Re: How come its OK to mix impedances in a radio system??


<jmeyer@xxxxxxxxxxx> wrote in message
news:eofjd19b1a0p7i6ljm81o0oakl03q78ipr@xxxxxxxxxx
> On Sat, 16 Jul 2005 18:46:15 -0700, Jim Thompson <thegreatone@xxxxxxxxxxx>
> wroth:
>
>
>>>
>>>* In other words, if the stages are not all matched to the same
>>>reference impedance, then the output power would be less than what it
>>>should be.
>>
>>Welcome to reality :-) Impedance matching is only necessary when you
>>are working with fixed impedance modules.
>>
>> ...Jim Thompson
>
>
> Another way to look at the situation...
>
> Consider the AC power grid. If the generator at the power plant were
> matched to the impedance of the grid, then they would have to generate 200
> watts
> for every 100 watts delivered to the grid. Half of their power would go
> into
> heating the generator. That's not realistic. And it illustrates the
> fallicy of
> the idea that maximum power transfer in a system always depends on matched
> impedances.
>
> There are other reasons why impedance matching is a good thing,
> sometimes, but not everywhere.
>
> Jim "The other one" Meyer
>
>
Interestingly, the one place where one might think you want to match
impedances for sure doesn't work out either. In connecting an antenna to the
input of a low noise amplifier, maximum gain occurs for the matched case.
BUT, often, if not usually, best signal to noise occurs when there is a
slight mismatch.

Tam


.

Relevant Pages

  • Re: rotating magnetic field
    ... | I pointed out your misunderstanding of the maximum power transfer theorem ... a system with a higher impedance than one delivering X watts with voltage ... It's not a miniscule part if your dealing with a Faraday-like generator ...
    (sci.physics.electromag)
  • Re: rotating magnetic field
    ... |> then the source system impedance is 20 ohms. ... | And that would mean that the load is 0 so the power delivered to the load ... If you know the actual series impedance, ...
    (sci.physics.electromag)
  • Re: Efficiency and maximum power transfer
    ... Let us regard a transmitter as an ideal RF generator with a resistance ... It is well known that, for maximum power transfer, the load ... To achieve a higher efficiency, the load resistance should be made ... If the equivalent source impedance is not important, ...
    (rec.radio.amateur.antenna)
  • Re: rotating magnetic field
    ... |> | This is a misconception of the maximum power transfer theorem. ... |> | matches the source impedance. ... please note that the resultant expression for power is inversly proportional to resistance. ... impedance (of the generator, not of the the whole grid). ...
    (sci.physics.electromag)
  • Re: 47uf decoupling caps?!
    ... Further when they couple with the power plane capacitance, ... do create a parallel resonance which *increases* the impedance at ... a low impedance path at high frequencies. ...
    (sci.electronics.design)