[SI-LIST] Re: Question on EMI radiated power
- From: "Charles Hill" <chuck@xxxxxxxxxxx>
- To: <a.ingraham@xxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Sat, 5 Nov 2005 19:41:26 -0700
Andrew,
Well, your view of the radiation resistance is supported by John D. Kraus
see "Antennas" page 136 copyright McGraw-Hill 1950 where it says the average
of the Poynting vector (power transfer) is .. the average power ...
streaming out of a sphere surrounding the dipole." Your view of the
radiation resistance being a function of frequency is supported by Robert S.
Elliott "Antenna Theory and Design" ISBN 0-13-03836-2 copyright 1981
Prentice-Hall page 302 (where is shows a plot)of the resistance and
reactance of a center fed dipole versus length parameterized by diameter.
There is no narrowband approximation in the analysis, so the radiation
resistance concept holds over frequency. This is also consistent with my
experience in building and using antennas.
My interpretation of a reactive component is energy storage in the near
field surrounding the antenna, and of course there will be no power transfer
due to this. This is completely consistent with energy storage in
capacitors and inductors. Therefore, there is no such thing as a complex
radiation impedance.
Regards,
Chuck Hill
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Andrew Ingraham
Sent: Friday, November 04, 2005 6:32 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Question on EMI radiated power
> In short, the answer is no.
Maybe it's just me ... but I would have worded it differently: that the
answer to Doug's question is essentially yes, but that (like so many things
in RF) it is a frequency-dependent resistance. At any one frequency, you
could come up with a resistance in the circuit that represents the transfer
of energy from the circuit into EMI rather than heat. I don't know how
useful it would be to determine this radiation resistance, even at a single
frequency, unless the EMI is large. So, conceptually yes, but in practice
maybe not that useful.
But ... Is there such a thing as a complex radiation impedance? If it has
an imaginary component, what does it represent? Radiation resistance is a
circuit stand-in for the energy (power) that gets radiated from the
radiating element. An imaginary component doesn't pass any net (average)
power. While it makes sense to have imaginary components of the circuit's
impedance due to the structure of the physical device, standing waves, etc.,
I don't see this as a component of the radiation impedance, which ought to
be pure real, shouldn't it? Radiation implies energy radiated outwards,
which means power loss from the circuit.
Regards,
Andy
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