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| From | Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> |
|---|---|
| Newsgroups | comp.dsp |
| Subject | Re: Wavelength Dependency in RF Propagation? |
| Date | 2022-06-03 12:30 -0400 |
| Organization | A noiseless patient Spider |
| Message-ID | <bbf50c68-b38c-56af-7850-07749a080c61@electrooptical.net> (permalink) |
| References | <87lgc7j7oo.fsf@digitalsignallabs.com> <peb4t9$i7o$1@gwaiyur.mb-net.net> <peet3s$2oi$1@dont-email.me> <d25f6441-3f48-7b20-49ef-17535ad49c28@electrooptical.net> |
(This weirdly came up as a new message--silly me.) Phil Hobbs wrote: > Les Cargill wrote: >> Marcel Mueller wrote: >>> On 26.05.18 07.40, Randy Yates wrote: >>>> I was miffed initially by this statement since, as far as I know, >>>> there is nothing inherent in wavelength that impacts how RF waves >>>> travel through space. >>> >>> If you are talking about vacuum then yes. In all other media the >>> velocity of propagation depends on the frequency. E.g. water >>> molecules in the air interact frequency dependent. >>> >>>> But I guess this was just a way (a confusing one, IMO) of referring >>>> to the wavelength dependency of antenna aperture, as explained >>>> nicely in this article on the Friis equation? >>> >>> The coupling of the antenna to the free space also introduces a >>> frequency dependent group delay. >> >> All necessary apologies in advance. >> >> All group delay is inherently frequency dependent: >> >> " Group delay is the actual transit time of a signal through a device >> under test as a function of frequency." >> >> http://na.support.keysight.com/pna/help/latest/Tutorials/Group_Delay6_5.htm >> >> >> A reasonable definition. > > But unfortunately dead wrong because it ignores causality. > Group delay != true delay, in general. > > Group delay is d phi / d omega, and is useful as a leading-order > approximation to how a nice wide smooth pulse propagates through a > network. It's exactly analogous with group velocity in radio or optical > propagation, which is d(omega)/d k, where k is the wave vector. > > You can see the distinction in two ways. First, group delay can be > negative, which true delay cannot. > > Second, networks can have group delay without having true delay. You > can undo the effect of a 1-pole RC lowpass with an RC highpass, for > instance. > >> >> I have the conceit that I'm not picking nits here so much as heading >> off one potentially confusing interpretation of that >> sentence :) The "quantifiers" for "a group delay" sort of leaves >> the phrase "for all group delay" dangling. >> >> >>> And last but not least a short >>> distance link has some frequencies with poor performance due to >>> eigenvalues of the overall geometry. >>> >> >> Aka comb filtering/multipath/cosite interference? >> >>> >>> Marcel >> > Cheers > > Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
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Re: Wavelength Dependency in RF Propagation? Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2022-06-03 12:26 -0400 Re: Wavelength Dependency in RF Propagation? Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2022-06-03 12:30 -0400
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