FreeLists / si-list / [SI-LIST] Re: Question on EMI radiated power

[Ihsan Erdin <erdinih@xxxxxxxxx>]

Andrew,
The radiation resistance is indeed a complex quantity due to the reactive
component of the radiated power, more so in the near field of an antenna. In
the far zone, the reactive component of the power attenuates more rapidly
and the resistance is approximated by its real component only. The imaginary
part simply has the same physical meaning as of any reactive entity in a
circuit: stored energy.

Getting to the original question, yes, we can assume a radiation resistance
from a circuit under the same restrictions that apply to antenna structures.
This is because any current carrying conductor is an antenna (not
necessarily an efficient one...) and the literature is prolific with studies
that use the analogy between traces and antennas to estimate the radiated
emissions from printed circuit structures. When the circuit size is
electrically small a single radiation resistance concept will hold in a
relatively broadband. As the wavelength gets comparable to trace lengths,
the previously raised concerns will be valid and the resistance will get
frequency dependent. Indeed, the earlier objections are not to the concept
itself but its validity over a broadband, which becomes true at high
frequencies.

But the practicality of such a resistance for PCBs should also be
questioned. The radiation resistance in antenna applications is usually used
as an indicator of antenna efficiency. In PCBs, however, we're more
interested in the radiated field intensity for concerns of meeting standards
and regulations. Provided that the current distribution on the tracks is a
known quantity (at least for differential mode currents), there're simple
formulations to estimate the radiated emissions from a PCB as employed by
HyperLynx and (former) Specctraquest. So, I'm not sure how the concept of
radiation resistance could offer more useful information.

Regards.

Ihsan

On 11/4/05, Andrew Ingraham <a.ingraham@xxxxxxxx> wrote:
>
> > 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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