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Standing Waves (and Impedance)
I've just begun (and I do mean begun!) a little refresher reading on the
subject of transmission lines from a time *long* ago. I'm trying to make some sense out of why the emphasis of standing waves. The idea is familiar. Is it that somehow knowing something about the standing wave on the line that one can construct some sort of stub to smooth out the input impedance? If so, wouldn't the stub need to be tuned depending on the length of a cable? Can this be done somehow by the xmitter? BTW, is there any analog of electrical impedance in hydraulics or other areas where waves are widely studied? 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 |
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 |
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 |
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 |
Standing Waves (and Impedance)
On Tue, 20 Dec 2005 03:41:41 GMT, "W. Watson"
wrote: I'm trying to make some sense out of why the emphasis of standing waves. Here is the short version: A matched transmission line behaves like the theory books say it does. The rated power from the transmitter goes through the transmission line with the lowest possible loss to the antenna where it is radiated just like the book says. A mismatched transmission just MIGHT work OK. If there is any possibility of generating interference, especially TVI, it will. The currents and voltages on a mismatched line are extreme... There MIGHT even be some sparks. Power loss will be at its worst for a given line. RF finds its way every where. Getting zapped once in a while eventually grows old to everyone. I remember the good old days when desk mikes were the only way to go. If you got too close, you got an RF zap on your lip. Solid state rigs don't tolerate a high SWR. They either protect themselves by reducing power or they require a lot of maintenance. You can learn to tolerate high SWR's, but I find it worthwhile to try to keep things matched. The energy has to go somewhere, I prefer it leave here through the antenna... John Ferrell W8CCW |
Standing Waves (and Impedance)
On Wed, 21 Dec 2005 20:18:05 GMT, John Ferrell
wrote: On Tue, 20 Dec 2005 03:41:41 GMT, "W. Watson" wrote: I'm trying to make some sense out of why the emphasis of standing waves. Here is the short version: A matched transmission line behaves like the theory books say it does. The rated power from the transmitter goes through the transmission line with the lowest possible loss to the antenna where it is radiated just like the book says. It is true that reducing SWR for a given line does reduce the loss if the line is long enough. (There are some scenarios where a short line with high VSWR has less loss than matched line of the same length.) But is matched line the real goal? If low loss is the goal, there are often cost effective lower loss solutions possible with lower loss line operated at high VSWR. A mismatched transmission just MIGHT work OK. If there is any possibility of generating interference, especially TVI, it will. The Why? How is TVI "generated" by line mismatch? Owen -- |
Standing Waves (and Impedance)
John Ferrell wrote:
You can learn to tolerate high SWR's, but I find it worthwhile to try to keep things matched. The energy has to go somewhere, I prefer it leave here through the antenna... John Ferrell W8CCW I assume that you realize there is a high SWR on a standing- wave antenna, like a resonant 1/2WL dipole? :-) -- 73, Cecil http://www.qsl.net/w5dxp |
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. |
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 |
Standing Waves (and Impedance)
I was simply sharing my experiences from the past.
If you have followed a few of my earlier posts you are aware I am simply a student who should have been studying this many years ago. I welcome any corrections. I never gave the swr on the radiator any thought. That is a good point. .. On Wed, 21 Dec 2005 22:18:26 GMT, Cecil Moore wrote: John Ferrell wrote: You can learn to tolerate high SWR's, but I find it worthwhile to try to keep things matched. The energy has to go somewhere, I prefer it leave here through the antenna... John Ferrell W8CCW I assume that you realize there is a high SWR on a standing- wave antenna, like a resonant 1/2WL dipole? :-) John Ferrell W8CCW |
Standing Waves (and Impedance)
On Wed, 21 Dec 2005 21:29:37 GMT, Owen Duffy wrote:
On Wed, 21 Dec 2005 20:18:05 GMT, John Ferrell wrote: On Tue, 20 Dec 2005 03:41:41 GMT, "W. Watson" wrote: I'm trying to make some sense out of why the emphasis of standing waves. Here is the short version: A matched transmission line behaves like the theory books say it does. The rated power from the transmitter goes through the transmission line with the lowest possible loss to the antenna where it is radiated just like the book says. It is true that reducing SWR for a given line does reduce the loss if the line is long enough. (There are some scenarios where a short line with high VSWR has less loss than matched line of the same length.) I will have to take your word for it, I cannot think of any examples. But is matched line the real goal? If low loss is the goal, there are often cost effective lower loss solutions possible with lower loss line operated at high VSWR. A mismatched transmission just MIGHT work OK. If there is any possibility of generating interference, especially TVI, it will. The Why? How is TVI "generated" by line mismatch? Owen I really don't know why there is more TVI with a high swr. But my experience has been that there is, especially on 6 meters. John Ferrell W8CCW |
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. |
Standing Waves (and Impedance)
On Thu, 22 Dec 2005 04:17:11 GMT, John Ferrell
wrote: It is true that reducing SWR for a given line does reduce the loss if the line is long enough. (There are some scenarios where a short line with high VSWR has less loss than matched line of the same length.) I will have to take your word for it, I cannot think of any examples. John, It was really a bit of an aside, a lead into the more important point that followed, however... In most practical lines at HF, loss is dominated by the series resistance. In the regions of a current minimum on a line with high VSWR, the I^2R losses are less than for the same net power on a flat line, and conversely, in the region of a current maximum on a line with high VSWR, the I^2R losses are more than for the same net power on a flat line. Over a half wave of line, the total losses are higher than a flat line, but a short line in the regions of a current minimum may have losses less than the matched line loss. Whilst that is not often to our benefit as we rarely have relatively short lines with high Z loads, the converse is true of the more common situation of a short line with a low Z load. For example, 3m of RG58 with a 5+j0 load (eg mobile antenna) on 3.5MHz has a matched line loss of 0.08dB, and an actual loss of 0.66dB. Many charts and formulas would predict the mismatched loss to be only 0.39dB, but it is worse because the line is short and in the region of a current maximum. But is matched line the real goal? If low loss is the goal, there are often cost effective lower loss solutions possible with lower loss line operated at high VSWR. A mismatched transmission just MIGHT work OK. If there is any possibility of generating interference, especially TVI, it will. The Why? How is TVI "generated" by line mismatch? I really don't know why there is more TVI with a high swr. But my experience has been that there is, especially on 6 meters. If you can't explain the mechanism by which SWR causes TVI, perhaps they correlate by some other cause. For example, an antenna may develop loose oxided connections which both change the load impedance (and hence VSWR), and create intermodulation causing TVI. If VSWR *does* cause TVI, surely someone will be able to explain how? Lots of people operate feedlines at high VSWR by design, and they do not necessarily cause TVI. When you dismiss the TVI myth, you get closer to understanding how the transmission lines work and perform, and that for example, operating a feedline at high VSWR can be part of an efficient and effective multiband HF antenna. Such a solution should not be dismissed out of hand because of high VSWR alone. Owen -- |
Standing Waves (and Impedance)
John Ferrell wrote:
. . . I really don't know why there is more TVI with a high swr. But my experience has been that there is, especially on 6 meters. John Ferrell W8CCW It might be that the same phenomenon, or related ones, have caused both the high SWR and the TVI. But high SWR doesn't cause TVI. Or feedline radiation, which is another mistaken idea. Roy Lewallen, W7EL |
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 |
Standing Waves (and Impedance)
John Ferrell wrote:
Here is the short version: A matched transmission line behaves like the theory books say it does. The rated power from the transmitter goes through the transmission line with the lowest possible loss to the antenna where it is radiated just like the book says. Mismatched transmission lines also behave like the theory books say. The rated power from the transmitter goes through the transmission line to the antenna where it is radiated just like the book says. I deleted only "with the lowest possible loss" because increased SWR does increase line loss. But if the line loss is low when matched, the increased loss due to high SWR is often negligible. . . . Roy Lewallen, W7EL |
Standing Waves (and Impedance)
Owen Duffy wrote:
I really don't know why there is more TVI with a high swr. But my experience has been that there is, especially on 6 meters. If you can't explain the mechanism by which SWR causes TVI, perhaps they correlate by some other cause. For example, an antenna may develop loose oxided connections which both change the load impedance (and hence VSWR), and create intermodulation causing TVI. If VSWR *does* cause TVI, surely someone will be able to explain how? When people report "high SWR", they are usually talking about a coax-fed system, and they usually mean "a higher SWR than I expected for this antenna". That is a big clue that the antenna is not performing correctly... but the high SWR is only a symptom. It shouldn't be mistaken for a cause. One very common cause of RFI is common-mode RF current on the outside of the coax - in other words, the coax has become an unintended part of the antenna. The outside of the coax comes back down into the house, and can be a potent conductor of RFI. The higher than expected SWR is simply because the addition of the coax makes this a *different* antenna from the one you thought you were using. With something like a 6m yagi, the cure is generally to change to a truly balanced feed system, and to add a feedline choke. Obviously common-mode current is not the *only* possible connection between "high SWR" and RFI, but it's more common than many people suspect. Just caught Roy's second post about the mistaken belief that high SWR and feedline radiation. It should be clear from the above that higher-than-expected SWR and feedline radiation are two separate *results* of unwanted common-mode currents. Once again, SWR should not be mistaken for a cause. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Standing Waves (and Impedance)
Sorry, the last paragraph of my previous posting should have included the word _causes_. It should have read: Just caught Roy's second post about the mistaken belief that high SWR _causes_ feedline radiation. It should be clear from the above that higher-than-expected SWR and feedline radiation are two separate *results* of unwanted common-mode currents. Once again, SWR should not be mistaken for a cause. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Standing Waves (and Impedance)
I am glad I chose to display my ignorance, I have learned a lot.
I will modify my earlier position that High SWR causes TVI to Roy's better explanation that they seem to have the same causes. I have noted that other people seem to get along with High SWR's and Non resonant antennas. Neither has worked well for me in the past. I used to operate ATV on 440mhz with a 48 element collinear antenna. (I did not do the initial construction) It was basically a collection of dipoles and reflectors tied together with open wire phasing harnesses. I could never quite figure out what the harness might be losing. The antenna seemed to be worth about 21db with very primitive measuring schemes. I speculated that it had a theoretical potential of 48db. I always felt that the phasing harness was the major contributor to the losses. With each driven element being resonant, some measure of success was assured with each one. At 440, feed line losses get critical in a hurry. The transmitter final was a military surplus cavity with a 4CX250 that had been "stuffed" to get it up to 440. As a side effect the output Z was determined to be pretty low by trying several home brew quarter wave coaxial matching sections. When a good match was made, a lot of problems went away. Not only was I managing a better signal, but the polyethylene cooling ducting was taking a longer time to melt down. Hence, my position that SWR IS important. Please note: I am not here to pick an argument, I recently took W4RNL's antenna course, bought Roy's EZNEC+ program, downloaded (still studying) Reg's many programs and have taken note that Cecil is a motorcycle fan like my Dad was. When I provoke an argument, I am trying to learn something. BTW, I do have a pretty good library of reference books but you guys explain it better in terms I can grasp! On Wed, 21 Dec 2005 22:45:20 -0800, Roy Lewallen wrote: John Ferrell wrote: Here is the short version: A matched transmission line behaves like the theory books say it does. The rated power from the transmitter goes through the transmission line with the lowest possible loss to the antenna where it is radiated just like the book says. Mismatched transmission lines also behave like the theory books say. The rated power from the transmitter goes through the transmission line to the antenna where it is radiated just like the book says. I deleted only "with the lowest possible loss" because increased SWR does increase line loss. But if the line loss is low when matched, the increased loss due to high SWR is often negligible. . . . Roy Lewallen, W7EL John Ferrell W8CCW |
Standing Waves (and Impedance)
John Ferrell wrote:
I never gave the swr on the radiator any thought. That is a good point. Hi John, it was supposed to be humorous. I find it amusing that someone will say, "I hate high SWRs.", while running a resonant dipole with an SWR of 20:1 (or whatever) on the radiating elements. Superposition and interference between the forward waves and reflected waves on the radiating elements is what brings the feedpoint impedance of a 1/2WL dipole down to 50 ohms (or whatever). If it were not for standing waves on a 1/2WL dipole, the feedpoint impedance would be in the neighborhood of 1200 ohms, similiar to an infinite dipole. -- 73, Cecil http://www.qsl.net/w5dxp |
Standing Waves (and Impedance)
John Ferrell wrote:
I really don't know why there is more TVI with a high swr. But my experience has been that there is, especially on 6 meters. SWR can have an indirect effect on common-mode currents by causing a malfunction of the balun. For an SWR of 1:1, the balun is probably functioning in the impedance environment for which it was designed. That balun may cease to choke properly when exposed to the impedances present in an SWR 1:1 environment and allow more common-mode signals to develop and radiate. -- 73, Cecil http://www.qsl.net/w5dxp |
Standing Waves (and Impedance)
Roy Lewallen wrote:
But the antenna doesn't have an SWR, ... Actually it does, if it is a standing-wave antenna like a 1/2WL resonant dipole. The reflections from the ends of the dipole are what lowers the virtual feedpoint impedance so that coax is a good match. -- 73, Cecil http://www.qsl.net/w5dxp |
Standing Waves (and Impedance)
John Ferrell wrote:
. . . The transmitter final was a military surplus cavity with a 4CX250 that had been "stuffed" to get it up to 440. As a side effect the output Z was determined to be pretty low by trying several home brew quarter wave coaxial matching sections. When a good match was made, a lot of problems went away. Not only was I managing a better signal, but the polyethylene cooling ducting was taking a longer time to melt down. Hence, my position that SWR IS important. . . . All this demonstrates is that impedance match is important to the transmitter final. The quality of impedance match is often indicated as SWR on an SWR meter when in fact the meter reading often has little or nothing to do with the SWR on any transmission line. Even when it does, the problems with the transmitter are due solely to the poor impedance match and not at all due to the SWR on connected transmission lines. Let me give an example. Connect your transmitter through a half wavelength of 300 ohm transmission line to a 50 ohm (resistive) load. The transmitter sees 50 ohms, so an SWR meter at the transmitter will read 1:1, even though the SWR on the line is in fact 6:1. The transmitter can't tell the difference between this setup, a direct connection to the 50 ohm load, or connection to it through a half wavelength of cable with any impedance and therefore having any SWR. In all cases, the transmitter sees 50 ohms, which is all that matters. The line's SWR makes no difference at all. If for some reason you were really interested in finding the SWR on the 300 ohm line, you'd have to insert a 300 ohm SWR meter at the transmitter-line junction. It would correctly read 6:1. Roy Lewallen, W7EL |
Standing Waves (and Impedance)
On Thu, 22 Dec 2005 15:07:45 GMT, Cecil Moore wrote:
Owen Duffy wrote: Lots of people operate feedlines at high VSWR by design, and they do not necessarily cause TVI. Here's one example where a high VSWR might cause more TVI. Your example suggests a simplified method of analysing the effectiveness of a balun in reducing common mode current under two different load scenarios (that happen to have a transmission line operating at different VSWR). If the line length of your example was an even number of quarter waves, then by your own analysis method, in the second case, the balun with 50 ohm load and "3000 ohm choking impedance" would be more effective at 12:1 VSWR than the flat line? So that goes to the meaning of "high VSWR *might* cause more TVI". Perhaps there is some other factor, and perhaps VSWR is not a root cause at all. There is nothing in what you have said that suggests to me that VSWR is the cause of TVI (or feedline radiation in the more general case). Owen -- |
Standing Waves (and Impedance)
Roy, you are, at least, on the right track.
To measure SWR on the feedline, it is necessary to climb up the mast or a ladder and insert an SWR meter, of the correcct impedance, between the antenna and the feedline? Then you have to come down safely to ground level, switch on the transmitter, and view the meter reading through an astronomical telescope, bearing in mind that the field of view with an astronomical telescope is inverted with respect to normal. In its usual position the SWR meter does not measure SWR on any line. It merely indicates whether or not the transmitter is correctly loaded with a resistive 50 ohms. Which is all anyone may wish to know. After 50 years or more of ignorance, it is about time this hoax was exposed to the world. Then, all that is necessary to prevent the instrument from telling lies, is to leave it where it is and change its name to TLI (Transmitter Loading Indicator). ---- Reg, G4FGQ. |
Standing Waves (and Impedance)
Reg Edwards wrote:
Roy, you are, at least, on the right track. To measure SWR on the feedline, it is necessary to climb up the mast or a ladder and insert an SWR meter, of the correcct impedance, between the antenna and the feedline? No. You can insert the SWR meter of the correct impedance at the input end of the feedline. Stay inside, nice and warm. Of course, if your line has a significant amount of loss, the SWR will vary along the line, so you'll have to put the meter at the point where you want to know the SWR. Then you have to come down safely to ground level, switch on the transmitter, and view the meter reading through an astronomical telescope, bearing in mind that the field of view with an astronomical telescope is inverted with respect to normal. That's surely a novel way of doing it, although unnecessary. On the one hand, that method might seem more plausible after finishing off a bottle of wine. On the other, that would be a bad time to be climbing the mast. In its usual position the SWR meter does not measure SWR on any line. It merely indicates whether or not the transmitter is correctly loaded with a resistive 50 ohms. Which is all anyone may wish to know. After 50 years or more of ignorance, it is about time this hoax was exposed to the world. Then, all that is necessary to prevent the instrument from telling lies, is to leave it where it is and change its name to TLI (Transmitter Loading Indicator). Have you had any luck in selling Agilent (HP), Narda, Anritsu, and those other ignorant companies into not specifying the input impedances of their precision RF measurement equipment, terminations, and other components in terms of SWR? Once you get them to see the light, hams will surely enlist in your jihad. Otherwise, we'll have postings from hams that go something like this: "My TLI says my precision termination resistor has an impedance of 1.02 Reggies. But the manufacturer specifies a maximum SWR of 1.05:1. Is it ok? Reg says there are 6 dB in an S-Unit, so are there 6 SWRs to a Reggie?" Roy Lewallen, W7EL |
Standing Waves (and Impedance)
"Reg Edwards" wrote Then, all that is necessary to prevent the instrument from telling lies, is to leave it where it is and change its name to TLI (Transmitter Loading Indicator). ========================================== This will have the effect of reducing SWR on the line to a more appropriate and realistic level of importance. Who cares what is the SWR on the transmission line provided the transmitter is loaded with its correct load resistance? ---- Reg, G4FGQ. |
Standing Waves (and Impedance)
Your reply was very fast. You didn't have time to think about it.
The only way to "measure" SWR is to place the meter at the antenna end of the line. You know that as well as I do. The SWR does not apply to any particular point on the line. It applies to the WHOLE line. ---- Reg, G4FGQ. |
Standing Waves (and Impedance)
Reg Edwards wrote:
. . . The only way to "measure" SWR is to place the meter at the antenna end of the line. You know that as well as I do. The SWR does not apply to any particular point on the line. It applies to the WHOLE line. I disagree with both of those statements, and both can be shown to be incorrect. If a line is lossless, the SWR is the same all along the line. An SWR meter of the line's impedance will measure the SWR correctly when placed anywhere along the line, including at either end. If a line has loss, the SWR varies along the line, being the greatest at the load and decreasing toward the source. (The concept of SWR at a single point is well understood and widely used and accepted, even though it deviates from the original literal definition.) In that case, the meter will correctly read the SWR at the position where it's placed. That position can be anywhere along the line including either end. Roy Lewallen, W7EL |
Standing Waves (and Impedance)
On Thu, 22 Dec 2005 16:07:56 -0800, Roy Lewallen
wrote: If a line has loss, the SWR varies along the line, being the greatest at the load and decreasing toward the source. (The concept of SWR at a single point is well understood and widely used and accepted, even though it deviates from the original literal definition.) In that case, the meter will correctly read the SWR at the position where it's placed. That position can be anywhere along the line including either end. .... and for most practical purposes, with knowledge of the matched line loss, the VSWR at any other point on that line can be estimated with reasonable accuracy from the measurement at a point on the line. Owen -- |
Standing Waves (and Impedance)
Well, I think this thread went a bit beyond where I had intended
(expected?), but it looks like it's provided some useful discussions, as it did me. Here's kind of a twist on all this. I went back to my very old physics book to see what they had to say about standing waves. Actually, I had more fun reading about waves created by a string attached to various objects at the remote end. The authors considered a case with an infinite mass at the end, and another with the end of the string fastened to a ring, which was place on a pole. Finally, they discussed what would happen if another string followed the first and the second the string was denser or less dense than the string on which the wave begin. Another time. :-) 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 |
Standing Waves (and Impedance)
I forgot to mention the physics book even got into something called virtual
waves, which I believe are the mirror image of an incident wave. The mirror is perpendicular to the travel, and one can look down it from the side. 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 |
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 |
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. |
Standing Waves (and Impedance)
Roy, you surprise me. Try a jug of Moonshine. Placing the SWR meter at the start of the feed-line terminated by the antenna, will tell you NOTHING about the SWR on that line. It is the antenna input impedance which determines the SWR on the line, and the meter doesn't have the foggiest idea what THAT is. The unknown antenna impedance is at the other end of a line of unknown length, unknown impedance and unknown loss. Unknown, that is, to the meter. YOU might have that knowledge. But then you can CALCULATE what the SWR is on the line. Meter readings having been discarded as useless. I repeat - the meter tells you only whether or not the transmitter is loaded with a resistive 50 ohms. No more and no less. If it is not 50 ohms the ambiguous meter will not even tell you the actual value of Z. Intoxicated or not, if you insist on a meter reading, there is no alternative to climbing the antenna mast. ---- Reg, G4FGQ. PS. The use of SWR by American plug and socket manufacturers to describe unrelated characteristics of their products is a small indication of the abysmal depths to which engineering has descended. Technical specifications are reduced to Camm's Comics. But they look good to the uninitiated. ---- Reg. ========================================== |
Standing Waves (and Impedance)
On Thu, 22 Dec 2005 16:07:56 -0800, Roy Lewallen
wrote: Reg Edwards wrote: . . . The only way to "measure" SWR is to place the meter at the antenna end of the line. You know that as well as I do. The SWR does not apply to any particular point on the line. It applies to the WHOLE line. I disagree with both of those statements, and both can be shown to be incorrect. If a line is lossless, the SWR is the same all along the line. An SWR meter of the line's impedance will measure the SWR correctly when placed anywhere along the line, including at either end. If a line has loss, the SWR varies along the line, being the greatest at the load and decreasing toward the source. (The concept of SWR at a single point is well understood and widely used and accepted, even though it deviates from the original literal definition.) In that case, the meter will correctly read the SWR at the position where it's placed. That position can be anywhere along the line including either end. Roy Lewallen, W7EL I am absorbing this, but slowly. I have understood that a "matched line" would indicate the same SWR at every point you might measure it with a directional coupler. The Swr we are discussing is that which we can measure with a directional coupler, is it not? The SWR on a mis-matched line will vary with the position you choose to measure it. This can be indicated by varying the transmission line length to get an acceptable match for the system. This will satisfy the need to match a transmitter for a given frequency. A directional coupler placed at different places on the line will still indicate a non uniform SWR. Any feed line losses due to insulation or radiation are effectively hidden from the transmitter end. This is why I have gone to the antenna/feed line to measure the power level and the SWR. If your feed line has become an effective dummy load or a better radiator than your antenna it would nice to know. I don't have an answer as to how to measure the instruments insertion effects. Please tell me where I am in error! John Ferrell W8CCW |
Standing Waves (and Impedance)
To all and sundry,
The length and directions and off-shoots of this and other threads, the resulting arguments, confusion and misunderstandings prove my basic point - The SWR meter is grossely mis-named. It leads old-timers, novices, CB-ers and professional engineers severely astray. It overstretches imaginations. Dis-educational! ---- Reg. |
Standing Waves (and Impedance)
"Reg Edwards" wrote Technical specifications are reduced to Camm's Comics. But they look good to the uninitiated. ========================================== Insert between "they look good" and "to the uninitiated" - "and sell". ---- Reg. |
Standing Waves (and Impedance)
John Ferrell wrote:
I am absorbing this, but slowly. I have understood that a "matched line" would indicate the same SWR at every point you might measure it with a directional coupler. A matched line is one which is terminated with its characteristic impedance. The SWR on a matched line is 1:1 at all points along the line. The Swr we are discussing is that which we can measure with a directional coupler, is it not? Yes and no. To measure the SWR requires an SWR meter or directional coupler which is designed for the particular characteristic impedance of the line. If a directional coupler is the proper impedance, it can be used to calculate the SWR from the forward and reverse powers. If it isn't, it can't. The SWR on a mis-matched line will vary with the position you choose to measure it. No, it won't, unless it has loss. If it has loss, the SWR will be greatest at the load and will monotonically decrease toward the source. This can be indicated by varying the transmission line length to get an acceptable match for the system. This will satisfy the need to match a transmitter for a given frequency. A directional coupler placed at different places on the line will still indicate a non uniform SWR. Any feed line losses due to insulation or radiation are effectively hidden from the transmitter end. It appears that you're assuming that what you measure with an SWR meter or calculate from directional coupler readings is the SWR. Unless the coupler or meter is designed for the Z0 of the line, it isn't. If the coupler or meter isn't of the proper impedance for the line, you'll get different readings as you move along the line. Those readings aren't, however, the line's SWR. . . . Roy Lewallen, W7EL |
Standing Waves (and Impedance)
This is pretty strange.
Suppose Reg has a 50 ohm line of some length connected to an antenna whose impedance is 100 + j0 ohms. After putting away his evening's bottle of wine, he climbs the tower and inserts a 50 ohm SWR meter at the antenna. He climbs back down, gets out his vintage brass telescope and keys the transmitter. Then, steadying himself, he peers through the telescope and sees that the SWR meter reads 2:1. (Being a clever person, he mounted the meter upside down so it would be right side up in the telescope, obviating the need for the added challenge of mental inversion.) I have an identical antenna, feedline, and SWR meter. I sit in my warm shack sipping my moonshine, connect the SWR meter to the input end of the line, hit the key, and note that the meter reads 2:1. Or perhaps slightly less if the line is noticeably lossy. Reg says: Placing the SWR meter at the start of the feed-line terminated by the antenna, will tell you NOTHING about the SWR on that line. I guess the 2:1 reading from the meter at the input end of the line is telling Reg nothing, while the 2:1 reading at the antenna is. Strange. The fact is that it's the SWR on the line, and it can be measured at any point along the line. I like my method better, but each to his own. Roy Lewallen, W7EL Reg Edwards wrote: Roy, you surprise me. Try a jug of Moonshine. Placing the SWR meter at the start of the feed-line terminated by the antenna, will tell you NOTHING about the SWR on that line. It is the antenna input impedance which determines the SWR on the line, and the meter doesn't have the foggiest idea what THAT is. The unknown antenna impedance is at the other end of a line of unknown length, unknown impedance and unknown loss. Unknown, that is, to the meter. YOU might have that knowledge. But then you can CALCULATE what the SWR is on the line. Meter readings having been discarded as useless. I repeat - the meter tells you only whether or not the transmitter is loaded with a resistive 50 ohms. No more and no less. If it is not 50 ohms the ambiguous meter will not even tell you the actual value of Z. Intoxicated or not, if you insist on a meter reading, there is no alternative to climbing the antenna mast. ---- Reg, G4FGQ. PS. The use of SWR by American plug and socket manufacturers to describe unrelated characteristics of their products is a small indication of the abysmal depths to which engineering has descended. Technical specifications are reduced to Camm's Comics. But they look good to the uninitiated. ---- Reg. ========================================== |
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