Robert11 wrote:

Hi Roy,

Thanks for reply re my older post on db question. Very clear explanations
by all.
Will put this as a new thread, though.

New at this, and relize I'm not thinking about this the correct way,
probably.

Relative to a receiving antenna's signal: what does the receiver actually
respond to; power or voltage at its input ?

Bob

--------------------------------------------------------------

(snip)

Boy, that almost demands a philosophical answer.

But only almost.

Power. You can (and most really low noise receivers do) transform the
impedance at the antenna into some ideal source impedance for the RF
preamp (or mixer if you're operating in a band where atmospheric noise
exceeds thermal noise by a good amount). Assuming an ideal* impedance
transformation each time you can go from almost any source impedance,
and the noise figure (which is a power ratio) will remain the same.

* Almost never a match. Low noise amplifiers usually do _not_ want a
matched source for best noise figure, although there are ways to force
it to happen.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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