[ARC5] The maximum power transfer theorem. [Was: Receiver input impedance...)

[Leslie Smith]

  Hello Richard,
  After reading your last posting, below, I'm trying to figure whether
  you're a nice bloke (informative) or a nasty bloke (giving
  mis-information)!
  So I'm going to spend what spare time I have today thinking about the
  impedance matching theorem, and how it relates to radio signals (that
  transfer voltage not power).
  It's a long time since I read about this, and anyway, I'm not sure I
  every really understood the deeper meaning of the theorem.

  I'm quite certain power stations don't dissipate 1/2 the power
  supplied to the national grid!
  If my memory is correct, at maximum power transfer, 1/2 the power is
  dissipated in the generator!
  I think your posting depends on similar thinking - the receiver is
  interested in "seeing" voltage, not power.
  But, if that's true, don't we want a good impedance match to get
  voltage transfer?   (Hmmm .... tick, tick, tick)
  (I should have listened more carefully in Physics 101).

  73 de Les Smith
  vk2bcu at operamail.com

  PS.  I'm writing 100 times:  I must listen carefully in my physics
  101.
  I must listen carefully in physics 101 ...
  I must not look out the window in physics 101.
  I must not be distracted by beautiful women during physics lectures
  ....
  (Physics lectures were held just before lunch, and the building was
  near the cafeteria.  All  the "girls" walked past the window.  Bad
  planning, I think.)
  
  (This is getting difficult ....)


On Tue, Oct 21, 2014, at 05:37, Richard Knoppow wrote:
> 
> ----- Original Message ----- 
> From: "Fuqua, Bill L" <wlfuqu00 at uky.edu>
> To: <kgordon2006 at frontier.com>; "ARC5" 
> <arc5 at mailman.qth.net>
> Sent: Monday, October 20, 2014 10:10 AM
> Subject: Re: [ARC5] Receiver input impedance...
> 
> 
> >  Generally, impedance matching is not very useful at below 
> > 30 MHz, especially below 10 MHz due to the atmospheric 
> > noise. Any signal you can receive must be above the 
> > atmospheric noise.
> > That is why ferrite antennas are just fine for AM 
> > broadcast receivers, but useless for transmission of 
> > signals. Reciprocity still applies it is just that you 
> > don't need much sensitivity at low frequencies.
> > 73
> > Bill wa4lav
> 
>    Well again the important thing at the receiver is to 
> maximize the ratio of signal _voltage_ to noise voltage. 
> Power transfer is of little importance so impedance matching 
> is not very critical. Generally, if the receiver impedance 
> is higher than the source impedance of the antenna and 
> feeder it will be good for SNR. However its not a simple 
> relationship because part of the receiver noise is the 
> equivalent noise resistance. This is mostly generated in the 
> input device whether its a tube or solid state. This becomes 
> very important at frequencies much beyond about 30 mhz and 
> maybe below that too.
> 
> 
> --
> Richard Knoppow
> Los Angeles
> WB6KBL
> dickburk at ix.netcom.com 
> 
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