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

In test devices the losses are not usually a problem as long as the levels
are accounted for if absolute numbers are needed. The impedances must match
if any tuning is done. As teh impedance of a receiver is not usaully 50
ohms over a wide frequency range , you use a 6 db pad or so to isolate the
generator from the receiver . If tuned circuits are involved, the impedance
missmatch (swr) can cause many problems . You tune a device for a 50 ohm
source or load and then replace it with a 70 ohm device and the tuning will
usually change . By using the pads, it is an attempt to keep the impedance
changes at a minimum when the test instruments are removed and the device
put back into normal operation.