Dave Shrader wrote: 
"Help me understand what you are trying to say." 
 
I`ll elaborate. 
 
Efficiency is output / input. 
 
1/2 or more of the power received by a receiving antenna is re-radiated. 
 
Nearly all of the power received by a transmitting antenna is 
transmitted. 
 
Considering the energy available to the antenna, the job done by the 
transmitting antenna system as compared with the job done by the 
receiving antenna system, the transmitting system is better. 
 
A receiving antenna must be resonant to enable full acceptance of 
available energy, and it must be matched to avoid re-radiation of more 
than 50% of the energy it is able to grab. 
If off-resonance, the receiving antenna has too-high impedance for 
significant induced current. Of course, we have such good receivers we 
can do without good efficiency. 
 
A transmitting antenna will radiate energy proportional to the current 
in the antenna. 
 
Ronold W.P. King says in "Transmission Lines, Antennas, and Wave 
Guides": 
 
"---the power (Io squared)(Ro) supplied to a highly conducting antenna 
(of Copper), with Ro taken from the curves of Sec. 10, is for practical 
purposes all radiated to the more or less closely coupled universe 
outside the antenna, while that used in heating the antenna itself is 
negligible." This information is on page 113. 
 
Inefficiency is to be found elsewhere from the transmitting antenna 
itself. We may use inefficient transmission lines and our wave 
generator, the transmitter, may be inefficient. We usually try to keep 
their losses low. 
 
It is not uncommon to produce RF in a Class C amplifier with an 
efficiency of 70%. With reasonable lines and antennas, nearly 100% of 
this power output can be radiated, producing appropriate millivolts per 
meter at one mile from the antenna. 
 
This is not completely reversible due to re-radiation of 1/2 or more of 
all the power a receiving antenna can grab. 
 
The hope for point to point wireless power transmission is in using 
antennas like large dishes, for example, which concentrate power within 
such a small angle that the receiving antenna captures all the 
transmitted beam. Similarly, all re-radiated power is beamed back to the 
transmitting antenna for another trip to the receiver. 
 
Best regards, Richard Harrison, KB5WZI 
 
		 
		
		
		
		
		
		
		
		
	
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