Home |
Search |
Today's Posts |
#11
![]() |
|||
|
|||
![]()
Roger wrote:
In the last 24 hours, Roy posted a revised analysis that contains results useful here. He presented a voltage example that resulted in a steady state with steady state voltages 4 time initial value. Under superposition, this would equate to 4 times the initial power residing on the transmission line under the conditions presented. This concurs with other authors who predict power on the transmission line may exceed the delivered power due to reflected waves. Your equation "Ptot = Ps + Pr + 2*SQRT(Ps*Pr)cos(A)" is taken from illumination and radiation theory to describe power existing at a point in space near a reflecting surface. If we consider space to be a transmission media, and the reflecting surface to be a discontinuity in the transmission media, then we have a situation very similar to an electrical transmission line near a line discontinuity. It is entirely reasonable to consider that the reflection ratio between the space transmission media and the reflective surface would result in an storage factor equaling 4 times the peak power of the initial forward wave. By storage factor, I simply mean the ratio of forward power to total power on the transmission media under standing wave conditions. Under open circuit conditions, a half wavelength transmission line will have a storage factor of 2. Roy presented an example where the storage factor was 4. Perhaps the space transmission media also has a storage factor of 4 under some conditions, as described by "Ptot = Ps + Pr + 2*SQRT(Ps*Pr)cos(A)". Power is neither stored nor conserved, so a power "storage factor" is meaningless. Consider a very simple example. Let's charge a capacitor with a constant current DC source. We'll apply 1 amp to a 1 farad capacitor for 1 second. During that time, the power begins at zero, since the capacitor voltage is zero, then it rises linearly to one watt as the capacitor voltage rises to one volt at the end of the one second period. So the average power over that period was 1/2 watt, and we put 1/2 joule of energy into the capacitor. (To confirm, the energy in a capacitor is 1/2 * C * V^2 = 1/2 joule.) Was power "put into" or stored in the capacitor? Now we'll connect a 0.1 amp constant current load to the capacitor, in a direction that discharges it. We can use an ideal current source for this. The power measured at the capacitor or source terminals begins at 0.1 watt and drops linearly to zero as the capacitor discharges. The average is 0.05 watt. Why are we getting less power out than we put in? "Where did the power go?" is heard over and over, and let me assure you, anyone taking care with his mathematics and logic is going to spend a long time looking for it. So in this capacitor problem, where did the power go? It takes 10 seconds to discharge the capacitor, during which the load receives the 1/2 joule of energy stored in the capacitor. Energy was stored. Energy was conserved. Power was neither stored nor conserved. Roy Lewallen, W7EL |
Thread Tools | Search this Thread |
Display Modes | |
|
|
![]() |
||||
Thread | Forum | |||
Standing Wave Phase | Antenna | |||
Standing wave on feeders | Antenna | |||
Dipole with standing wave - what happens to reflected wave? | Antenna | |||
Newbie ?: I've Built A Simple 1/4 Wave Dipole for 2 Mtrs. Could IMake a1/2 Wave? | Homebrew | |||
What is a traveling-wave antenna? | Antenna |