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Dipole with standing wave - what happens to reflected wave?
RF transmitter output has impedance of 50 ohms and is connected to dipole
with a feedpoint impedance of 50 ohms via feeder with characteristic impedance of 50 ohms. System is perfectly matched. I expect SWR meter to show perfect match of 1:1. Dipole has a standing wave on it. Ends of dipole are at high voltage. Dipole is centre-fed with centre being high current point. Standing wave means that a reflected wave exists. Wave is reflected from open ends of dipole. What happens to the reflected wave? How does it vanish at centre of dipole? Why does reflected wave not go along feeder into transmitter output? There cannot be a reflected wave on feeder because SWR is 1:1. |
#2
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Dipole with standing wave - what happens to reflected wave?
David wrote:
RF transmitter output has impedance of 50 ohms and is connected to dipole with a feedpoint impedance of 50 ohms via feeder with characteristic impedance of 50 ohms. System is perfectly matched. I expect SWR meter to show perfect match of 1:1. Dipole has a standing wave on it. Ends of dipole are at high voltage. Dipole is centre-fed with centre being high current point. Standing wave means that a reflected wave exists. Wave is reflected from open ends of dipole. What happens to the reflected wave? How does it vanish at centre of dipole? Why does reflected wave not go along feeder into transmitter output? There cannot be a reflected wave on feeder because SWR is 1:1. If the reflection is exactly in phase with the next wave arriving through the feed line, then it just raises the impedance the line sees. In other words, the reflection acts as a large part of the feed energy for the next cycle. It doesn't bounce into and out of the feed line, it bounces back and forth from end to end of the dipole. Actually there are two reflected waves going in opposite directions, end to end, simultaneously. |
#3
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Dipole with standing wave - what happens to reflected wave?
On Mon, 4 Sep 2006 20:44:17 +0100, "David" nospam@nospam wrote:
RF transmitter output has impedance of 50 ohms and is connected to dipole with a feedpoint impedance of 50 ohms via feeder with characteristic impedance of 50 ohms. System is perfectly matched. I expect SWR meter to show perfect match of 1:1. Accepting the figures as an example and not necessarily a reality... You have just described a point which is a junction between: - a load where the ratio of voltage to current is 50+j0; - a feedline whe * the ratio of the voltage to current due to the forward travelling wave must each be in the ratio 50+j0; * the ratio of the voltage to current due to the reflected travelling waves must each be in the ratio 50+j0. Having regard for the sign of the traveling waves, the only solution to those constraints / conditions is that the reflected travelling wave must have zero amplitude. Dipole has a standing wave on it. Ends of dipole are at high voltage. Dipole is centre-fed with centre being high current point. Standing wave means that a reflected wave exists. Wave is reflected from open ends of dipole. What happens to the reflected wave? How does it vanish at centre of dipole? Why does reflected wave not go along feeder into transmitter output? There cannot be a reflected wave on feeder because SWR is 1:1. Lets be clear that we are now talking about a single frequency. At any point, the forward and reflected waves resolve to a single voltage at that point, and a single current flowing at that point, and the ratio of voltage to current is the impedance (and these are all complex quantities, ie they have real and imaginery parts). If the point you consider is the feedpoint, and the ratio of voltage to current is 50+j0, then that is the impedance, it fully describes the load at that frequency. You talk of the "reflected wave" as if it has inertia, that it must keep travelling when it reaches the junction of the feedline and the antenna? You are not alone in speaking that way, but thinking that way will get in the way of understanding what is happening. Next thing, you will be thinking that the reflected wave must travel back to the PA anode and will be absorbed there causing overheating. Owen -- |
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Dipole with standing wave - what happens to reflected wave?
"Owen Duffy" wrote:
You talk of the "reflected wave" as if it has inertia, that it must keep travelling when it reaches the junction of the feedline and the antenna? You are not alone in speaking that way, but thinking that way will get in the way of understanding what is happening. Next thing, you will be thinking that the reflected wave must travel back to the PA anode and will be absorbed there causing overheating. _______ If you write of reflected power existing on the transmission line between the tx output connector and the antenna input connector, then yes -- with sufficient tx output power and a poor enough match at the antenna feedpoint, that reflected power can cause transmission line and/or tx PA component failure. I've analyzed and repaired many such failures of these installed systems in my pre-retirement career, and know this first hand. Conventional theory also supports this result. RF Visit http://rfry.org for FM transmission system papers. |
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Dipole with standing wave - what happens to reflected wave?
On Mon, 4 Sep 2006 17:03:39 -0500, "Richard Fry"
wrote: "Owen Duffy" wrote: You talk of the "reflected wave" as if it has inertia, that it must keep travelling when it reaches the junction of the feedline and the antenna? You are not alone in speaking that way, but thinking that way will get in the way of understanding what is happening. Next thing, you will be thinking that the reflected wave must travel back to the PA anode and will be absorbed there causing overheating. _______ If you write of reflected power existing on the transmission line between the tx output connector and the antenna input connector, then yes -- with sufficient tx output power and a poor enough match at the antenna feedpoint, that reflected power can cause transmission line and/or tx PA component failure. The mechanism is not "that the reflected wave must travel back to the PA anode and will be absorbed there causing overheating". Take an example of the 50 ohms load discussed, and an electrical half wave of 70 ohm line connected to a transmitter designed for a 50 ohm load. The transmitter behaves exactly as if that line were 50 ohms. Though there is a reflected travelling wave on the line, it does not travel back to the PA anode where it is absorbed and converted to heat. At the tx end of the line, the forward and reflected wave components resolve to a 50+j0 load, and the transmitter sees the same 50 ohm load as it would were 50 ohm line used. Increasing power or increasing line Zo for a higher VSWR will not change the outcome of this example. Owen -- |
#6
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Dipole with standing wave - what happens to reflected wave?
Owen Duffy wrote:
Take an example of the 50 ohms load discussed, and an electrical half wave of 70 ohm line connected to a transmitter designed for a 50 ohm load. The transmitter behaves exactly as if that line were 50 ohms. Though there is a reflected travelling wave on the line, it does not travel back to the PA anode where it is absorbed and converted to heat. Of course not. Destructive interference redirects the energy back toward the load at the impedance discontinuity. The same thing happens with a 1/4WL thin-film on non-reflective glass. -- 73, Cecil http://www.w5dxp.com |
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Dipole with standing wave - what happens to reflected wave?
"Owen Duffy" wrote
The mechanism is not "that the reflected wave must travel back to the PA anode and will be absorbed there causing overheating". Take an example of the 50 ohms load discussed, and an electrical half wave of 70 ohm line connected to a transmitter designed for a 50 ohm load. The transmitter behaves exactly as if that line were 50 ohms. Though there is a reflected travelling wave on the line, it does not travel back to the PA anode where it is absorbed and converted to heat. ____________ Really, the mechanism is there -- only the unique circumstance you describe protects the PA from seeing reflected power in such cases. Other line lengths in this scenario could stress PA components beyond their ratings. And even if the PA saw no reflected power because of a fortunate length of transmission line connecting it to a mismatched antenna/load, that reflected power still exists in the transmission line, and may cause its failure. Manufacturers of the rigid coaxial line used in broadcast stations (e.g., Dielectric) require derating its maximum power rating inversely by the value of the SWR existing in it . A power derating factor related to SWR also applies to Andrew Heliax® and RG-type coax line. Deliberately setting up, or tolerating reflected power on a transmission line is not done without risk. RF |
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Dipole with standing wave - what happens to reflected wave?
Richard Fry wrote:
Really, the mechanism is there -- only the unique circumstance you describe protects the PA from seeing reflected power in such cases. Other line lengths in this scenario could stress PA components beyond their ratings. Yes, the exposure of the PA to reflected power depends upon the phase of the reflected wave referenced to the phase of the source wave. SWR doesn't tell the whole story because phase is not reported by the SWR measurement. What is happening with 1/2WL of Z0=600 ohm feedline connected to a 50 ohm load on one end and a 50 ohm source on the other end is interference. Destructive interference toward the source causes constructive interference toward the load and the reflected energy at the source is re-directed back toward the load. What we want to avoid in our antenna systems is constructive interference toward the source. -- 73, Cecil http://www.w5dxp.com |
#9
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Dipole with standing wave - what happens to reflected wave?
On Tue, 5 Sep 2006 06:48:57 -0500, "Richard Fry"
wrote: "Owen Duffy" wrote The mechanism is not "that the reflected wave must travel back to the PA anode and will be absorbed there causing overheating". Take an example of the 50 ohms load discussed, and an electrical half wave of 70 ohm line connected to a transmitter designed for a 50 ohm load. The transmitter behaves exactly as if that line were 50 ohms. Though there is a reflected travelling wave on the line, it does not travel back to the PA anode where it is absorbed and converted to heat. ____________ Really, the mechanism is there -- only the unique circumstance you describe protects the PA from seeing reflected power in such cases. Other line lengths in this scenario could stress PA components beyond their ratings. Richard, There is not such a mechanism in the general case, the example I gave shows that the reflected wave does not necessarily travel back to the source where it is absorbed. If you re-read my words "Next thing, you will be thinking that the reflected wave must travel back to the PA anode and will be absorbed there causing overheating." Remembering the context was a wave on the dipole reflected from the o/c end, and the word "must" was used to mean "necessarily". Sure,transmission lines with VSWR may transform impedance, have higher losses (if they are long enough), operate at higher voltages in places (if they are long enough), operate with higher currents in places (if they are long enough). Nothing I have said is in conflict with that or suggests otherwise. Transmitters operated at other than their rated load impedance may operate at higher voltages, higher currents, different power etc and may damage components. Nothing I have said is in conflict with that or suggests otherwise. But, the mechanism is not that the reflected wave *necessarily* travels all the way back to the PA anode by virtue of some kind of momentum (as sometimes expressed by some amateurs). In the case raised by the OP, the reflected wave on the dipole and the forward wave resolve (as in resolution of phasors) to an impedance of 50+j0 (OP's hypothetical example), and the constraints / conditions at the feedline / feedpoint junction are fully satisfied with no reflected wave on the feedline. (I used the term resolve, Cecil must call it destructive interference.) The reflected wave on the dipole does not have a momentum that *must* carry it to the PA anode to be absorbed there. Owen -- |
#10
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Dipole with standing wave - what happens to reflected wave?
Owen Duffy wrote:
Having regard for the sign of the traveling waves, the only solution to those constraints / conditions is that the reflected travelling wave must have zero amplitude. Sorry Owen, that's not true. Assuming a 1/2WL dipole, the reflected voltage has traveled 180 degrees. The reflected current has traveled the same 180 degrees plus experienced a 180 degree phase shift at the tip of the dipole. Assuming 100 watts is being applied to the antenna, the following conditions satisfy the observed conditions on the dipole at the feedpoint. Pfor = 500W, Vfor = 548V, Ifor = 0.91A Pref = 400W, Vref = 490V, Iref = 0.81A Pnet = 100W, Vnet = 58V, Inet = 1.72A You talk of the "reflected wave" as if it has inertia, that it must keep travelling when it reaches the junction of the feedline and the antenna? You are not alone in speaking that way, but thinking that way will get in the way of understanding what is happening. It is well known that all EM waves contain energy and momentum (inertia) so David is correct. The thing that reverses the momentum of the reflected wave is destructive interference at the Z0-match point. Anyone interested in understanding how/why it happens is invited to peruse my energy analysis article at: http://www.w5dxp.com/energy.htm -- 73, Cecil http://www.w5dxp.com |
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