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I am studying RF communications and learned that antenna length is proportional to a wavelength of a signal. And many literature say that a high frequency is preferred in wireless communication because a length of antennas can be short, hence, a size of a device can be small.

From here I can deduce a reason that a baseband signal can't be sent through the air. The reason is that, a length of antennas for baseband signals will be infinity, but in a real world, infinity length of antennas is impossible. Hence, sending a baseband signal through the air is impossible.

However, as I am a learner, I can't tell the above statement is valid.

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    \$\begingroup\$ If by baseband you refer to the audio signal itself then even a very low frequency of 10Hz is a wavelength of 30 000 km - big but certainly not infinite. Very low frequency transmissions are used to communicate with submarines. (so no, its not impossible). The main reason (IMHO) is that if the transmission was only the base band you would only be allowed ONE radio station. Using higher carrier frequencies allows us to divide up the RF spectrum into lots of different channels. \$\endgroup\$ – JIm Dearden Sep 25 '16 at 14:34
  • \$\begingroup\$ Correct and let me expand on that a little more in my answer. \$\endgroup\$ – Bimpelrekkie Sep 25 '16 at 15:10
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    \$\begingroup\$ If the baseband signal has necessary components at DC or simply low enough in frequency to make the antenna impractical you are correct, also see the bandwidth issue raised by Alex. In the analog era of capacitve and magnetic stage coupling basebands schemes were designed to avoid the "hole at DC". But today's digital processing can represent 0 Hz with the same fidelity as any other frequency, so today 0-centered IQ baseband designs are the norm. When accurately tuned an AM or FM signal has a critical-to-definition DC component in such a scheme; if mistuned it will be at another frequency. \$\endgroup\$ – Chris Stratton Sep 25 '16 at 19:55
  • \$\begingroup\$ Interestingly, LTE excludes channel 0, which would contain DC in the baseband, apparently to allow cheaper transceivers. \$\endgroup\$ – Simon Richter Sep 26 '16 at 7:07
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Sort of. Certainly antenna length is proportional to wavelength, so if you take the limit going to DC then you'll need an infinitely long antenna. However, even if you make an antenna infinitely long, it won't work very well. Antennas have limited bandwidth. Say you design a 7.5 cm long antenna for a 1 GHz signal. This antenna will work very well at 1 GHz, but at 2 GHz it's too long and the radiation pattern will change. At 500 MHz, it will be too short and won't radiate efficiently. One of the features of RF transmissions is they usually have a small bandwidth relative to the transmit frequency. Say, a 10 MHz wide band sitting at 1 GHz. The frequency only varies by 1% in this case, so you can pretty much ignore that variation when designing the antenna and design it for a single frequency. So, if you wanted to design a 10 MHz baseband antenna that works from DC up to 10 MHz, you need to design an antenna that works properly not over a 1% bandwidth but over 7 orders of magnitude, which is pretty much impossible. Actually, it's not even 7 orders of magnitude-that would be 1 Hz to 10 MHz-it's infinite orders of magnitude. The infinitely long antenna that you would need for DC will not work well at all at 10 MHz.

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The ressaon is that, a length of antenas for a baseband signal will be infinity, but in a real world, infinity length of antennas is impossible. Hence, sending a baseband signal through the air is impossible.

This statement is nonsense !

Ideally an antenna should have a length of 1/4 the wavelength you want to transmit/receive, or a multiple of that 1/4 the wavelength. If the antenna is shorter or folded to be smaller, it will still work yet less efficiently.

So you can transmit/receive low frequencies (with large wavelengths) through the air (or space) but it is not efficient. Also, if you send a 1 kHz signal, no-one else in range can use that frequency. Additionally propagation of such 1 kHz signal is such that that range could be very large.

The solution is to modulate that 1 kHz (baseband) signal onto a carrier. For example 100 MHz. That 100 MHz travels less far as it behaves more like light, it has more difficulty in going around corners etc. But someone else in the same region as you could use 101 MHz and another 102 MHz each of you without disturbing the others (assuming the receivers can filter out the other signals).

So it is possible to transmit a baseband signal through the air but it is very impractical and not limited by antenna size but by efficiency.

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    \$\begingroup\$ At whoever -1 this answer: I don't care about the -1 but I do care what your reasoning behind you disagreeing with this anwer is. So please explain so we can al learn something. \$\endgroup\$ – Bimpelrekkie Sep 25 '16 at 15:11
  • \$\begingroup\$ Your explanation is only correct for a signal which has a single or narrow band of frequency components not including DC. The poster's question has no such constraint, so your answer is wrong and your dismissal of the poster's factually correct statement is unhelpful. Overall, Alex's answer is more correct and picks up what you miss. Also, a 1 KHz signal carrying any information is one which has already been modulated. If you want to de-conflict it, you can choose a different low frequency within the range of what your signal processing can handle without frequency translation. \$\endgroup\$ – Chris Stratton Sep 25 '16 at 17:33
  • \$\begingroup\$ I do not consider DC a signal as it does not carry information. And what would be the point in transmitting a DC value ? Alex's answer might be more correct but I doubt if it gives OP more insight into the matter. To explain the basics I prefer to simplify and not try to cover everything. If you disgree then write your own answer and explain in terms OP can understand why it is true if you include DC and why one would include DC. \$\endgroup\$ – Bimpelrekkie Sep 25 '16 at 18:28
  • \$\begingroup\$ "And what would be the point in transmitting a DC value" - this may be a primary source of your error. Many signaling schemes will break if the DC component or very low frequency AC components are removed. For example, the output of a common serial port. \$\endgroup\$ – Chris Stratton Sep 25 '16 at 18:30
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    \$\begingroup\$ The very essence of the question is why baseband signals are not suitable for wireless transmission. Fortunately you seem to be catching on to this important reason that applies in many cases. And FYI, once upconverted as you propose, your 1 KHz baseband signal component is not different than a 0 Hz baseband signal component, apart from landing at a different frequency and having a displaced image. They carry the same amount of information! \$\endgroup\$ – Chris Stratton Sep 25 '16 at 18:42
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Any antenna, no matter how short, will radiate energy. The key point is that as the antenna becomes small compared to the wavelength of the signal, the efficiency of the antenna decreases. Thus, the short antenna is not useful.

AM radio (with frequencies of 550 KHz - 1.6 MHz or so) is effectively baseband for a system like Bluetooth. (Bluetooth uses a baseband bandwidth of about 2 MHz). AM requires large radio towers for an efficient antenna. Bluetooth - sitting on a 2.4 GHz carrier - only needs a small piece of metal in your cell phone.

Of course, the other problem with transmitting at baseband is that all signals will be on the same "channel" and interfere with each other.

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