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Comment by Johnythree | Hacker News Reader

A more likely theory is that the fob transmitter (and antenna) is tuned to put out max signal when near the body. This is it's normal use.

If however you tested it with the fob somewhat isolated it would then be out of tune.

eg in the factory it would be tuned at a set distance from a "dummy body".

It's the same with a walkie-talkie radio. The antenna must be tuned when held in the hand, as your body provides the missing earth or ground-plane.

You can easily demonstrate this. Mount an antenna on a ground-plane with a length of coax, a SWR meter and a transmitter. Tune the antenna so the SWR is 1:1 and then move you hand close to the antenna. Once your hand gets withing a wavelength or so, the effect of the "detuning" can be readily seen on the SWR meter.

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This reminds me of a class in college where one project was to make a waveguide WiFi antenna. You know the ones; often built out of a Pringles can.

On the day the project was due, we each demo'd our antennas and the instructor recorded their range, SNR, etc. Most in the class followed the Pringles can tutorials, but I was always kinda extra. So I used some kind of roof flashing coiled into a cylinder to get the length and radius tuned just right, and I fashioned a comfy handle to aim it using some scrap aluminum flat, and round wood stock.

Mine did really well, but it also had the widest variance in performance. We figured out that my wood handle could be held in a way in which you didn't actually touch any metal, but if you put your thumb up on the aluminum part of the handle, it instantly got way better SNR!

The instructor told us that touching the housing improved the quality of the ground because the capacitance of our meat acts like a short at those high frequencies.

Yes - and not surprising, it's functionally acting like a ground plane.

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.

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.

More likely than what? SE comments provide at least three distinct theories. IMO this one (also provided in the comments) seems fairly conclusive https://www.remcom.com/examples/keyless-entry.html

The scenarios being compared are:

- Fob in hand

- Fob in hand, held against head

Both of these already fulfill your “near body” requirement. Your point around tuning would not explain the specific difference being questioned.

I always did hold the fob close to my palm (with the keyring parts between my fingers) and pointing the fob "up", hopefully aligning a whip in there. It does work better this way, and I can also hold it high up. My theory was that my hand acts as a ground-plane.

Different body positions are going to affect the antenna tuning differently.

And of course different surroundings will also effect the directivity (eg the radiaton pattern).

So some will give better results than others. Why is hard to understand?

So your direct point is manufacturers have tuned fobs for best connectivity when held /against the user’s forehead/? I must’ve never seen the user manuals…

Even simpler, tune in an FM radio and move your hand near the antenna. It will sound better or worse depending on your hand position and how close it is to the antenna.

Back in the early 00's when we still had analogue VHF TV signals, we wanted to watch the football world cup at school. Of course there was no cable or anything at the school and no VHF antenna and streaming was not a possibility yet so we had to be creative. We found an antenna cable and tried to manipulate the cable and use whatever as an antenna. We tried the sheet metal roofing, but that didn't work very well, we tried some metal rods and that didn't work great either. By chance we found out that if someone held the cable in one hand and leaned out of the window with their other hand stretched out we got a signal that was good enough to watch the game. Well, for everybody except the poor bastard that had to stand behind the TV halfway out the window.

Fond memories here of watching Babylon 5 on a Performa 5300 with an Apple TV Tuner Card. There was no antenna to the room with the computer, but I found that I was able to get an acceptable picture by holding the coax cable attached to the antenna input in the right way while keeping one hand attached to the central heating radiator...

A metal fork is the best antenna (among non-antennas)

So a homemade theremin?

Reminded me of discovering this when I was a kid and thought I was magic. I had forgotten about that.

You might be right about this, it makes sense. So standing on your tippytoes with the remote-arm stretched out to try and get better range is actually counter productive. Take 2 steps closer, and hold the remote close to your body.

How does it detect the presence of a nearby human body?

OP means that while developing the radio and antenna part of the fob the engineers have adjusted the radio and antenna to work the best when in close proximity to a body. The human body has a certain effect on the radio signals, so adjusting the radio and antenna to work best while close to the human body will mean that it won't work as optimally when further away from a human body.

It doesn't detect the human body, it is just designed to work the best when it is physically close to the human body.

> A more likely theory is that the fob transmitter (and antenna) is tuned to put out max signal when near the body. This is it's normal use.

This is true.

Or pointing it at your head, is the same as holding it up high in the air.

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