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Friis Transmission Equation Frequency Dependence Question

 
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scubasteve
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Joined: 19 Feb 2017
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PostPosted: Mon Feb 20, 2017 2:13 am    Post subject: Friis Transmission Equation Frequency Dependence Question Reply with quote

On this page you say that an important result of the Friis Transmission Equation is the frequency dependence of path loss, that more path loss is seen at higher frequencies.

I disagree with this I believe the frequency dependence has nothing to do with path loss but only the dimensions of the physical antennas themselves. Rather than attempting to pose my argument in this text box I will link here to a great article that I think expertly sums up the issue.

This is something I have been confused about for a while and I feel that the above linked article is correct over your antenna-theory page.

Can you please provide direction as to why I/the article might be wrong in this case? I would be very eager to hear!
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admin
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PostPosted: Mon Feb 20, 2017 7:21 pm    Post subject: Reply with quote

Yes that referenced article is correct.

A more complete statement for pathloss is that "for fixed gain antennas, the pathloss increases with frequency". But you could also say "for a fixed aperture antennas, the pathloss is constant with frequency".

But whatever you are doing, you should expect more pathloss at higher frequencies. 60 GHz is used for point to point communication over small distances, and much lower frequencies are used for long wave propagation.
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scubasteve
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PostPosted: Wed Mar 01, 2017 5:30 pm    Post subject: Reply with quote

Quote:
But whatever you are doing, you should expect more pathloss at higher frequencies.


Can you explain why this is in the context of the rest of your comment? It has nothing to do with free space loss, and assuming the antenna isn't changing size why would this happen?


And also would you agree that the page I posted is somewhat misleading? Though it doesn't explicitly state so it certainly implies that frequency increases loss as a necessity, and that it's from free space losses. It mentions nothing about it being dependent also on fixed gain vs fixed aperture.

Would you consider revising
that page so it includes the same information in the previously linked article and in your forum reply in this thread?
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coundpake
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Joined: 03 Aug 2016
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Location: San Jose, CA

PostPosted: Mon Mar 13, 2017 8:58 pm    Post subject: Reply with quote

You're right, it has nothing to do with free-space loss. If you were designing a communication systems between satellites in space you would just consider different frequencies to propagate equally. However, when in presence of a medium/obstacles (atmosphere, buildings, clothing, trees, etc.) you have to consider how propagation (refraction, reflection, diffraction, absorption, polarization, scattering) changes with frequency. In the RF/MW spectrum these properties usually increase attenuation at higher frequencies.

Quote:
But you could also say "for a fixed aperture antennas, the pathloss is constant with frequency".

For 2 fixed aperture antennas, wouldnt the pathloss decrease with increasing frequency (gain, power received increases ~ f^2)? However, if you had one G=1 and one fixed Aperature, then pathloss would be constant with frequency.
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