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December 21st 04, 10:45 PM

On 21 Dec 2004 14:23:10 GMT, (PDRUNEN) wrote:

Hi All,

I was reviewing a 75 to 50 ohm resistive matching network using two resistors,
the insertion lost was 5.7 db.

If we have a 100Vrms source with 50 ohm source impedance and it is driving a
matched 50 ohm load then the load takes 1A and the power in the load is 50
watts.

If the load is replaced with 75 ohm, then 0.8 amps will flow and the power is
48 watts. (I*I*R) == (0.8)*(0.8)*75.

I guess I must be not be taking something in account, but 2 watts does not
equal 5.7 db.

I know there must be a good reason to put the matching pad in line for the
sprectrum analyizer but I don't under why.

Thanks,

de KJ4UO

Dear KJ4UO,

First, I must apologize for not noticing that your posting DOES
mention that the application is a Spectrum Analyzer. That being the
case, I can be much more enlightening about WHY we want a flat line
(no standing waves).

A spectrum analyzer is of use primarily in showing WHAT frequencies
are present in the specturm AND what their relative levels are. Unless
the line is flat, the relative levels will be distorted or inaccurate.
This comes about due to the fact that a mismatched line becomes an
impedance transformer that is very frequency dependant. That is, some
frequencies will be artificially enhanced and others artificially
subdued due to the varying impedance impedance seen by the source.
Conversely, when the source sees a perfect match across its acceptance
spectrum, only the built-in non-linearities will degrade the accuracy.

In short, the line must be properly matched in order that the spectrum
analyzer give the accuracy that it is capable of.

BTW, it should be understood that the matching L-Pad, in order to be
effective, must be physically located at the Spectrum Analyzer end of
the line - not at the source end of the line.

Interestingly enough, hams do not usually take this into account when
tuning an antenna system, because they are only interested in a flat
line for that short little piece of coax between the tuner and the
transmitter. They don't normally do anything about the horrible SWR
that exists on the feedline - Hi!

Bob, W9DMK, Dahlgren, VA
http://www.qsl.net/w9dmk

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