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
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