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What causes this?
I just put up an inverted V for 30 meters.
I started out with each leg being 24'0". This gave me a low SWR at 9.5665 mhz which works out to 229.6 instead of the usual 234/F. As I trimmed, I decided to keep track of how much I trimmed and what the nnn/F number would be. As I got closer to my goal of 10.15, the number went down, eventually ending up at 227.28/10.1955=22.292' Also, the 2:1 swr bandwidth went up - it started at 567 kc and ended up at 655 kc. Either way, I got the antenna up and it's working fine - I'm just curious why the formula for length and the bandwidth changed as the antenna got shorter. Ken KG0WX |
On Wed, 1 Dec 2004 10:58:35 -0600, "Ken Bessler"
wrote: works out to 229.6 instead of the usual 234/F. ending up at 227.28 I'm just curious why the formula for length and the bandwidth changed as the antenna got shorter. Hi Ken, The normal 5% shortening (234/F) is due to what is called "end effect." Your ends are closer together than for the standard dipole, and get closer yet when the V is shortened - I suppose. 73's Richard Clark, KB7QHC |
Ken Bessler wrote:
I just put up an inverted V for 30 meters. I started out with each leg being 24'0". This gave me a low SWR at 9.5665 mhz which works out to 229.6 instead of the usual 234/F. Just about right for insulated wire. Did you use insulated wire? -- 73, Cecil http://www.qsl.net/w5dxp |
"Cecil Moore" wrote in message ... Ken Bessler wrote: I just put up an inverted V for 30 meters. I started out with each leg being 24'0". This gave me a low SWR at 9.5665 mhz which works out to 229.6 instead of the usual 234/F. Just about right for insulated wire. Did you use insulated wire? -- 73, Cecil http://www.qsl.net/w5dxp Nope, I used leftovers from the two 150' rolls I bought to make my 160m antenna. It's your standard issue 14/7 stranded bare copper. Ken KG0WX |
Just curious, Ken. Your question seems to be about resonance at
different frequencies. Yet your reported measurements were about SWR. Are you equating minimum SWR with resonant frequency? Chuck NT3G Ken Bessler wrote: I just put up an inverted V for 30 meters. I started out with each leg being 24'0". This gave me a low SWR at 9.5665 mhz which works out to 229.6 instead of the usual 234/F. As I trimmed, I decided to keep track of how much I trimmed and what the nnn/F number would be. As I got closer to my goal of 10.15, the number went down, eventually ending up at 227.28/10.1955=22.292' Also, the 2:1 swr bandwidth went up - it started at 567 kc and ended up at 655 kc. Either way, I got the antenna up and it's working fine - I'm just curious why the formula for length and the bandwidth changed as the antenna got shorter. Ken KG0WX |
"chuck" wrote in message k.net... Just curious, Ken. Your question seems to be about resonance at different frequencies. Yet your reported measurements were about SWR. Are you equating minimum SWR with resonant frequency? Chuck NT3G No. I've been down that road before - an antenna can pose a 200 ohm impedence at resonant frequency resulting in a SWR of 4:1. I'm simply trying to get the lowest SWR in the middle of the target range. Ken KG0WX |
Ken Bessler wrote:
I just put up an inverted V for 30 meters. I started out with each leg being 24'0". This gave me a low SWR at 9.5665 mhz which works out to 229.6 instead of the usual 234/F. As I trimmed, I decided to keep track of how much I trimmed and what the nnn/F number would be. As I got closer to my goal of 10.15, the number went down, eventually ending up at 227.28/10.1955=22.292' Also, the 2:1 swr bandwidth went up - it started at 567 kc and ended up at 655 kc. Either way, I got the antenna up and it's working fine - I'm just curious why the formula for length and the bandwidth changed as the antenna got shorter. Ken KG0WX 234/f is just a starting point. Dave WD9BDZ |
David G. Nagel wrote:
Ken Bessler wrote: I just put up an inverted V for 30 meters. I started out with each leg being 24'0". This gave me a low SWR at 9.5665 mhz which works out to 229.6 instead of the usual 234/F. As I trimmed, I decided to keep track of how much I trimmed and what the nnn/F number would be. As I got closer to my goal of 10.15, the number went down, eventually ending up at 227.28/10.1955=22.292' Also, the 2:1 swr bandwidth went up - it started at 567 kc and ended up at 655 kc. Either way, I got the antenna up and it's working fine - I'm just curious why the formula for length and the bandwidth changed as the antenna got shorter. Ken KG0WX 234/f is just a starting point. The "starting point" in question was a low 40m dipole, strung in the back alley of the old ARRL HQ building. For any other antenna, anywhere else in the universe, the "magic number 234" is going to be slightly different. The difference in SWR bandwidth between 2:1 points is a bit more complicated, and probably can't be explained in a one-liner. It will be mostly determined by the interplay between two factors: 1. What the resonant impedance is (in relation to 50 ohms), which determines the minimum SWR. 2. How quickly the reactive part of the feedpoint impedance changes with frequency, for different dipole lengths. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
You've gotten a number of good answers, but maybe I can still add a
little helpful information. The resonant length and the bandwidth of an antenna are determined by some basic electromagnetic principles. Although simple in concept, the exact solution for the impedance (and therefore the resonant frequency and bandwidth) of even an elementary dipole is actually very complex. The most common method involves solution of a triple integral equation, which can't be done directly at all, but requires a computer to numerically approximate the result.(*) The formulas you see in handbooks are just a rough approximation that's more-or-less good over a limited range of conditions. The actual resonant frequency and bandwidth are affected by wire diameter, height above ground, and angle between the wires, as well as just the wire length. And the relationships aren't really simple at all. So the bottom line is that the formulas work well enough to get you into the ballpark, from which you've usually got to do some trimming -- just as you did. You can't expect more than that from them. Readily available, inexpensive or free, computer programs can do the complex calculations from fundamental electromagnetic principles with rather astounding accuracy, in a small fraction of a second for a simple antenna. The computed results can still differ from reality, though, due to differences between the model antenna and the real one, like nearby objects or wire insulation not included in the model, wire sag, capacitance of end insulators, common mode feedline current, and so forth. But they'll still get you much closer than the simple handbook formulas. However, the simple formulas and a bit of cut and try are perfectly adequate for many simple antennas, and might easily be faster in the long run for someone not familiar with the programs. (*) Before the ready availability of computers, many different methods were devised to approximate the solution, with varying degrees of complexity and accuracy. Roy Lewallen, W7EL Ken Bessler wrote: I just put up an inverted V for 30 meters. I started out with each leg being 24'0". This gave me a low SWR at 9.5665 mhz which works out to 229.6 instead of the usual 234/F. As I trimmed, I decided to keep track of how much I trimmed and what the nnn/F number would be. As I got closer to my goal of 10.15, the number went down, eventually ending up at 227.28/10.1955=22.292' Also, the 2:1 swr bandwidth went up - it started at 567 kc and ended up at 655 kc. Either way, I got the antenna up and it's working fine - I'm just curious why the formula for length and the bandwidth changed as the antenna got shorter. Ken KG0WX |
Ken, I guess I'm still confused.
As I understand it, one cannot reliably determine the exact resonance of a dipole by finding the point of minimum SWR. Until this measurement issue is resolved, there would seem to be little benefit to seeking an explanation of why the formula appeared not to work. It is probably too late now, but if you had used an impedance bridge (MFJ or Autek, for example) you could have found resonance at the point of zero reactance. All within the limits of the instruments, of course. Some of the posts suggest other reasons why the formula might not work, but it is not yet evident to me that it didn't work. Sorry my earlier post was not more clear. (I'm even sorrier for this post if you actually used an impedance bridge! Hi.) 73, Chuck NT3G |
I'm just curious why the formula for length
and the bandwidth changed as the antenna got shorter. Ken KG0WX =============================== The resonant length of an antenna depends on - Length of wire. But also to a far smaller extent on - Increasing conductor diameter. Increasing conductor insulation thickness. Reducing height above ground. Close proximity to antenna supports, buildings, trees and indoors. All of which can reduce the resonant length to very slightly less than the theoretical maximum value of 150/MHz metres for a halfwave dipole. Or 492/MHz feet, pruned by a very few percent. Behaviour versus the included angle of an inverted-V is slightly peculiar. As the included angle approaches zero (which nobody ever uses) the resonant length approaches that of an ordinary open-wire transmission line, 150/MHz again. "Very slightly less" is of the order of 1 or 2 or 3 percent unless you have a VERY low antenna. Everybody's antenna is slightly different. Just keep a pair of pruning shears handy. Or bend the wire back on itself. ---- Reg, G4FGQ |
Ken, KG0WX wrote:
"I`m just curious why the formula for length and the bandwidth changed as the antenna got shorter." In an inverted V, the capacitance effect at its high-voltage ends is enhanced by their nearness to earth. Best regards, Richard Harrison, KB5WZI |
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"Richard Clark" wrote in message ... On Thu, 2 Dec 2004 13:12:15 -0600, (Richard Harrison) wrote: In an inverted V, the capacitance effect at its high-voltage ends is enhanced by their nearness to earth. Hi Richard, Then that is dashed by his having trimmed the ends (presumably everything at the same height, and as such in a direction away from earth) and the multiplier shrank more. More likely the capacitance between the ends grew instead (corresponds to all factors observed). 73's Richard Clark, KB7QHC Now THAT makes sense! I was worried that at the rate I observed the nnn/f formula decline, your average 10m dipole would be stupidly short! And, for the record, I never touched the height of the feedpoint or the angle of the legs. Instead of tying to the tips of the legs, I attached the guy lines (heavy duty natural twine) about 8" up from the ends. That way, I could trim the ends without having to redo the supports. I wrapped the twine around the ends to keep everything straight. So, only 2 things changed - the height of the ends (10') and the length of the legs. Thanks everyone for the ideas..... Ken KG0WX |
There's good reason for confusion.
Resonance is defined as the frequency at which the reactance is zero. Antenna resonance is the frequency at which the feedpoint reactance is zero. At the input end of a feedline, resonance will occur at the same frequency as antenna resonance only if the antenna resistance is the same as the feedline characteristic impedance, or if the feedline is a multiple of a quarter wavelength (yes, quarter) long. So resonance at the input end of a feedline is often at a different frequency than antenna resonance. (You could call that "antenna system resonance".) The reading on an SWR meter is a function of the both the resistance and reactance of the load presented to the meter. As the frequency changes, both the feedpoint resistance and reactance change. So if the resistance is closer to the SWR meter's Z0 when there's a slight reactance, the SWR meter reading at the feedpoint can be better than at resonance, when the reactance is exactly zero. When measured at the feedline input, there's the additional factor of impedance transformation that can modify the reactance and therefore resonance. In practice, though, the point of minimum SWR is nearly always very close to resonance, at the antenna feedpoint. This is because for most antennas, the reactance changes much faster with frequency than the resistance does. And, when the feedpoint resistance is roughly equal to the feedline Z0, the transformation by a mismatched feedline isn't extreme, so again the point of minimum SWR is usually close to resonance. Where you're likely to see a noticeable difference between resonance and lowest SWR is with antennas with several coupled elements (where feedpoint R can change more rapidly with frequency), or severely mismatched feedlines, like when using open wire line to feed a dipole on several bands. In the end, it really doesn't matter. There's nothing at all magical about resonance, so there's no need to try and achieve it. What you're usually interested in doing is matching the rig to the input of the feedline, and the common measure of the quality of that match is the reading on an SWR meter. So the common, and valid, practice is to prune the antenna for lowest SWR meter reading. If you do that, there's no need to worry about where resonance might be. Roy Lewallen, W7EL chuck wrote: Ken, I guess I'm still confused. As I understand it, one cannot reliably determine the exact resonance of a dipole by finding the point of minimum SWR. Until this measurement issue is resolved, there would seem to be little benefit to seeking an explanation of why the formula appeared not to work. It is probably too late now, but if you had used an impedance bridge (MFJ or Autek, for example) you could have found resonance at the point of zero reactance. All within the limits of the instruments, of course. Some of the posts suggest other reasons why the formula might not work, but it is not yet evident to me that it didn't work. Sorry my earlier post was not more clear. (I'm even sorrier for this post if you actually used an impedance bridge! Hi.) 73, Chuck NT3G |
So the common, and valid, practice is to prune
the antenna for lowest SWR meter reading. If you do that, there's no need to worry about where resonance might be. =================================== An excellent description, Roy. There's a minor omission. You omitted to say that the SWR meters is redundant because the actual reading is disregarded. There are other reasons of course. Only a TLI is needed. ---- Reg |
No, the SWR meter is useful in determining the quality of the match. I
was careful to say "SWR meter reading" to distinguish it from the SWR on a transmission line. Roy Lewallen, W7EL Reg Edwards wrote: So the common, and valid, practice is to prune the antenna for lowest SWR meter reading. If you do that, there's no need to worry about where resonance might be. =================================== An excellent description, Roy. There's a minor omission. You omitted to say that the SWR meters is redundant because the actual reading is disregarded. There are other reasons of course. Only a TLI is needed. ---- Reg |
The quality of the match is NOT the SWR on ANY line. Quality is the degree
of conformance to specified requirements. Or how closely the transmitter load matches the required value. The meter does NOT measure the SWR on the line between itself and the antenna. It does NOT measure the SWR on any real line. If only because no other real line exists The meter is redundant. As discussed elsewhere, other quantities which an SWR meter purports to measure are Forward and Reflected power, both at the same place and same time. Which are just as imaginary as the transmission line the meter assumes they exist on. Imaginary quantities can be useful at times. But at least the name of the imaginary number "j" does not cause anywhere near as much confusion about what is really happening as the name "SWR Meter" does. Witness the arguments amongst otherwise sane, logical, intelligent, educated people. ---- Reg. ====================================== "Roy Lewallen" wrote in message ... No, the SWR meter is useful in determining the quality of the match. I was careful to say "SWR meter reading" to distinguish it from the SWR on a transmission line. Roy Lewallen, W7EL Reg Edwards wrote: So the common, and valid, practice is to prune the antenna for lowest SWR meter reading. If you do that, there's no need to worry about where resonance might be. =================================== An excellent description, Roy. There's a minor omission. You omitted to say that the SWR meters is redundant because the actual reading is disregarded. There are other reasons of course. Only a TLI is needed. ---- Reg |
Thanks for the information and explanation, Roy.
I do agree that the error introduced by using an SWR minimum as a proxy for zero reactance would not alone account for Ken's results. I also agree that resonance is important here only insofar as it is a definitional element in the nnn/f = L formula. I understand the formula is an approximation. At issue was whether the approximation held constant over a frequency excursion of approximately 5%, not an unreasonable expectation. Ken offered measurements purportedly showing that the approximation did not hold over that range. I will try to show that the uncertainty in the reported measurements does not provide confidence in the conclusion that the "constant" has changed. Whether the "constant" actually changed, and if so why, does not concern me here. Ken initially reported a change to the "constant" (i.e., 234) of 2.32 in going from 9.5665 to 10.1955 MHz by trimming the length of a dipole. He based this on his measurements of SWR used to determine resonant frequency and on his measurements of wire length. What is the required precision in SWR measurement needed to determine resonant frequency? An error in measuring the resonant frequency that would produce a "constant" error of 2.32, is roughly 0.097 MHz. For a 30 meter half-wave dipole, the feed-point SWR ranges from 1.35 at 9.97 MHz to 1.35 at 10.02 MHz, with 1.34 being the minimum at 10.0 MHz (from Reg Edwards' swr_freq.exe program). This SWR range corresponds to a frequency difference of 0.06 MHz. More directly, an SWR measurement error of 0.01 at the antenna could produce an error in computed resonant frequency of 0.06 MHz. Measuring at the end of a transmission line is not likely to improve this relationship. So how likely is it that SWRs were actually read with precisions on the order of 0.01, when the measurements were taken hours or days apart? I can't do that on my Bird. I think only a digital SWR meter can provide that kind of precision. On a calm day. With no inherent lsd jitter. And would it be repeatable after hours or days between measurements? Try it on your antennas. In any case, to achieve the 500 Hz precision reported would require something like five significant digits of SWR measurement. Maybe in a lab with a cutting-edge network analyzer. The issue is not accuracy, of course, but whether SWR measurements of the required precision are feasible. Please, no lectures, folks, on how unimportant such SWR measurements are in normal practice. This is a very unusual application of SWR measurement. What is the effect of errors in length measurement on calculated frequency using the formula? For the length measurement, Ken reports a precision of 0.001 foot. That is 0.012 inches, less than 1/64 inch. I don't believe this kind of precision was achieved either. An inch in a 24 foot length of wire is actually pretty good, considering the difficulty in holding it taut without stretching it, etc. But a one inch difference will produce an equivalent frequency error of 0.03 MHz (by the formula). Simply adding the 0.06 Mhz and 0.03 MHz errors gives a total uncertainty of 0.09 MHz, an amount roughly equal to that required to generate an error of 2.32 in calculating the "constant". I know, I know. These are not rms errors. But they're what we have. In other words, no cigar for for showing that the "constant" changes by 2.32 when the frequency changes by five percent. Perhaps someone can provide a more sophisticated analysis of the measurement uncertainty involved. Perhaps with a better analysis, we will find that we can all go out and measure SWR to within 0.01 so we can calculate resonant frequency to within 500 Hz. Yeah, I know all that precision is imaginary, thanks to umpteen digit calculators. And sure, changing the height of the antenna ends could also explain a change in the "constant". At least the change would be in the reported direction. So there it is! I looked at the problem and tried to understand where the data came from and how they were measured and how much confidence I should give them. Meanwhile, I missed the whole point of the exercise, which seemed to be hypothetical: if one were to do such and such and found that the "constant" changed, what might have caused it. That's what's great about this group. The rest of you pretty much thought about the question and went to what is probably the correct answer. Do the data Ken reported really support that answer? Who knows. Gotta love this stuff. 73 to all and thanks for your patience on this tediously long post. Chuck NT3G |
chuck wrote:
I do agree that the error introduced by using an SWR minimum as a proxy for zero reactance would not alone account for Ken's results. I have only just now read this thread so I am ignorant of anything that has been posted before. I just want to add that, using my MFJ- 259B, the minimum SWR is not usually at the same frequency as the purely resistive reading. Draw your own conclusions from that measurement. -- 73, Cecil http://www.qsl.net/w5dxp |
Sorry to hack just a very short piece out of context, but it's pretty
fundamental to the whole issue. I'll try to comment on the rest later, as time permits. . . chuck wrote: . . . What is the required precision in SWR measurement needed to determine resonant frequency? . . . There is no amount of precision which will make that possible. The resonant frequency cannot be determined by measurement of SWR alone (or more accurately, by an SWR meter reading), except in one very special case: If the resistance at resonance is equal to the SWR meter's characteristic impedance, then a reading of exactly 1:1 indicates resonance at the SWR meter terminals. This is seldom the case for an antenna or antenna system. In other words, if the SWR meter reads 1:1, then the load it's connected to is 50 + j0, which is resonant (assuming a 50 ohm SWR meter). There's no way to determine resonance with any other reading or for any other resistance -- if the meter dips to, say 1.2:1 at some frequency, you can't know if resonance has occured at that frequency unless you have some additional information. Again, in practice, the point of minimum SWR meter reading is often, but not always, very close to the point of resonance (at the SWR meter). And again, there's seldom any need to determine resonance. Roy Lewallen, W7EL |
Ian White, G3SEK wrote:
Now if you measure the complex (R-X) impedance of an antenna across a very wide range of frequencies, and plot it on the Smith chart, you will typically find that it goes in a series of orbits around the chart. For resonance not to occur at minimum SWR, the orbit must not be a circle centered on the pure resistance line. The 'R' in the R+jX impedance looking into my transmission line changes much faster around resonance than does any SWR circle. -- 73, Cecil http://www.qsl.net/w5dxp |
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