08-09-2017, 04:50 PM
(13-07-2017, 04:22 PM)ppppenguin Wrote: A paradox of the impedance matching for power transfer is that half of the total power is dissipated uselessly in the source. The other half usefully in the load.
Maybe I've misunderstood something, because I'm sure transmitters have been made that give better than 50% efficiency from electricty in to power in the aerial.
You haven't missed anything BUT the use of the rule of thumb is misapplied. If you have a power generator that produces 100 mega watts and feeds the power grid it will not consume 100 mega watts at full load. 60 years ago more or less when I was taught Z matching and the Zs = Zl at max power transfer it was to show the wasted power in the source (batteries). It was a bit of common since but seems to have been misused and distorted. At Zs = Zl the voltage across the load is 50% of supply. In other words if I load a 12 volt battery to get max power out I have 50 % of the voltage dropped internally in the battery and only 6 volts at the load. Any further loading will produce less voltage on the output at higher current. Since power = current X voltage we hit the point of diminishing returns at Zs = Zl. In the '60s this would lead into a rule of thumb that Zs should be a minimum of 1/10th of Zl. Even with the 10 to 1 ratio the source would dissipate 9% which is a lost in a power circuit.
Today you will see recommendation for a 200 to 1 ratio in some manufacturer's data.
Look at any tube datasheet for plate resistance and plate load. Are they equal? Look at the grid resistor, is it equal to the plate load of the previous stage?
Well I've rambled enough.
Well one other thing. Look up an 811 tube and look at the plate inut power vs the output. It outputs about 5 times what it dissipates. (the only reason I said 811 is Collins used them in equipment I worked on)
