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End goal: I wish to use a smartphone NFC transmitter (high-frequency RFID @ 13.56 MHz) to send data to a low-frequency RFID @ 120–150 kHz door reader to open the door.

Illustrative example:

  • HF transmitter frequency = 5x LF receiver frequency
  • HF transmitter sends 5 signal units of high amplitude
  • HF transmitter then sends 5 signal units of low amplitude
  • wishful end goal: LF receiver receives a high and then a low signal unit

Perhaps I'm thinking about it too digitally (ones and zeros), but could the transmitter emit the signal in such a pattern (e.g. repeated "high"s/"on"s and then repeated "low"s/"off"s) that the receiver can resonate to that and receive an intended piece of information?

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  • $\begingroup$ What is the bandwidth spec for the NFC transmitter? If broad enough, one just modulates the 13.56MHz with your desired 120-150kHz. But, while only 1% of the carrier frequency, that likely is higher than an NFC transmitter wants to do. (But this likely isn't an antenna problem, more the rest of your electronics). $\endgroup$
    – Jon Custer
    Dec 5, 2019 at 14:31
  • $\begingroup$ On the other hand, per wiki: "They operate within the globally available and unlicensed radio frequency ISM band of 13.56 MHz. Most of the RF energy is concentrated in the ±7 kHz bandwidth allocated for that band, but the emission's spectral width can be as wide as 1.8 MHz[39] in order to support high data rates." - Looks like you are good. The door read will ignore the carrier and pick up the modulated signal, depending on strength. $\endgroup$
    – Jon Custer
    Dec 5, 2019 at 14:33
  • $\begingroup$ I am at a very entry-level in this part of physics (Computer Science degree with a course in physics and one in electrotechnics), but I can get up to date given the reading material. I'll therefore look these things up, thanks! $\endgroup$
    – clausavram
    Dec 5, 2019 at 15:18
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    $\begingroup$ There seem to be several unrelated questions grouped together here, and only some of them are physics questions. I would suggest you edit the question to eliminate the material about modulation (which is engineering, not physics), and sound (which is a separate topic). $\endgroup$
    – user4552
    Dec 5, 2019 at 16:50
  • $\begingroup$ what do you mean by an antenna "emulating" a lower frequency signal? $\endgroup$
    – hyportnex
    Dec 5, 2019 at 20:20

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I don't think this will work. An antenna has a resonant frequency. For example, a lot of antennas are cut to a length of about 1/4 of a wavelength. They will still work at frequencies off resonance, but they will be much less efficient. E.g., AM radio receiving antennas are much shorter than the resonant length, and they pay for this by being inefficient, but this is made up for with amplification (and thermal noise is small, so this works). In your case, I don't think you'll get any significant amount of signal at a frequency that's off by a factor of 100 compared to the antenna's resonant frequency.

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  • $\begingroup$ Is there no way to bypass this limitation? To get more of the transmitted higher-frequency energy into the receiving antenna, but without increasing the signal strength by 100 fold? $\endgroup$
    – clausavram
    Dec 6, 2019 at 23:06

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