about matching impedances

[mmarwan]

I realy have problems in some issues in the matcing networks and matching impedances..
suppose that i have a transmision line of impedance Z1 on the left hand side and i want to match it to T.L of Z2 on the R.H.S so i place a matching network inbetween to avoid any reflections back on the Z1 line, but does that mean that all incident power on the Z1 line is transmitted to the Z2 line through the matching network..In other words, i know that there is a standing wave in the matching network itself,does this mean that the matching network disipates power..
is there any refrences that is talking about matching networks but from the power transfere point of view.
i will be so gratefull if anyone would help me...

 

[flatulent]

If there are no losses in the intermediate network, no power is lost. If the matching is done by a distributed network, like a quarter wave of line, there are reflections within the center line, but they all cancel out at the point where the first and the middle line meet.

If it is a lumped element network, there are voltages across the L and C components making current flow through them and producing losses from any series or shunt resistances. The inductors usually have most of the loss.

If you want some ballpark values, a test of HF networks conducted by the ARRL showed at least 10% loss for all models.

 

[biff44]

Think of it like a car with a standard transmission. If you are going 5 miles per hour, and you have a 200 HP engine, IF you are in first gear you can get most of those 200 HP to the tires to make the car accelerate, but IF you are in 4th gear at 5 MPH the 200 HP is inefficiently wasted and the engine sputters and jerks.

But in the later case, the transmission is not absorbing the unused horsepower and getting red hot! The engine is simply not able to deliver its designed power to the wheels because of a load mismatch.

Same thing with a RF transmitter. It may be rated at 100 watts output, for instance, but can only deliver that power when the correct load is provided. Your matching network takes the antenna impedance and converts it to something that the transmitter likes to see. Mismatched power, for the most part, is felt at the transmitter, not the matching network.