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Standing Waves (and Impedance)
There are several possible reasons for being interested in standing
waves (on transmission lines). Some are valid, some are not, and you'll even get plenty of, um, discussion about what's valid and what isn't. If your goal is to get maximum power delivered to a load, then it's good to minimize standing waves on the line delivering power to that load, because a line delivering a particular amount of power to a load will have greater power lost in the line with greater standing waves. If the line is being used near its maximum power or voltage rating, standing waves are a concern because for a given power delivered to the load, the rms current at current nodes and the peak voltage at voltage nodes both increase with increased standing waves. And a high standing wave ratio on a line which is long compared with a wavelength suggests that the input impedance to the line will vary rapidly with frequency, whereas a line with low standing wave ratio will present a relatively constant impedance to the driving source, assuming the load is reasonably "flat" with frequency. As an example of this last point, a 30 meter (~100 foot) 50 ohm line with 0.8 velocity factor and very low loss, delivering power to a 50 ohm load at 450MHz, will present a 50 ohm load to the driving source. But delivering power to a 200 ohm load, the source will "see" almost 200 ohms at frequencies where the line is an integer number of electrical half-waves long, and it will "see" just over 12.5 ohms midway between those frequencies. You get 200 ohms at 440MHz, 12.5 ohms at 442MHz--and reactive in between. It's possible to use stubs and series line sections to effect an impedance match between a load and a line. For example, the right length and impedance series section will give you a match at one particular frequency, at least, and multiple sections can give you a "perfect" match at multiple frequencies, with (perhaps) quite acceptable match over a range of frequencies. There are lists of analogs among electrical, mechanical, acoustic, and other media. "electrical hydraulic impedance analog" in a Google search will give you many hits. Cheers, Tom |
#2
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Standing Waves (and Impedance)
K7ITM wrote:
There are several possible reasons for being interested in standing waves (on transmission lines). Some are valid, some are not, and you'll even get plenty of, um, discussion about what's valid and what isn't. If your goal is to get maximum power delivered to a load, then it's good to minimize standing waves on the line delivering power to that load, because a line delivering a particular amount of power to a load will have greater power lost in the line with greater standing waves. If the line is being used near its maximum power or voltage rating, standing waves are a concern because for a given power delivered to the load, the rms current at current nodes and the peak voltage at voltage nodes both increase with increased standing waves. And a high standing wave ratio on a line which is long compared with a wavelength suggests that the input impedance to the line will vary rapidly with frequency, whereas a line with low standing wave ratio will present a relatively constant impedance to the driving source, assuming the load is reasonably "flat" with frequency. As an example of this last point, a 30 meter (~100 foot) 50 ohm line with 0.8 velocity factor and very low loss, delivering power to a 50 ohm load at 450MHz, will present a 50 ohm load to the driving source. But delivering power to a 200 ohm load, the source will "see" almost 200 ohms at frequencies where the line is an integer number of electrical half-waves long, and it will "see" just over 12.5 ohms midway between those frequencies. You get 200 ohms at 440MHz, 12.5 ohms at 442MHz--and reactive in between. It's possible to use stubs and series line sections to effect an impedance match between a load and a line. For example, the right length and impedance series section will give you a match at one particular frequency, at least, and multiple sections can give you a "perfect" match at multiple frequencies, with (perhaps) quite acceptable match over a range of frequencies. There are lists of analogs among electrical, mechanical, acoustic, and other media. "electrical hydraulic impedance analog" in a Google search will give you many hits. Cheers, Tom Thanks for your reply. I have a few questions. When you say "standing waves", I take it that one can have more than one on the line? I follow your example, but I may come back to it once I've done the calcs. How does one know they want to improve their impedance match? Why doesn't there seem to be a need for this (probably through a balun) on a standard AM radio with a 1/2 wave line antenna or even some ferrite coil? Is there some auto-balun that works this all out? Wayne T. Watson (Watson Adventures, Prop., Nevada City, CA) (121.015 Deg. W, 39.262 Deg. N) GMT-8 hr std. time) Obz Site: 39° 15' 7" N, 121° 2' 32" W, 2700 feet "He who laughs, lasts." -- Mary Pettibone Poole -- Web Page: home.earthlink.net/~mtnviews |
#3
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Standing Waves (and Impedance)
W. Watson wrote:
Thanks for your reply. I have a few questions. When you say "standing waves", I take it that one can have more than one on the line? Standing waves are created by two coherent traveling waves moving in opposite directions in a transmission line. In a conventional system of source, transmission line, and load, one of the traveling waves moves from the source toward the load and is called the forward wave. The other traveling wave moves from the load toward the source as a reverse or reflected wave. The reflected wave is usually the result of a load being mismatched to a transmission line. If no mismatch exists, no standing waves are created and the system is considered to be "flat", i.e. one forward traveling wave. How does one know they want to improve their impedance match? For a transmitted signal, we establish a Z0-match to our transmitter often at the input of an antenna tuner. When reflected energy is eliminated on the coax between the tuner and transmitter, we know we have a Z0-match by the SWR meter reading of 1:1. We also use our antenna tuners to tune for maximum received signal on our S-meters. At the Z0-match point, maximum available energy is transferred. If you know the input impedance to a receiver, you can match your antenna system to it to achieve maximum available energy transfer from the antenna. -- 73, Cecil http://www.qsl.net/w5dxp |
#4
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Standing Waves (and Impedance)
Cecil Moore wrote:
W. Watson wrote: Thanks for your reply. I have a few questions. When you say "standing waves", I take it that one can have more than one on the line? Standing waves are created by two coherent traveling waves moving in opposite directions in a transmission line. In a conventional .... How does one know they want to improve their impedance match? .... antenna tuners to tune for maximum received signal on our S-meters. At the Z0-match point, maximum available energy is transferred. If you know the input impedance to a receiver, you can match your antenna system to it to achieve maximum available energy transfer from the antenna. Thanks. A standing wave is the sum of an incident added to the reflective wave. Isn't it possible to send two incident waves down an xline with different frequences, and produce two different standing waves by having some multiplicative relationship between the two incident waves and the xline length? Not a bad explanation from Wikipedia: SWR has a number of implications that are directly applicable to radio use. 1. SWR is an indicator of reflected waves bouncing back and forth within the transmission line, and as such, an increase in SWR corresponds to an increase in power in the line beyond the actual transmitted power. This increased power will increase RF losses, as increased voltage increases dielectric losses, and increased current increases resistive losses. 2. Matched impedances give ideal power transfer; mismatched impedances give high SWR and reduced power transfer. 3. Higher power in the transmission line also leaks back into the radio, which causes it to heat up. 4. The higher voltages associated with a sufficiently high SWR could damage the transmitter. Solid state radios which have a lower tolerance for high voltages may automatically reduce output power to prevent damage. Tube radios may arc. The high voltages may also cause transmission line dielectric to break down and/or burn. Abnormally high voltages in the antenna system increase the chance of accidental radiation burn if someone touches the antenna during transmission. |
#5
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Standing Waves (and Impedance)
W. Watson wrote:
A standing wave is the sum of an incident added to the reflective wave. Isn't it possible to send two incident waves down an xline with different frequences, and produce two different standing waves by having some multiplicative relationship between the two incident waves and the xline length? Sure, it's possible but one wonders about the application. 2. Matched impedances give ideal power transfer; mismatched impedances give high SWR and reduced power transfer. A middle ground - Conjugately matched impedances give ideal power transfer in the presence of high SWR. A feedline doesn't have to be flat to be "matched". All that is required is that maximum available power (actually energy) be transferred. -- 73, Cecil http://www.qsl.net/w5dxp |
#6
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Standing Waves (and Impedance)
To anybody who may be reading -
(1) There's far too much importance attached to standing waves on transmission lines. But see (4). (2) There are colossal standing waves on antennas which are seldom taken any notice of. (3) Anyway, of what use does anybody make of standing waves after taking the trouble to measure them. And the measurements themselves are the most inacurate in the field of radio engineering. (4) And to cap it all, the common or garden SWR meter does NOT measure standing waves on the feedline to the antenna where they might conceivably be of interest. It's all a gigantic hoax! ---- Season's Greetings from Reg, G4FGQ. |
#7
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Standing Waves (and Impedance)
.. "Cecil Moore" wrote in message om... W. Watson wrote: A standing wave is the sum of an incident added to the reflective wave. Isn't it possible to send two incident waves down an xline with different frequences, and produce two different standing waves by having some multiplicative relationship between the two incident waves and the xline length? Sure, it's possible but one wonders about the application. Cecil: Think about a 6 MHz wide analog TV channel. Those antennas aren't flat, and there are 2 transmitters, visual and aural. Putting an analog TV station on the air the first time, particularly low VHF, is a real interesting exercise. Or at least it was back in the stone (vacuum tube) age, the last time I did one. 73, George W5VPQ My real address is my ham call atARRL.NET The ATTGlobal is a SPAM trap snip |
#8
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Standing Waves (and Impedance)
Crazy George wrote:
Those antennas aren't flat, and there are 2 transmitters, visual and aural. The audio is not mixed with the main carrier? -- 73, Cecil http://www.qsl.net/w5dxp |
#9
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Standing Waves (and Impedance)
"Cecil Moore" wrote in message om... W. Watson wrote: A standing wave is the sum of an incident added to the reflective wave. Isn't it possible to send two incident waves down an xline with different frequences, and produce two different standing waves by having some multiplicative relationship between the two incident waves and the xline length? Sure, it's possible but one wonders about the application. Some repeaters use one antenna for two repeaters on 144 and 440 mhz. Comercial transmitters do this all the time. Usually the transmitters are in the same band. |
#10
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Standing Waves (and Impedance)
W. Watson wrote:
. . . Not a bad explanation from Wikipedia: SWR has a number of implications that are directly applicable to radio use. 1. SWR is an indicator of reflected waves bouncing back and forth within the transmission line, and as such, an increase in SWR corresponds to an increase in power in the line beyond the actual transmitted power. This increased power will increase RF losses, as increased voltage increases dielectric losses, and increased current increases resistive losses. I go along with that. 2. Matched impedances give ideal power transfer; mismatched impedances give high SWR and reduced power transfer. That's oversimplified and a misapplication of the rule of maximum power transfer. Suppose I have a 50 ohm source connected to a 50 ohm load and adjust the source so it puts 100 watts into the load. Then I put a half wavelength of 300 ohm line between the source and the load. The transmission line will have a 6:1 SWR. There will be a 6:1 impedance mismatch at the transmission line-load junction. Yet -- The load power will be 100 watts as before. -- The power produced by the source will be 100 watts as before. -- The system efficiency will be the same as it was before. -- 100 watts will be transferred from the source to the line. -- 100 watts will be transferred from the line to the load. So in no way did the high SWR result in reduced power transfer. Now change the load impedance to 300 ohms. -- There is now a 6:1 mismatch between the source and the line. -- The line SWR is now 1:1. -- The load power will be reduced. The mismatch between source and line didn't cause a high SWR on the line. In fact, changing the line impedance degraded the match at the same time it improved the SWR. 3. Higher power in the transmission line also leaks back into the radio, which causes it to heat up. That's demonstrably false. For some examples and explanations, see http://www.eznec.com/misc/food_for_t...se%20Power.txt. (You might have to splice this URL back together if your browser splits it.) 4. The higher voltages associated with a sufficiently high SWR could damage the transmitter. Solid state radios which have a lower tolerance for high voltages may automatically reduce output power to prevent damage. Tube radios may arc. The high voltages may also cause transmission line dielectric to break down and/or burn. That's true. Some transmitters can be damaged from a number of causes when the load impedance isn't approximately what the transmitter was designed for. Only one of those possible causes is increased voltage. Of course, a high SWR can also cause the voltage at the transmitter to be lower than it otherwise would have been. Abnormally high voltages in the antenna system increase the chance of accidental radiation burn if someone touches the antenna during transmission. But the antenna doesn't have an SWR, the transmission line does. If you do have an open wire transmission line, it's best not to touch the line regardless of the SWR. But if you have a high line SWR, there's just a good of a chance that the voltage at the point you touch is lower due to the high SWR than it is than the voltage is higher. I'll bet if you search the web you can find just about any kind of possible misinformation about SWR, just as you can about any other topic. Roy Lewallen, W7EL |
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