- #36
vk6kro
Science Advisor
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Yes, but that wasn't the point of it. The point was that the radiated power is not zero when the matched condition exists.
If the antenna feedline looks like 50 ohms, then this maximum power will be delivered to the feedline.
If there were no losses, this power would also be delivered to the antenna and all of it would be radiated.
conway said:The statement at issue was my claim that the re-radiated power is not only non-zero, it is equal to the absorbed power.
berkeman said:I think I have a good counterargument to an RX antenna (well matched) re-radiating anything. If it did, then antenna array steering would be much more complicated (if not impossible), which it isn't.
vk6kro said:You might like to rephrase this though:
I don't actually know what the wasted power is in a 10-kW AM radio transmitter, but there's no technical reason why you'd want to match the amplifier impedance to the load.
This is not like loading a power station until its output voltage dropped to half. RF power amplifiers really do operate like this and impedance matching is very important with them.
conway said:This is logical, but remember you're talking about practical receivers where the antenna current is only a tiny fraction of the theoretical matched current.
I took issue with the notion of a RX antenna "drawing power" from a TX antenna when they are well separated. Naturally, when they are close, their mutual impedance can affect the input impedance of the TX antenna (as in a Yagi type array, with parasitics) but any model which implies the "drawing out" process at great distance should work over any distance - hence, I took it a bit further to where propagation time is a significant factor. As a thought experiment I don't think it was over the top to consider ("take sides with") both RX and TX; after all, they are both involved in the process. Where was the problem with that?.GODISMYSHADOW said:Someone should establish some rules for "thought experiments."
You're talking about astronomical distances. Here, you take sides
with the receiver and switch to the viewpoint of the transmitter.
Then you jump back to the receiver. Seems dangerous to me,
because information itself can't travel faster than light. I don't like
your thought experiment.
berkeman said:I think I have a good counterargument to an RX antenna (well matched) re-radiating anything. If it did, then antenna array steering would be much more complicated (if not impossible), which it isn't.
In antenna arrays, we phase shift the RX signals to steer the direction of the gain lobe. For a 2-element array, you can steer the direction of the gain lobes by phase-shifting the RX signals before combining them. And if you don't phase shift them, you get your lobes right where you expect them, front-to back, with gain determined by the RX signals only, not mitigated by some reflected energy passing between the two elements. We use this every Field Day, and antenna array equations are pretty well understood.
Whew. Glad that's settled!
sophiecentaur said:Actually, that only applies to active receiving arrays. For a number of elements, situated close to each other, if you want to direct the main beam by passive phasing of the outputs, the element currents will affect each other (more of the mutual impedance matrix values become significant). The local (resonant / reactive) fields around the elements (E and H in quadrature at this point) can be quite significant and they talk to each other*. Many active arrays deliberately use 'voltage probe' elements rather than matched elements so that you can control them without needing to account for their effect on each other. Steering multi-element TX arrays is a real problem because you need to shift as much actual power from each element as possible. Try designing a simple steerable two element mf mast array with 1/2 wave spacing. For some beam angles the matching / driving is very hard.
*Things seem to change from I and V in a dipole and associated fields, which are in quadrature and E and H fields in the distant radiated wave, which are in phase.
A Yagi antenna is not steerable - it is designed to have lots of current in its parasitic elements in order to to drive it in one particular direction. It has only one driven element and it is not what I was describing. To steer with two or more driven elements is the problem. Then the mutual impedance between two elements is a nuisance. You can end up chasing your tail if you want to fire in certain directions.berkeman said:Not true. Why are the reflector and director elements in a Yagi not terminated? If they were, your Yagi wouldn't work so well...
sophiecentaur said:A Yagi antenna is not steerable - it is designed to have lots of current in its parasitic elements in order to to drive it in one particular direction. It has only one driven element and it is not what I was describing. To steer with two or more driven elements is the problem. Then the mutual impedance between two elements is a nuisance. You can end up chasing your tail if you want to fire in certain directions.
sophiecentaur said:I took issue with the notion of a RX antenna "drawing power" from a TX antenna when they are well separated. Naturally, when they are close, their mutual impedance can affect the input impedance of the TX antenna (as in a Yagi type array, with parasitics) but any model which implies the "drawing out" process at great distance should work over any distance - hence, I took it a bit further to where propagation time is a significant factor. As a thought experiment I don't think it was over the top to consider ("take sides with") both RX and TX; after all, they are both involved in the process. Where was the problem with that?.
conway said:I'm going to support Sophie on this issue. When the receiver is far away from the transmitter, it does not in any way "load down" the transmitter, and basically for the reasons Sophie said. Of course there is a very small reflected wave but that's not what we mean by loading, even in principle. The energy transfer from A to B does not depend on the detection of the reflected wave; the receiver and transmitter would work exactly the same if the reflected wave could somehow be caught and destroyed before it got back to the transmitter.