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Why does holding a key fob to your head increase its range?

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Re: Why does holding a key fob to your head increase its range?

#141

Post by HnUser12 »

mcv wrote: post #102
xp84 wrote: post #43

The answer posted with all the experiments is conducted with a 2009 GTI, which reminds me of my first car, a Jetta from a generation before that. Its manual included this helpful illustration of the full expected range of the remote: https://imgur.com/a/1Pdfyg4 Yes, apparently they were only willing to promise it would work within about arm's length of the car! It wasn't that bad, but also it wasn't a heck of a lot b…

Short range isn't necessarily bad. Modern cars unlock automatically if you try to open the door while the keyfob is in range, so long range could mean someone could unlock your car while you're out of sight. I don't know if that system uses the same range as when you press the button on the keyfob, but I have read about a hack (probably here) where thieves used a transmitter to extend the range of both the car and th…

I don't know the inner workings of the fob, but my car has a pretty long range when unlocking with the key fob click, but the automatic unlock without click(keyless entry) works only when I'm next to the handle. It doesn't work even if I'm at the back of the car and someone tries to unlock driver door.

Re: Why does holding a key fob to your head increase its range?

#142

Post by zro »

srcreigh wrote: post #99
LargeTomato wrote: post #52

Earlier quoted context omitted.

One more anecdote, this has not consistently worked for me. Raising the height of the receiver may have an effect but I'm not sure.

We need to cross analysis people across the axes of tall/short and large/small head for posterity

Tall with large head here, works pretty consistently for me

Re: Why does holding a key fob to your head increase its range?

#143

Post by ht85 »

jimnotgym wrote: post #109
ht85 wrote: post #92

From the top answer: > 1/2 m ≈ 1.5 ft Thanks US units for the quote of the day: "Meters are for waves, feet are for antennas"

Excuse me, feet are US units??? Surely you mean feet are units that were adopted in the US... and then dropped by the rest of the world!

You are correct, wikifeet confirms it.

Re: Why does holding a key fob to your head increase its range?

#144

Post by class3shock »

martijnvds wrote: post #58

Sixty Symbols did a video on this almost a decade ago: https://www.youtube.com/watch?v=0Uqf71muwWc

I was looking to see if someone had posted this, as it was what I immediately thought of when I saw the title. Lovely video, wild to think it's been that long since it's release.

Re: Why does holding a key fob to your head increase its range?

#145

Post by toast0 »

fragmede wrote: post #135
eitally wrote: post #131

Earlier quoted context omitted.

I had a 2001 GTI and it similarly only reliably worked within probably 25' of the vehicle. In contrast, my 2017 F150 fob works as much as 150-200' from the vehicle. I tremendously appreciate this, especially with remote start to warm it in the winter.

If range is desirable, Tesla's, and I imagine other cars have this as well, have a cell modem and an app, so you can control it via the Internet, which is effectively infinite range. It's occasionally handy to be able to unlock the doors to let someone into the car from elsewhere in the world. Eg friend left their bag in the car but I'm not around to let them in.

My Ford has that. Just need to be sure to park in range of ATT 3g. (Well, I pulled the modem this weekend). They had replaced the 2g modem with a 3g modem in 2017... because networks turning off older signals was unfathomable.

Re: Why does holding a key fob to your head increase its range?

#146

Post by Johnythree »

yunohn wrote: post #90
Johnythree wrote: post #42

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 mis…

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.

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?

Re: Why does holding a key fob to your head increase its range?

#147

Post by Johnythree »

wallaBBB wrote: post #94
Johnythree wrote: post #42

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 mis…

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…

> 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.

Re: Why does holding a key fob to your head increase its range?

#149

Post by Johnythree »

greggsy wrote: post #96
wallaBBB wrote: post #94

Earlier quoted context omitted.

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…

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.

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.

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