JPN occupancy is taken together with the power into the calculations, it's a very crowded spectrum...).

If a spread spectrum is used at 8xx/9xx range, there there is no worry of hitting the limit, here usually battery life and max peak current are limiting, and fine tuning is often done for individual samples at EOL in the plant. It doesn't necessarily mean that all at 8xx/9xx are spread spectrum, but the "premium" (usually half-duplex) are. They provide a lot better performances at low SNR (and thus great range) compared to the standard FSK/Manchester 1 or 2 CH fobs.

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