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#1




SWR  wtf?
I don't see the original posting here on rec.radio.amateur, but there
are a few misconceptions in the followups which should be addressed. Lancer wrote: On Tue, 28 Jun 2005 19:13:35 GMT, james wrote: On Tue, 28 Jun 2005 18:31:58 GMT, Lancer wrote: To have the measured SWR change with coax length, means you have current flowing on the outside of the coax. Your coax then becomes part of the antenna, so changing its length is changing the antenna length. This would change the feedpoint impedance and the SWR. That's correct, except that coax loss will also cause the SWR to change with coax length. Loss will cause the SWR at the antenna (load) to always be greater than at the transmitter (source). Unless the line is carrying common mode currents that affect antenna impedance, changing coax length won't change the SWR, even if the antenna isn't matched. Again correct except for overlooking the effect of coax loss. But there's a real problem in communicating this. If you hook a 50 ohm SWR meter to the input of a 75 ohm, 300 ohm, or line of any impedance other than 50 ohms, the meter reading won't be the SWR on the transmission line. That can mislead people into thinking that the SWR is changing with line length when it actually isn't. ******** BS Common mode currents on the shield of coaxial cables do not alter the feed impedance. Repeat ofter me. Common mode currents on the shield of coaxial cables do not alter the feed impedance. Why repeat it if it isn't true? The explanation given by Lancer was correct. If you change the length of the antenna, the feedpoint impedance will change. When you have common mode current flowing on the feedline, the feedline is part of the antenna; changing its length is changing the antenna's length. The feed impedance of an antenna is solely determined by its physical length and any load impedances within the antenna structure. Load impedances can be stray capacitance with ground via metal objects within the near field of the antenna or even a building. You have to realize that a radiating feedline (one carrying common mode current) IS part of the antenna structure. The "Magic" of an electrical halfwave transmission line is at a precise frequency, the reflection of the load to the transmistter is equal to the characteristic impedance of the transmission line irregardless of what impedance it is terminated with. This is true only of a lossless line. If the load impedance isn't far from the line's characteristic impedance (i.e., the line's SWR is low), a small amount of loss won't make much difference. However, if the line SWR is high, even a small amount of loss can make a major change in the impedance seen at the line's input. The effect is to skew the impedance toward the line's Z0. Other lengths have the load impedance reflected back and transformed by the length of the coax. The coax then acts as a transformer. It will either step up or step down the impeadnace of the load depending on the load itself and the electrical length of the coax. It's a little more complicated than that. The line doesn't simply multiply or divide the impedance by a constant, like a transformer  except in the special case of a quarter electrical wavelength line or odd multiples thereof. In other cases, the line does transform the impedance, but in a complex way in which the resistance and reactance are transformed by different factors. And reactance can be present at a line's input even when the load is purely resistive. A Smith chart is a good visual aid in seeing what happens. Assuming a lossless line, the impedance traverses a circle around the origin. The radius of the circle corresponds to the line's SWR. With the chart, you can see all the combinations of R and X which a given line can produce with a given load by changing its length. Incidentally, loss causes the impedance to spiral inward toward the origin as the line gets longer, showing how loss skews the input impedance toward Z0. All a tuner does is electrically lengthen or shoten the coax by introducing a lumped LC constant that helps present a resistive load to the transmitter. The SWR at the feedline does not change. By placing various different lengths of coax inline, you do the same thing a tuner does, add a lumped LC constant. As can be seen from the Smith chart, you can produce only particular combinations of R and X by changing the length of a line which has a given load impedance. Unless you're unusually lucky or have planned things carefully, none of these combinations will result in 50 + j0 ohms, the usual goal, at the line's input. In contrast, a tuner is able to adjust both R and X to produce, if designed right for the application, 50 + j0 for a wide range of load impedances. It requires at least two adjustable components to achieve an impedance match from an arbitrary load impedance, because there are two separate quantities, R and X or impedance magnitude and phase, which have to be adjusted. Changing the line length is only one adjustment, so it can't be guaranteed to provide a match. If you could also change the line's Z0, for example, or the length of a stub, you'd have two adjustments and you could guarantee a match providing you have enough adjustment range. james So thats all my tuner does, lengthen or shorten the coax? Are you sure about that? Rest assured, that's not all it does. Roy Lewallen, W7EL 
#2




(I've snipped parts of Roy's original posting, indicated by ..., that I
hope are not particularly relevant to my added comments.) Roy Lewallen wrote: I don't see the original posting here on rec.radio.amateur, but there are a few misconceptions in the followups which should be addressed. .... Unless the line is carrying common mode currents that affect antenna impedance, changing coax length won't change the SWR, even if the antenna isn't matched. Again correct except for overlooking the effect of coax loss. But there's a real problem in communicating this. If you hook a 50 ohm SWR meter to the input of a 75 ohm, 300 ohm, or line of any impedance other than 50 ohms, the meter reading won't be the SWR on the transmission line. That can mislead people into thinking that the SWR is changing with line length when it actually isn't. In addition, most hams (and other nonprofessionals  and even many professionals) don't bother to check that their SWR meter is properly calibrated to the impedance they think it is. Most are nominally 50 ohms, but they can be built for any practical line impedance. Checking calibration is not all that difficult, if you take the time to do it. In addition, your nominally 50 ohm line (or 75 or whatever) can have an actual impedance 10% or more from the nominal value. If you have properly calibrated your meter to 50 ohms, and your line is 60 ohms, you would read 1.2:1 SWR when your line is actually 1:1. And if the SWR on the 60 ohm line is 1.2:1, that 50 ohm SWR meter can read anything between 1:1 and 1.44:1, depending on the line length and its load. Finally, though you may have checked that the meter to reads 1:1 with a 50 ohm load and infinity to 1 with a short or open load, the construction of inexpensive meters may cause them to have significant errors at other load impedances. .... The "Magic" of an electrical halfwave transmission line is at a precise frequency, the reflection of the load to the transmistter is equal to the characteristic impedance of the transmission line irregardless of what impedance it is terminated with. This is true only of a lossless line. If the load impedance isn't far from the line's characteristic impedance (i.e., the line's SWR is low), a small amount of loss won't make much difference. However, if the line SWR is high, even a small amount of loss can make a major change in the impedance seen at the line's input. The effect is to skew the impedance toward the line's Z0. The piece that Roy quoted is so outrageous that I can easily believe he didn't read it right, but I've reread it several times, and it keeps coming out the same: the "magical" halfwave line does NOT reflect an impedance to the source (transmitter) equal to the LINE impedance as the quoted section says, but it reflects the LOAD impedance (altered by line loss as Roy says). .... about tuners, Roy went on to write: It requires at least two adjustable components to achieve an impedance match from an arbitrary load impedance, because there are two separate quantities, R and X or impedance magnitude and phase, which have to be adjusted. Changing the line length is only one adjustment, so it can't be guaranteed to provide a match. If you could also change the line's Z0, for example, or the length of a stub, you'd have two adjustments and you could guarantee a match providing you have enough adjustment range. In addition, two adjustable components in a particular configuration, even if they are infinitely adjustable (and reasonably close to lossless!!a very tall order!) won't necessarily give you the ability to transform any arbitrary impedance to 50 ohms. There may be whole practical areas of the complex impedance plane left untransformable. Also, the efficiency of a particular tuner topology for a given load impedance may be very good or may be terrible, when using practical components in the tuner. To reiterate what Roy wrote, it's important to use the right topology for the job you need to do. Cheers, Tom james So thats all my tuner does, lengthen or shorten the coax? Are you sure about that? Rest assured, that's not all it does. Roy Lewallen, W7EL 
#3




Thanks to Tom for the comments and additions.
. . . [I've lost track of who said this:] The "Magic" of an electrical halfwave transmission line is at a precise frequency, the reflection of the load to the transmistter is equal to the characteristic impedance of the transmission line irregardless of what impedance it is terminated with. [Roy:] This is true only of a lossless line. If the load impedance isn't far from the line's characteristic impedance (i.e., the line's SWR is low), a small amount of loss won't make much difference. However, if the line SWR is high, even a small amount of loss can make a major change in the impedance seen at the line's input. The effect is to skew the impedance toward the line's Z0. [Tom:] The piece that Roy quoted is so outrageous that I can easily believe he didn't read it right, but I've reread it several times, and it keeps coming out the same: the "magical" halfwave line does NOT reflect an impedance to the source (transmitter) equal to the LINE impedance as the quoted section says, but it reflects the LOAD impedance (altered by line loss as Roy says). . . . Wow, I certainly read that (top quote) too quickly. Tom is absolutely right, as written it's very wrong, and I misread it. I retract my statement about it's being "true only of a lossless line"  of course it's not true at all, but works as Tom says. Roy Lewallen, W7EL 
#4




On 28 Jun 2005 17:51:10 0700, "K7ITM" wrote in
. com: snip But there's a real problem in communicating this. If you hook a 50 ohm SWR meter to the input of a 75 ohm, 300 ohm, or line of any impedance other than 50 ohms, the meter reading won't be the SWR on the transmission line. That can mislead people into thinking that the SWR is changing with line length when it actually isn't. In addition, most hams (and other nonprofessionals  and even many professionals) don't bother to check that their SWR meter is properly calibrated to the impedance they think it is. Most are nominally 50 ohms, but they can be built for any practical line impedance. Checking calibration is not all that difficult, if you take the time to do it. In addition, your nominally 50 ohm line (or 75 or whatever) can have an actual impedance 10% or more from the nominal value. If you have properly calibrated your meter to 50 ohms, and your line is 60 ohms, you would read 1.2:1 SWR when your line is actually 1:1. And if the SWR on the 60 ohm line is 1.2:1, that 50 ohm SWR meter can read anything between 1:1 and 1.44:1, depending on the line length and its load. Finally, though you may have checked that the meter to reads 1:1 with a 50 ohm load and infinity to 1 with a short or open load, the construction of inexpensive meters may cause them to have significant errors at other load impedances. Impedance matching of an SWR meter is generally unimportant since most SWR meters used for HF have a directional coupler that is much shorter than the operating wavelength. Regardless, I'm not a big fan of SWR meters  they are good for detecting a major malfunction but that's about it. Antenna tuning/matching is best done with a field strength meter. == Posted via Newsfeeds.Com  UnlimitedUncensoredSecure Usenet News== http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups = East and WestCoast Server Farms  Total Privacy via Encryption = 
#5




Frank Gilliland wrote:
Impedance matching of an SWR meter is generally unimportant since most SWR meters used for HF have a directional coupler that is much shorter than the operating wavelength. Point is that they are usually calibrated for Z0=50 ohms and are in error when used in Z0 environments differing from Z0=50 ohms, e.g. Z0=75 ohms.  73, Cecil http://www.qsl.net/w5dxp == Posted via Newsfeeds.Com  UnlimitedUncensoredSecure Usenet News== http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups = East/WestCoast Server Farms  Total Privacy via Encryption = 
#6




On Tue, 28 Jun 2005 15:53:03 0700, Roy Lewallen
wrote: To have the measured SWR change with coax length, means you have current flowing on the outside of the coax. Your coax then becomes part of the antenna, so changing its length is changing the antenna length. This would change the feedpoint impedance and the SWR. That's correct, except that coax loss will also cause the SWR to change with coax length. Loss will cause the SWR at the antenna (load) to always be greater than at the transmitter (source). Would the changing of the coax lead to moving the SWR meter to a different voltage point on the coax?  73 for now Buck N4PGW 
#7




On Tue, 28 Jun 2005 20:49:46 0700, Frank Gilliland
wrote: I'm not a big fan of SWR meters  they are good for detecting a major malfunction but that's about it. Antenna tuning/matching is best done with a field strength meter. A local retired instructor of some sort (military, i believe) has the same opinion. He doesn't like SWR meters but instead measures all his antennas by field strength meter. I used to tune my Swan with one. I found when I used an SWR meter, the minimum SWR dip was NEVER the maximum field strength reading. I always had to raise the SWR to about 1.3:1 or so. Around here, most of us know not to mention the performance of an antenna to him if we only used an SWR meter or antenna analyzer. His first question is "How did it do with the FSM?" I believe he is right. Radios drop power when they don't like the SWR and raise it when it does. 73 N4PGW  73 for now Buck N4PGW 
#8




On Wed, 29 Jun 2005 01:04:59 0400, Buck wrote:
I used to tune my Swan with one. I found when I used an SWR meter, the minimum SWR dip was NEVER the maximum field strength reading. I always had to raise the SWR to about 1.3:1 or so. The probable reason for maximum power output not coinciding with the plate current dip is imperfect neutralisation (whether or not the stage is neutralised). If you have a neutralisation adjustment, you can get the two to coincide by properly adjusting the neutralisation. You can use this coincidence as a quite sensitive indication of optimal neutralisation if you use a digital meter to monitor plate current, and the power out indicator to monitor RF output power. Adjust the tuning and loading for rated power into a dummy load, check the tune cap is peaked for max Po, observe the plate current, carefully dip the plate current, noting whether the dip was left or right of max Po. Now tweak the neutralisation until the two coincide. When it is all done properly, the "dip" of the plate current should be symmetric, ie it should be as "sharp" approaching from one side as the other. (Asymetric dips are another symptom of nonomptimal neutralisation. Owen  
#9




Buck wrote:
Would the changing of the coax lead to moving the SWR meter to a different voltage point on the coax? Sort of, but not exactly. Let's take an example. I'll keep the values purely real to help folks who aren't familiar with complex math, but keep in mind that these are special cases and a full treatment would be somewhat more involved. I'll also make all the transmission lines lossless to simplify things. An SWR meter really just provides another way of reporting the impedance it sees. You can verify this by connecting pure resistances of various values to its output. For example, a (properly calibrated and operating) 50 ohm SWR meter will report 1:1 if you connect its output to a 50 ohm resistor. If you connect it to either a 25 or 100 ohm resistor, it reports 2:1. It does this despite the fact that there's no transmission line at all connected to its output. Some people can put up a huge smokescreen and waving of hands about reflected waves of one kind or another, but at the end of the day the SWR meter can't tell the difference between a resistor and a transmission line terminated with a load, if the impedances the meter sees are the same. It's sensitive only to impedance; it has no way of knowing even if a transmission line is connected to its output, let alone what the transmission line's SWR or even characteristic impedance is. Now put a half wavelength piece of 50 ohm coax between the SWR meter and those resistors. The SWR meter will still see the same impedances as before, so it'll report the same SWRs. Now, though, there really is a transmission line connected to its output. And because the meter is a 50 ohm meter and the line has a 50 ohm Z0, the SWR meter reading is the same as the actual SWR on the line. When the load is 50 ohms, the line's SWR is 1:1 and the meter sees 50 ohms so it reports 1:1. When the load is 25 ohms, the line's SWR is 2:1, and the meter sees 25 ohms and reports 2:1. When the load is 100 ohms, the line's SWR is 2:1, and the meter sees 100 ohms and reports 2:1. Next experiment: Connect the SWR meter through a *quarter* wavelength of 50 ohm line to a 100 ohm load. Now the impedance looking into the line is 25 ohms instead of 100. But the SWR meter reads 2:1 when it sees 25 ohms as well as 100, so it still reads 2:1, which is also still the SWR on the 50 ohm line. You can change the length of the 50 ohm line all you want and, if it's lossless, the line's actual SWR stays the same  but the impedance at the input end of the line changes. For a 100 ohm load, when the line is any even number of half wavelengths long, the input Z is 100 ohms. When the line is any odd number of quarter wavelengths long, the input Z is 25 ohms. At other lengths, the impedance is both resistive and reactive, but the line's SWR is always 2:1. And the SWR meter interprets all these possible impedances as 2:1, and that's what it reads. The line SWR doesn't change as you change its length, and the SWR meter reading doesn't change, either. Now instead of a 50 ohm line, let's connect a half wavelength 100 ohm line to the output of the same 50 ohm SWR meter and hook that to a 50 ohm resistive load. The line's actual SWR is 2:1 and, just like any lossless line, the SWR stays the same regardless of its length. If the transmission line is an even number of half wavelengths long we'll have 50 ohms at the input and the SWR meter will read 1:1, since it's a 50 ohm meter and interprets 50 ohms as 1:1. If we change the line length to a quarter wavelength, the input impedance will be 200 ohms, which the 50 ohm SWR meter will interpret and report as 4:1. So by changing the line length from a half to a quarter wavelength we've changed the SWR meter reading from 1:1 to 4:1, even though the line's actual SWR was 2:1 all along. That's what I was talking about. The SWR meter makes assumptions about the SWR on the line from the impedances it sees. The line transforms the load impedance in a different way than a 50 ohm line would. The SWR meter then assumes an incorrect SWR value for the line, and this incorrect value changes as the line length changes. The same thing happens if the line has a 50 ohm characteristic impedance and the meter is designed for some other Z0. The lesson is that an SWR meter shows the actual SWR on a transmission line connected to its output only if the SWR meter is designed for the same Z0 as the line. Too often, people say "The SWR is. . .", but really mean "The SWR meter reading is. . .". As you've seen, the two can often be very different. When you see the SWR reading changing as you change the line length, it doesn't necessarily mean that the line's SWR is actually changing. Remember, in the preceding discussion I've assumed for simplicity that all lines were lossless. In the real world, no line is, so the actual line SWR will always be higher at the load than the source (unless of course it's 1:1 at the load). Roy Lewallen, W7EL 
#10




Roy, to cut things short, why don't you just say SWR meters don't
measure SWR on anything. All they do is indicate whether or not the transmitter is terminated with its correct load resistance. So they are quite useful. They won't even tell you what the load resistance actually is unless the load is exactly correct. Stop fooling and confusing yourselves. The solution to everybody's problems is simple  just change the name of the thing to TLI. (Transmitter Loading Indicator).  Reg, G4FGQ 
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