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What I can I think about is:

  • Power spectrum density at MW frequencies is very poor at a point it is unable to penetrate obstacle without losing a lot of power.
  • Multipath fading at those frequencies are very high to a point the receiver is unable to filter them out.
  • Radiation pattern of MW is not isotropic, with the main loop is very important than other directions.

Are those assumptions correct? What else?

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  • \$\begingroup\$ MW is not an abbreviation of microwave - it is the ITU designation for "medium wave" which is 300kHz to 3MHz range. Where in the 300MHz to 300GHz region of "microwaves" are you interested. \$\endgroup\$ – Tom Carpenter Sep 22 '15 at 0:57
  • \$\begingroup\$ No antenna has a truly isotropic radiation pattern (it would need to be a point). An isotropic antenna is a theoretical construct (at all frequencies) used to quantify the effective gain of a real antenna (for the direction it has gain). \$\endgroup\$ – Peter Smith Sep 22 '15 at 7:06
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Here's how it works and why it works: -

  1. Choose an operating frequency that is high enough to make a parabolic dish antenna produce a beam width that is nice and tight.
  2. Aim the transmit antenna at the receive antenna very carefully i.e. align them up.

The transmission losses will be very small i.e. you get a great signal to noise ratio over vast distances because (theoretically) all the transmit power is largely pointed towards the receiver.

Sub microwave antennas are required to be very large to get a tight beam width but they do exist of course: -

enter image description here

With proper point to point microwave transmissions, there are no multipath fades because all the energy is transmitted within the tight beam width. However, if the link isn't designed correctly multipath fades can be an issue but, still much less of a problem that regular dipole to dipole transmissions.

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  • \$\begingroup\$ This answers it, thnx ;) \$\endgroup\$ – Malek Sep 22 '15 at 9:40
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Power spectrum density at MW frequencies is very poor at a point it is unable to penetrate obstacle without loosing a lot of power.

That's not what power spectral density means. PSD is just a measure of power at a given frequency.

But you're right that microwaves don't pass through objects as well as longer wavelengths do (in most cases; there are exceptions). They are more likely to interact with the objects — in particular, to be be absorbed or scattered — instead. That is the fundamental physical reason; everything else is just the engineering consequences.

Multipath fading at those frequency are very high to a point the receiver is unable to filter them out.

Multipath is not a separate phenomenon: it is a consequence of the wave interacting with obstacles.

Radiation pattern of MW is not isotropic, with the main loop is very important than other directions.

This is backwards. Radiation patterns are determined by antenna design. Any antenna design can built for any frequency, unless it has impractically large or small elements. What's going on here is that:

  1. Antennas with higher gain (directionality), at the same frequency, are larger.
  2. Antennas for higher frequencies are smaller.

Therefore, increasing frequency allows the use of higher-gain antennas in the same space. Higher-gain antennas are more efficient when feasible since less transmitter power is required. Thus, when the communication is between two fixed stations and there is a clear line of sight, using microwave frequencies is advantageous.

Microwave frequencies may be be used even in the presence of obstacles, simply because the higher frequencies allow for physically small antennas and large absolute bandwidths, and the less reliable propagation is an acceptable tradeoff; this applies to many modern data protocols such as Wi-Fi and Bluetooth in 2-5 GHz.

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  • \$\begingroup\$ Finally, he didn't ask about point-to-point, but about line of sight. Omnidirectional microwave communication would be possible - if there are no obstacles. In principle, microwaves behave like light and are absorbed by most materials. \$\endgroup\$ – sweber Sep 22 '15 at 9:05
  • \$\begingroup\$ Thanks this is informative, Kevin. @sweber, good point about absorption, agree. \$\endgroup\$ – Malek Sep 22 '15 at 9:35
  • \$\begingroup\$ @sweber Edited! \$\endgroup\$ – Kevin Reid Sep 26 '15 at 22:46

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