2
$\begingroup$

Is it possible to transfer data at a high rate, like 1-2 Mbps, on 0.5 - 30 MHz?

From Shannon's Theorem, with a bandwidth of just 300 kHz it is possible to reach 1.5 Mbps.

I want to use these bands because of longer ranges and lower losses in urban areas, but I do not know why these bands are not used for Wi-Fi connections. What is the problem with operating like this?

$\endgroup$
  • 1
    $\begingroup$ Can you please post your actual derivation of the 1.5 Mbps channel capacity? Note that Shannon-Hartley capacity theorem gives a theoretical upper bound. $\endgroup$ – Juancho Oct 25 '14 at 0:54
  • $\begingroup$ for HQ vedios for example . $\endgroup$ – abduoman11 Oct 25 '14 at 2:52
  • $\begingroup$ In addition to what other answers have said, the natural noise floor is higher at lower frequencies, and propagation is more unpredictable at HF and below. $\endgroup$ – Phil Frost - W8II Oct 26 '14 at 14:37
4
$\begingroup$

There are many reasons not to use low frequencies such as below 30 MHz. Here are just a few:

1) It is prohibited by law. These frequencies are assigned to other services. 2.4GHz is a shared service... Even your microwave oven cooks there.

2) They need large antennas to function. The shortest antenna at 30 Mhz, which still is effective, is about 5 meters long (while at 2.4GHz it only needs 6-7 cm).

3) Shannon's limit isn't (a limit, I mean): the formula $C=B\log_2(1+\frac{S}{N})$ just says that you can send more info over the same bandwidth if you improve the $\frac{S}{N}$ ratio.

4) Then, you should realize that if you want to be at Shannon's C rate, you have to have the channel for yourself. No other users can use the same frequency. If you realize that 2.4 GHz is a jungle with probably 100s of simultaneous users...

So, if you want to improve your channel capacity (C), you should concentrate on improving the S/N ratio, which is much easier to do at 2.4 or 5.2 GHz than at 0-30 MHz

$\endgroup$
  • $\begingroup$ I want increase my wifi range up to 30-60 km at heavy urban areas.... wich it is impossible at 5 GHz , 2,4GHz or even 900 MHz ! $\endgroup$ – abduoman11 Oct 25 '14 at 16:54
  • $\begingroup$ Well, I think you will have to either install a repeater (most APs can configured as such) somewhere, or, probably the most economical/reliable way, use the internet. $\endgroup$ – jcoppens Oct 25 '14 at 17:12
  • $\begingroup$ internet cant be used for mobile receiver $\endgroup$ – abduoman11 Oct 25 '14 at 19:08
  • $\begingroup$ Haven't you heard about WiFi? $\endgroup$ – jcoppens Oct 25 '14 at 20:52
  • $\begingroup$ I talking about HF wifi $\endgroup$ – abduoman11 Oct 25 '14 at 23:03
4
$\begingroup$

There are several reasons the low bands are not used for high data throughput:

  1. The problem is at HF frequencies, a 300kHz bandwidth is a lot of spectrum. At MF frequencies it's a whole lot of spectrum.

    1a. If you look at this spectrum chart you will see that 300kHz covers more that 10 assigned services at MF and covers most of 4 services at HF.

  2. From a legal standpoint (most countries have laws governing spectrum usage) you would not be authorized to transmit over such a wide chunk of spectrum.

    2a. To be legal you would have to lease the spectrum and even if you could get the governing agency to lease all that spectrum to you, you couldn't afford it.

  3. If you decided to ignore the legal aspect and forge ahead(see point 4 however), remember MF frequencies travel great distances via ground-wave propagation and HF frequencies bounce of the ionosphere and travel around the world. A 300kHz signal would cause a lot of people interference. Someone would hunt you down and stick a pin in your COAX. [tic]

  4. You would have to design and build a transmitter/receiver and antenna system capable of transmitting/receiving such a broad signal on those frequency bands which would be impractical.

I hope this clears things up a bit.

$\endgroup$
  • $\begingroup$ While I was typing this I got interrupted (took me an hour to get back to the post) and another answer was posted in the meantime which basically covers what I said, and covered the S/N ratio. $\endgroup$ – cyberlord8 Oct 25 '14 at 17:55
  • $\begingroup$ Thanks Kevin, I couldn't figure out how to indent the sub-paragraphs. $\endgroup$ – cyberlord8 Oct 25 '14 at 18:41
  • $\begingroup$ but for example the 1.8 - 2 MHz is 200 KHz , may be be enough for 0.5 - 1 Mbps . $\endgroup$ – abduoman11 Oct 25 '14 at 19:12
  • $\begingroup$ Yes, it may be, depending on conditions (S/N), but you are using that entire segment of spectrum all to yourself and that would be illegal for one, and frowned upon by all the users of that chunk of spectrum. Look at it this way, each 'band' has 10 times more bandwidth than the previous band. So in order to do HQ video the lowest band that would have enough bandwidth to reasonably support this activity would be the UHF band. $\endgroup$ – cyberlord8 Oct 25 '14 at 19:46
  • 1
    $\begingroup$ The other problem with your example is designing a transceiver/antenna system to handle such a wide transmission at such a low frequency, it would be impractical. $\endgroup$ – cyberlord8 Oct 25 '14 at 19:55

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.