Home |
Search |
Today's Posts |
#10
![]() |
|||
|
|||
![]()
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 |
Thread Tools | Search this Thread |
Display Modes | |
|
|
![]() |
||||
Thread | Forum | |||
Mobile Ant L match ? | Antenna | |||
A Subtle Detail of Reflection Coefficients (but important to know) | Antenna | |||
Reflection Coefficient Smoke Clears a Bit | Antenna | |||
Length of Coax Affecting Incident Power to Meter? | Antenna | |||
50 Ohms "Real Resistive" impedance a Misnomer? | Antenna |