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I have an electrical engineering degree, but little experience in building wireless equipment. I need to transmit a signal through about 100 feet of distance, including a concrete wall. I need something small that will transmit just a binary "I'm ON" signal when on, and nothing when off.

I don't really care how large the receiver is, but I'd like the transmitter to be relatively small. The transmitter would really only get a "power on" signal--be expected to transmit when it has power--and then stop transmitting when the power ceased.

Ordinarily for this application I would use some kind of large overkill solution just to use familiar off-the-shelf components. But very little seems to make it into this concrete enclosure (Cell/WiFi/GPS/etc.) What might I use?

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    \$\begingroup\$ we are about designing electronics. This is both a brainstorming/polling question and shopping. \$\endgroup\$ – Kortuk Jul 16 '11 at 23:41
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    \$\begingroup\$ @Kortuk I'd argue that you could have suggested an edit to improve it...and that would have made my day a little better instead of a bit worse. But I rewrote it. Happier? \$\endgroup\$ – HostileFork Jul 17 '11 at 0:47
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    \$\begingroup\$ I'd also suggest editing the title - which is very misleading. The title actually implies a 100 feet thick concrete wall, which is different from 100 feet air distance, including a small concrete wall. \$\endgroup\$ – sybreon Jul 17 '11 at 2:39
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    \$\begingroup\$ Lots of concrete is poured with a steel mesh inside of it to reinforce it. That mesh also frequently penetrates into the ground. This ought to produce a faraday cage, to some extent. So, wavelength needs to be smaller than the mesh size, but not so small that concrete itself blocks it? (Unless your concrete doesn't have any steel in it.) \$\endgroup\$ – Jay Kominek Jul 17 '11 at 3:24
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    \$\begingroup\$ After my car was stolen I had an old (broken screen) NOOK that I use for 3g communication using the free internet B&N gives me to ping home from my car with a gps receiver getting coordinates. \$\endgroup\$ – crasic Jul 24 '13 at 0:10
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You have to be careful with this kind of 'I'm on' signal.

A lot of regulations for un-licensed radio frequencies (such as the ISM band, etc) stipulate a maximum duty cycle on the transmission, meaning that you are only allowed to actively transmit for a certain % of the time. This prevents blocking of a particular frequency and allows better TDM of signals.

For example, for the ISM (Industrial, Scientific, Medical) range:

Dependent on the sub-band the transmission power is limited to 10 dBm … 27 dBm. The permitted time allocation (duty cycle) also varies with the sub-band. So interferences with other 868 MHz equipment are reduced and thus a better transmission quality can be achieved.

- ISM 433/868

So whatever solution you go for you will have to create some kind of periodic pulsed beacon that sends a short burst signal at pre-defined periods rather than just a constant 'on' signal.

This should also mean that you can save power in between the pulses as you can turn the transmitter off (a lot of TX chips have a 'sleep' or 'shutdown' mode) when it's not actively being used.

As has been mentioned in some of the comments, there is most likely a set of reinforcement bars in the concrete forming a 'Faraday cage'. This blocks a large number of signals from penetrating as they hit the bars and then get grounded by them. So, you need to pick a frequency that has a wavelength that is smaller than the space between the bars. Also, concrete can absorb the signal at certain frequencies.

From what I understand this is mostly due to the water content in the concrete. Water contains hydrogen. Hydrogen resonates at 2.4GHz. Many transmissions (WiFi for example) occur at 2.4GHz, so the hydrogen in the water in the concrete absorbs the transmission.*

So, the ISM 868MHz band has a wavelength of about 0.35m - this is probably going to be way too big to fit between the bars (I'm not sure what the regulations state about bar spacing). ISM 915 takes it down to 0.33m - still too big. The 2.4GHz is 0.125m - much more realistic but may not get through the concrete. So you'd be looking somewhere in the mid-to-high 1GHz range. Personally I'm not aware of a license free frequency range in that area. You'd need to check with the RF licensing people in your area (FCC, OfCom, etc).

*(This is purely my own conjecture - please correct me as I'd like to know the truth myself)

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  • \$\begingroup\$ Thank you for the detailed answer! Is there a good way to test this stuff? While I have an old rusty Cornell EE degree, the only "gear" I have is a radio shack multimeter...I don't even have my set of FRS radios anymore. I do know I can't pick up FM radio at all in the garage, so I wonder what the best proof-of-concept would be. \$\endgroup\$ – HostileFork Jul 18 '11 at 5:57
  • \$\begingroup\$ (Also, I haven't even thought of lobby the apartment people to let me run cable. I can't do it without their permission, but as my motivation for asking this is based on a chronic security problem, they might well enable me to do it...and that would sidestep what I'm doing altogether. This permission might not be all that easy to get, but also not impossible...) \$\endgroup\$ – HostileFork Jul 18 '11 at 6:18
  • \$\begingroup\$ I have never heard that 2.4 GHz wont go through concrete. I believe 60 GHz is a harmonic of water, but not 2.4. The lower the frequency the less losses you will have while still staying above the wavelength required to get through the wall. \$\endgroup\$ – Kellenjb Jul 18 '11 at 13:36
  • \$\begingroup\$ @Kellenjb 2.4GHz has something to do with hydrogen - they use it so much because it is a 'quiet' zone in the background noise. \$\endgroup\$ – Majenko Jul 18 '11 at 13:42
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    \$\begingroup\$ Water has lots of vibrational modes, like the ones exploited in your microwave that are not related to the actual atomic transition frequency, the 2.4Ghz is basically only used because its unlicensed under ISM regulations. The only transition in hydrogen near that frequency is actually at 1.4Ghz, corresponding to the 21cm "hydrogen line" or the ground state hyperfine transition, which will not impact your communications because its not a particularly strong transition. All other transitions are significantly higher frequency (first excited state is in the ultraviolet range) \$\endgroup\$ – crasic Jul 23 '13 at 22:35
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The biggest issue that needs to be looked at is how to improve the signal to noise ratio (SNR). WiFi is pretty well optimized, so if you are unable to get any data to this area, then you will probably struggle with most frequency bands. However, there are a few things you can do to improve your SNR.

One method of improving SNR is to place a directional antenna on your receiving end. By using a directional antenna the resulting noise should be less because you wont be picking up noises from other directions.

You should also consider where to place the transmitter in the car as the angle at which the signal goes through the wall can make a big difference. A 6" thick wall can turn into what appears as a much wider wall depending on what angle you are going at it.

And one final thing to think about, I am not up on all of the regulations, but I believe some frequency bands have an average power as well as a peak power associated with its limits. In your situation it would be best to transmit at the peak power and adjust the length of transmission to follow the average power requirements.

If all else fails you may need to consider placing a device in the middle to perform re-transmitting.

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