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December 21st 04, 10:16 PM

W9DMK

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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,

I have fundamental objections to responding to respondants, so I will
continue to address this thread directly. It has been suggested that a
virtue of the T pad would give a clean 6 dB loss instead of some
"not-so-nice" loss, like 5.7 dB. Well, that's not a problem because
the L-Pad can also be designed for exactly 6 dB. In fact, it can be
designed to provide ANY loss you want, so long as it's AT LEAST 5.7
dB. You see, the 5.7 dB L-Pad happens to be the MINIMUM loss design
for that particular mismatch.

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

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