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Given:

  • Transmit power 1 W
  • Receiver sensitivity -95 dBm to -85 dBm
  • Frequency 2.4 GHz
  • distance 2 km

I have used Friis equation to compute it. I will use 2 similar antennas.

$$ P_r = P_t + G_t +G_r - F $$

where F = free space attenuation.

$$ G = (P_r-P_t+F)/2$$

I got G<0, is it correct?

Klas-Kenny
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FrimHart64
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    You can't divide something in dB (P, G, etc) by 2 and get a rational answer, which is what you did here: G=(Pr−Pt+F)/2. Proper way is to subtract 3 dB. – SteveSh Feb 24 '22 at 12:53
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    FrHart64, as previously pointed out, you should accept an answer if it has serviced your needs. You should also upvote answers that have been useful to you. I don't suppose you want to be known as mean and, I don't suppose that gaining a mean reputation is a good for obtaining help on future questions. – Andy aka Mar 03 '22 at 09:39

1 Answers1

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The Friis path-loss equation using isotropic antennas can be expressed in decibels: -

Path-loss (dB) = 32.45 + 20\$log_{10}\$(f) + 20\$log_{10}\$(d)

Where \$f\$ is in MHz, \$d\$ is in kilometres and free-space is assumed.

At 2.4 GHz and 2 km, that's a path-loss of 32.45 dB + 67.60 dB + 6.02 dB = 106 dB.

Your transmit power is 1 watt (+30 dBm) and your receiver sensitivity might be -85 dBm so, that is a difference of 115 dB hence, you have a surplus of 9 dB and, you can use isotropic antennas in free space for this. Hence your value for antenna gain is negative.

However, over terrain (obstacles) and, with weather changes and man-made noise, a general approximate rule of thumb is to add 20 or 30 dB to your path loss to cater for these extremes.

This is called fade-margin.

Now, your path-loss is 136 dB (30 dB added) and, your transmit power and receiver sensitivity fall-short by 21 dB hence, you need antenna-gain in my humble opinion.

Another Q and A on a similar subject.

Andy aka
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    if i look for an antenna datasheet, I have never seen the gain with negative value. I got -19 dBi = 2*G, it means G = -8,5 dBi. NO antenna has this value – FrimHart64 Feb 24 '22 at 10:02
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    @FrHart64 please read my answer very carefully especially these bits: **you have a surplus** and **Hence your value for antenna gain is negative**. – Andy aka Feb 24 '22 at 10:03
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    What does "surplus" means? – FrimHart64 Feb 24 '22 at 10:56
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    [This is what surplus means](https://dictionary.cambridge.org/dictionary/english/surplus) – Andy aka Feb 24 '22 at 10:57
  • @FrHart64 you should [take the 2 minute tour](https://electronics.stackexchange.com/tour) to understand the motivation behind people giving you free help and apply that principle to all the questions you have raised that have given satisfactory answers. – Andy aka Feb 24 '22 at 10:59
  • @thank you for the translation :) I would like to ask what it means for gain calculation? – FrimHart64 Feb 24 '22 at 12:13
  • @FrHart64 I don't understand your question. – Andy aka Feb 24 '22 at 12:14
  • i understand I can use isitropic antenna for my case. Problem is I can't understand what gain of antenna I need then – FrimHart64 Feb 24 '22 at 12:40
  • You need to have an overall antenna gain of 21 dBi if you accept that you need a 30 dB fade margin (as I suggest). The 21 dBi can be at one end or both ends (10.5 dBi) or somewhere in between. – Andy aka Feb 24 '22 at 12:42
  • @FrHart64 - Isn't this a re-write of of your other question about the lander/rover antenna requirement? – SteveSh Feb 24 '22 at 12:49
  • @FrHart64 - isotropic antenna and (antenna) gain are mutually exclusive. – SteveSh Feb 24 '22 at 13:29
  • So...i need an antenna with about 10 dBi gain? – FrimHart64 Feb 24 '22 at 14:53
  • for example, patch antenna, its gain max is 9 dbi – FrimHart64 Feb 24 '22 at 15:09
  • @FrHart64 you need to apply antenna gain to both ends of the link that add up to a total of 21 dBi - that could mean 10.5 dBi at one end and 10.5 dBi at the other end. I can't make recommendations about antennas because requests for product recommendations are off-topic and, I'm not an expert in antennas anyway. Did you take the 2 minute tour as I requested? – Andy aka Feb 24 '22 at 15:17
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    @SteveSh - Hi, If you want to make (relevant) points to the OP, but they aren't related to *this* answer (e.g. regarding the OP's other question), then it's generally best to do so under their question rather than under a specific answer. Otherwise it effectively mixes two topics in one thread, making it harder for readers to follow either one. Mods can't move comments, so as the OP hasn't (yet) responded to yours, I suggest deleting & reposting your point about their previous question. Thanks for your cooperation. – SamGibson Feb 24 '22 at 15:53