Why microwave links are in the higher microwave spectrum (I don't know exactly but I guess it's around 20-40GHz). If by Friis Equation higher frequencies have lower range then why use higher frequency rather than low frequency?

  • \$\begingroup\$ Higher Frequency have higher bandwidth (how much data you can send per seconds). This is also why for instance WiFi new protocols goes from 2.4 to 5Ghz \$\endgroup\$ – Damien Sep 18 '18 at 3:49
  • \$\begingroup\$ How are bandwidth and frequency related? Is 1 Mhz equal to 1 Mbps? \$\endgroup\$ – ObsessionWithElectricity Sep 18 '18 at 3:56
  • \$\begingroup\$ See Why is channel capacity a factor of bandwidth instead of frequency? \$\endgroup\$ – The Photon Sep 18 '18 at 4:03
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    \$\begingroup\$ @Damien you are mistaken. Higher frequency does not have higher bandwidth. Bandwidth is a difference between two frequencies. Some higher frequency bands may have larger legal allocations but that is a matter of law, not physics. However, a given bandwidth is a smaller proportional difference at a higher frequency, so it is easier to design wideband systems at higher center frequencies. \$\endgroup\$ – Chris Stratton Sep 18 '18 at 4:12
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    \$\begingroup\$ @analogsystemrf what are talking about? \$\endgroup\$ – ObsessionWithElectricity Sep 18 '18 at 4:55

Microwave links require directionality or high gain antennas and they mainly use dishes (parabolic antennas) at each end. The high directionality means that a decent amount of the energy sent by the transmitter is picked up by the receiver. A higher operating frequency means more directionality for a given dish size: -

enter image description here

Notice the lambda term in the denominator; as frequency rises lambda falls and gain increases. The term D is the dish diameter. See this web page for more detail.

If by Friis Equation higher frequencies have lower range then why use higher frequency rather than low frequency?

First of all let's clear this misconception up; a transmitted radio wave has exactly the same "range" in free space irrespective of the frequency i.e. it reduces in power density as distance squares. But, of course, for a simple antenna such as a quarter wave monopole, its optimum length decreases with frequency hence the power it can extract from "the ether" also reduces. You have to start thinking about a receive antenna behaving like a fishing net. The wider the net, the more received energy (and fish) it can collect.

The Friis transmission equation has a lambda squared term in the numerator (causing link loss to increase with frequency) but, this effect is cancelled by the dish's lambda squared term in the denominator.

Friis link loss equation: -

enter image description here

  • \$\begingroup\$ So different kind of antennas behave differently to increase or decrease in f. But my question was that why do they use microwaves why can't they use radio waves with high gain antennas wouldn't they be better (face less attenuation)? \$\endgroup\$ – ObsessionWithElectricity Sep 18 '18 at 7:46
  • \$\begingroup\$ Secondly, Is it true that these waves not pass through walls of buildings? \$\endgroup\$ – ObsessionWithElectricity Sep 18 '18 at 7:47
  • \$\begingroup\$ An RF wave of (say) 100 MHz would need a parabolic dish the size of a house to get the same directionality as a regular sized dish at several GHz. Trying to make a high gain antenna at 100 MHz compete on performance with a dish at several GHz would mean higher cost and bigger size plus vulnerability in weather conditions. It's just not an economical option. Not sure about how deep they will penetrate - try googling. \$\endgroup\$ – Andy aka Sep 18 '18 at 8:29
  • \$\begingroup\$ Can I calculate dish size for a specific frequency. Can you link me calculator? \$\endgroup\$ – ObsessionWithElectricity Sep 18 '18 at 8:38
  • \$\begingroup\$ Calculator. Enter frequency and diameter and press calculate. If you've finished with the previous question we talked at length about how about formally accepting it? \$\endgroup\$ – Andy aka Sep 18 '18 at 9:19

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