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I know we use modulator to change the frequency of original wave, thus reducing the size of antenna. As I understand the frequency of carrier signal does not matter . But I don't understand why.

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  • \$\begingroup\$ now we're in a deadlock: I don't understand why the carrier frequency should matter. \$\endgroup\$ Commented Jan 23, 2021 at 11:31
  • \$\begingroup\$ So, please explain why you think it should matter. (There's a scientific-philosophical underlying principle here: Occam's razor: Make the least amount of assumptions possible. Not making the assumption that it matters is less assumptions) \$\endgroup\$ Commented Jan 23, 2021 at 11:32
  • \$\begingroup\$ I just do not know whether frequency of signal change the data or not \$\endgroup\$ Commented Jan 23, 2021 at 11:34
  • \$\begingroup\$ it doesn't, as whatever you learnt from tells you. \$\endgroup\$ Commented Jan 23, 2021 at 11:36
  • \$\begingroup\$ Context context context. <sub>fewer assumptions LOL</sub>. \$\endgroup\$
    – Andy aka
    Commented Jan 23, 2021 at 11:45

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The frequency of the carrier does matter. You need to pick one that's suitable for your antenna, and doesn't interfere with, or get interference from, neighbouring services.

However, at the receiver, when you demodulate the modulated carrier with respect to the specific frequency you chose at the transmitter, you remove the effect of the initial choice. You could choose to transmit with 100 MHz or 500 MHz carrier, and as long as you convert that signal down with the equivalent of a 100 MHz or 500 MHz reference, the two systems will function the same.

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The carrier doesn't carry any information (or intelligence). It is modulated by the message signal and as you quoted, the physical dimensions of antennas are related to the carrier frequency. In most cases, we often try to suppress the power of the carrier to have high efficiency. The modulated information is contained in the side bands: irrespective of whether it is DSB-SC, SSB-SC or VSB modulation. The sidebands are centred about the carrier frequency in case of a DSB-SC. Both carry the same information and of course the location of the sidebands depend on the carrier frequency. So, the carrier frequency does matter.

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