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Shorted 1/4 wave stub ?
I know that a shorted 1/4 wave stub exhibits a very high impedance. But
for the 2nd harmonic it's a 1/2 wave stub and exhibits a very low impedance or a short. There are claims that this can be used to filter the even harmonics. Shorts can't diisipate power and must reflect, so how does a stub work? -- 73 Hank WD5JFR |
"Henry Kolesnik" wrote in message ... I know that a shorted 1/4 wave stub exhibits a very high impedance. But for the 2nd harmonic it's a 1/2 wave stub and exhibits a very low impedance or a short. There are claims that this can be used to filter the even harmonics. Shorts can't diisipate power and must reflect, so how does a stub work? 73 Hank WD5JFR Put a Tee on a coax line and connect the stub there. At the H2 there is a short and reflectinonback up-stream. Common filters you see everywhere are "reflective" by nature / design as well. They have other than Zo off frequency and therefore reflect the unwanted stuff back to the source. It has to be able to handle it. Help any? -- Steve N, K,9;d, c. i My email has no u's. |
I know that a shorted 1/4 wave stub exhibits a very high impedance. But
for the 2nd harmonic it's a 1/2 wave stub and exhibits a very low impedance Shorts can't diisipate power and must reflect, so how does a stub work? Hi Hank, A 1/2 wave shorted stub looks like a series resonant circuit. It makes a good notch filter, which is one of the Amateur applications that I have used. You are right, they don't dissipate any power since Power=I**2*R and R equals zero. 73 Gary N4AST |
Hi Hank, A 1/2 wave shorted stub looks like a series resonant circuit.
It makes a good notch filter, which is one of the Amateur applications that I have used. You are right, they don't dissipate any power since Power=I**2*R and R equals zero. 73 Gary N4AST ============================ R is not zero. It can be several ohms. And lots of amps can flow. Just to add a 1/2-wave shorted stub also makes a mess of the tuning of the remainder of the antenna system at all other frequencies. It must be designed "in". |
R is not zero. It can be several ohms. And lots of amps can flow.
OK Reg, but in this ideal rf environment we Amateurs live in R is zero. Just to add a 1/2-wave shorted stub also makes a mess of the tuning of the remainder of the antenna system at all other frequencies. It must be designed "in". That should be fairly obvious to the most casual observer. :-). In the receive applications that I have used a 1/2 wave shorted stub as a notch filter, I can assure you I did not have lots of Amps flowing. 73 Gary N4AST |
Henry Kolesnik wrote:
I know that a shorted 1/4 wave stub exhibits a very high impedance. But for the 2nd harmonic it's a 1/2 wave stub and exhibits a very low impedance or a short. There are claims that this can be used to filter the even harmonics. Shorts can't diisipate power and must reflect, so how does a stub work? Consider the following configuration: Source----ideal 1 WL feedline-----------+-----matched load | |Stub | | open If the stub is 1/4WL, the forward voltage and reflected voltage in the stub is the same as on the feedline. At the mouth of the stub, they are 180 degrees out of phase and superpose to zero volts which obeys Ohm's law and delivers zero power to the load. Double the frequency. That makes the stub 1/2WL. The forward voltage and reflected voltage are essentially the same as in the 1/4WL stub but this time they are in phase and superpose to a maximum value which obeys Ohm's law and delivers maximum power to the load. Absolutely nothing except superposition and interference happens at the mouth of a stub. All the reflected action happens at the physical open circuit. Virtual impedances are only a V/I ratio and CANNOT cause reflections. Absolutely no reflections are happening at the mouth of the stub (unless a physical impedance discontinuity exists there). Consider this. If a stub really presented an infinite impedance, you could simply remove it and nothing would change. Something inside the stub happens to cause that infinite virtual impedance. Hint: Stubs have a near-infinite SWR. To understand that assertion, consider how much voltage exists at a voltage minimum inside a stub. It is nearly zero. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
JGBOYLES wrote:
Hi Hank, A 1/2 wave shorted stub looks like a series resonant circuit. It makes a good notch filter, which is one of the Amateur applications that I have used. You are right, they don't dissipate any power since Power=I**2*R and R equals zero. Stubs dissipate the maximum amount of power possible since the SWR on a stub is nearly infinite. If you don't believe it, hang a Bird Wattmeter halfway down a stub. The ratio of Vmax/Vmin and Imax/Imin is very high. If the stub were lossless, the SWR inside a stub would be infinite. After all, a stub is merely a shorted or open piece of transmission line. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
JGBOYLES wrote:
That should be fairly obvious to the most casual observer. :-). In the receive applications that I have used a 1/2 wave shorted stub as a notch filter, I can assure you I did not have lots of Amps flowing. Have you ever measured it? Have you any idea of the magnitude of current measurable at the shorted point in a stub during transmit? -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
Have you ever measured it? Have you any idea of the magnitude of
current measurable at the shorted point in a stub during transmit? No Cecil I don't, should I? The only experience I have with a shorted 1/2 wave stub is in receive applications. I have swept them with a homebrew spectrum analyzer and tracking generator and can say they look like series resonant circuits. At the micro watt level will I still have several Amps in the stub? How would you measure the current in the stub during transmit using equipment the average ham (like me) has laying around the shack ? 73 Gary N4AST |
JGBOYLES wrote:
Have you ever measured it? Have you any idea of the magnitude of current measurable at the shorted point in a stub during transmit? No Cecil I don't, should I? The only experience I have with a shorted 1/2 wave stub is in receive applications. I have swept them with a homebrew spectrum analyzer and tracking generator and can say they look like series resonant circuits. At the micro watt level will I still have several Amps in the stub? Of course not for receive. Here's the statement to which I was objecting: "... they don't dissipate any power since Power=I**2*R and R equals zero." R is not zero half-way into a shorted 1/2WL stub. For a lossless stub, R is infinite at the half-way point. Stubs have considerable I^2*R losses during transmit. Of course, if you are using them to notch filter out unwanted receive signals, you want them to be lossy. How would you measure the current in the stub during transmit using equipment the average ham (like me) has laying around the shack ? Wrap ten turns of pickup wire on a ferrite toroid. Use a dummy load to calibrate it by running the dummy load wire through it. Then install the shorted stub wire through the ferrite toroid and make the measurement. Roy Lewallen describes how to measure RF current in his article in "The ARRL Antenna Compendium", vol 1, in "Baluns: What They Do and How They Do It." Or install a one ohm resistor at the stub short and measure the voltage. An oscilloscope will do. You don't need super high accuracy. For the following stub, we can estimate the value of the current at the short in the stub assuming a lossless line. The source is a 100W Signal Generator equipped with a perfect Circulator and Load 100W SGCL----50 ohm feedline-----------+---50 ohm load | |1/2WL Stub |50 ohms | short No current reaches the load. The forward current is 1.414 amps and the reflected current is 1.414 amps on the feedline and inside the stub. 100 watts of reflected power is being dissipated in the circulator load resistor. The RMS current at the short is 2.828 amps, the in-phase sum of the forward and reflected currents. The peak-to-peak current at the short is 8 amps. The only reflection point in the entire above system is at the short at the end of the stub. No reflections occur at '+'. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
"Cecil Moore" wrote in message ... JGBOYLES wrote: Hi Hank, A 1/2 wave shorted stub looks like a series resonant circuit. It makes a good notch filter, which is one of the Amateur applications that I have used. You are right, they don't dissipate any power since Power=I**2*R and R equals zero. Stubs dissipate the maximum amount of power possible since the SWR on a stub is nearly infinite. If you don't believe it, hang a Bird Wattmeter halfway down a stub. The ratio of Vmax/Vmin and Imax/Imin is very high. If the stub were lossless, the SWR inside a stub would be infinite. After all, a stub is merely a shorted or open piece of transmission line. -- 73, Cecil http://www.qsl.net/w5dxp I would have to agree Cecil. The point is proven by the fairly shallow notch afforded by a single stub ( approx -20dB at VHF). I can do better with a quality L-C combination. Dale W4OP |
Uh.. I'm gonna do this anyway...
"Cecil Moore" wrote in message ... Henry Kolesnik wrote: ...shorted 1/4 wave stub... for the 2nd harmonic it's a 1/2 wave stub and exhibits a very low impedance or a short. There are claims that this can be used to filter the even harmonics. Shorts can't diisipate power and must reflect, so how does a stub work? Consider the following configuration: Source----ideal 1 WL feedline-----------+-----matched load | |Stub | | open Oops! Time to wipe the CRT clean... Henry wants to discuss a SHORTED stub...reset. end. stop. My original comments hold. Harmonics are "reflected" or however you want to look at the fact that they don't make it to the "matched load". ....... Unfortunately, I continue... Assuming Cecil's comments will be valid if I swap the 1xF (1/4W) and the 2xF (1/2W) comments, I proceed... If the stub is 1/4WL, [... RESPONDIND as though this was the 1/2W remark...] At the mouth of the stub, they are 180 degrees out of phase and superpose to zero volts which obeys Ohm's law and delivers zero power to the load. Yea, ok, so a little sneaks by since it ain't lossless. Double the frequency. That makes the stub 1/2WL. [...RESPONDING as though this was the 1/4W remark...] The forward voltage and reflected voltage are essentially the same ... they are in phase and superpose to a maximum value which obeys Ohm's law and delivers maximum power to the load. In my mental model, a hi Z across the line. Absolutely nothing except superposition and interference happens at the mouth of a stub. All the reflected action happens at the physical open circuit. Virtual impedances are only a V/I ratio and CANNOT cause reflections. Absolutely no reflections are happening at the mouth of the stub (unless a physical impedance discontinuity exists there). \\ Nit alarm on\\ (Though I consider this a clarification in an attempt to simplify the explanation and remove the extra complexity added by the double terms) I consider superposition and interference to be the same thing. Superposition is the addition of two quantities (voltages ot currents). They can be of similar sign or different. If they are closer to 'in-phase', then they add to a greater value and if they are closer to 180 degree phase difference, then they add to a smaller value. You can call that "algebraiclly add" if it makes you feel better. That is, a +1 and +1 _add_ to +2. A -1 and a +1 _add_ to 0.0 \\ end nit alarm \\ This IS a correct way to look at it. Think of the "main" line" (the one going to the "matched load" above) as being the thing which is "added to" the shorted system. Since it is added to a low Z point (on this high VSWR line), little goes down that path to the desired load. Any 50 ohm "thing" put in parallel at the point of the lowest voltage of the standing wave, sees a low/zero voltage, therefopre nothing goes there. Consider this. If a stub really presented an infinite impedance, you could simply remove it and nothing would change. At the fundamental (_OPEN_ 1/4W stub, remember) this IS TRUE. Are you saying that the 1/4W _shorted_ stub addition or removal makes a BIG difference at the fundamental -I think not, or you still talking about the 1/4W _open_ stub which kills the system at the fundamental when added. |
Steve Nosko wrote:
Oops! Time to wipe the CRT clean... Henry wants to discuss a SHORTED stub...reset. end. stop. Sorry, a shorted 1/2WL stub exhibits a short at the fundamental frequency and an open when it is 1/4WL at 1/2 the frequency of the first. Same principles apply. Absolutely nothing except superposition and interference happens at the mouth of a stub. All the reflected action happens at the physical open circuit. Virtual impedances are only a V/I ratio and CANNOT cause reflections. Absolutely no reflections are happening at the mouth of the stub (unless a physical impedance discontinuity exists there). \\ Nit alarm on\\ (Though I consider this a clarification in an attempt to simplify the explanation and remove the extra complexity added by the double terms) I consider superposition and interference to be the same thing. So do I. I probably should have said superposition/interference, which logically equals (superposition AND interference) since ONE AND ONE = ONE. Consider this. If a stub really presented an infinite impedance, you could simply remove it and nothing would change. At the fundamental (_OPEN_ 1/4W stub, remember) this IS TRUE. Are you saying that the 1/4W _shorted_ stub addition or removal makes a BIG difference at the fundamental -I think not, or you still talking about the 1/4W _open_ stub which kills the system at the fundamental when added. Any stub that presents an infinite impedance should be able to be removed and without anything changing. You have an infinite impedance before the removal and you have an infinite impedance after the removal, so nothing changes when you remove the stub. (Hint: Devil's advocate reasoning applied) Of course, something changes when one removes the stub - that's the entire point. There are forward components and reflected components flowing in and out of a stub. That's why removing it changes things. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
"Cecil Moore" wrote in message
... Steve Nosko wrote: Oops! Time to wipe the CRT clean... Henry wants to discuss a SHORTED stub...reset. end. stop. Sorry, a shorted 1/2WL stub ... Same principles apply. That's why I went ahead and continued on the "complementary situation" [...] Any stub that presents an infinite impedance should be able to be removed and without anything changing. You have an infinite impedance before the removal and you have an infinite impedance after the removal, so nothing changes when you remove the stub. (Hint: Devil's advocate reasoning applied) Your "Devil's advocate "meaning escapes me... Of course, something changes when one removes the stub - that's the entire point. There are forward components and reflected components flowing in and out of a stub. That's why removing it changes things. Yea, but in the practical sense, it does not change anything at the fundamental (desired) frequency [[high Z stub, power still to the load]]. It's only when you consider the harmonics, if they are there, that anything important changes, right. So I'm puzzled why you included that part in your comment. -- Steve N, K,9;d, c. i My email has no u's. |
Any stub that presents an infinite impedance should be able to be removed
and without anything changing. You have an infinite impedance before the removal and you have an infinite impedance after the removal, so nothing changes. There are forward components and reflected components flowing in and out of a stub. That's why removing it changes things. Is it because energy is stored in the stub? If the impedance is infinite then no power dissipation is removed from the system. Something is dynamic in nature in this example because if you remove an infinite impedance, nothing will happen. Could the change be due to the fact that the stub is in the near field of the antenna and subject to induced rf currents? If this were the case, then removal of the stub would change things. If the stub presents an infinite impedance, with no external influences, removing it should have no effect. If the stub were replaced with a quality parallel resonant LC circuit in a shielded box, would removing it make any difference? I don't know, I am asking. 73 Gary N4AST |
Henry Kolesnik wrote:
I know that a shorted 1/4 wave stub exhibits a very high impedance. But for the 2nd harmonic it's a 1/2 wave stub and exhibits a very low impedance or a short. There are claims that this can be used to filter the even harmonics. Shorts can't diisipate power and must reflect, so how does a stub work? It works exactly as you stated: "It reflects the second and all even harmonic energy back to the source." |
Steve Nosko wrote:
Yea, but in the practical sense, it does not change anything at the fundamental (desired) frequency [[high Z stub, power still to the load]]. It's only when you consider the harmonics, if they are there, that anything important changes, right. So I'm puzzled why you included that part in your comment. Some people believe that there is no current flowing into or out of a 1/4WL shorted stub. But all they have to do to change their minds is measure the current through the short at the end of the stub. It is sky high. The SWR inside a 1/4WL shorted stub is near infinite, i.e. the square root of Pr/Pf inside a stub is near unity. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
JGBOYLES wrote:
If the stub presents an infinite impedance, with no external influences, removing it should have no effect. Exactly! Therefore, the infinite impedance at the mouth of a stub is a virtual impedance, not a physical impedance. The physical impedance exists at the end of the stub where all the reflections are taking place. Reflections occur only at a physical impedance discontinuity. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
Dave Shrader wrote:
Henry Kolesnik wrote: I know that a shorted 1/4 wave stub exhibits a very high impedance. But for the 2nd harmonic it's a 1/2 wave stub and exhibits a very low impedance or a short. There are claims that this can be used to filter the even harmonics. Shorts can't diisipate power and must reflect, so how does a stub work? It works exactly as you stated: "It reflects the second and all even harmonic energy back to the source." Yes, but only after making a round trip to the short or open at the bottom end of the stub. V/I ratios don't cause reflections. Only physical impedance discontinuities cause reflections. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
"Henry Kolesnik" wrote in message ... I know that a shorted 1/4 wave stub exhibits a very high impedance. But for the 2nd harmonic it's a 1/2 wave stub and exhibits a very low impedance or a short. There are claims that this can be used to filter the even harmonics. Shorts can't diisipate power and must reflect, so how does a stub work? stubs work very nicely. you can get practical stub information at my web site, including how to build a 40m to 15m 3rd harmonic stub filter: http://www.k1ttt.net/technote/techref.html#filters as you may have noticed by now you have kicked the proverbial hornets nest. reflections are a touchy word in this group, usually attracting the endless argument that travels from thread to thread. in time this will deteriorate into name calling and endless argument over reflections, interference, virtual impedances, and a few other topics. |
Dave
Nice site, I like the "white paper" approach as I prefer the info without the glitter. I've only read a few items and I quote: "This is a plot of the attenuation provided by the stub. You can see that it provides about 32db of attenuation at 28.25Mhz. " I've noticed that the literature I've purused indicates that stubs either attenuate or reject. None say reflect! I don't want to get into a discussion of word definitions becasue reflect and feject are close but attenuate is not in the same class. Comments... -- 73 Hank WD5JFR "Dave" wrote in message ... "Henry Kolesnik" wrote in message ... I know that a shorted 1/4 wave stub exhibits a very high impedance. But for the 2nd harmonic it's a 1/2 wave stub and exhibits a very low impedance or a short. There are claims that this can be used to filter the even harmonics. Shorts can't diisipate power and must reflect, so how does a stub work? stubs work very nicely. you can get practical stub information at my web site, including how to build a 40m to 15m 3rd harmonic stub filter: http://www.k1ttt.net/technote/techref.html#filters as you may have noticed by now you have kicked the proverbial hornets nest. reflections are a touchy word in this group, usually attracting the endless argument that travels from thread to thread. in time this will deteriorate into name calling and endless argument over reflections, interference, virtual impedances, and a few other topics. |
Dave wrote:
stubs work very nicely. you can get practical stub information at my web site, including how to build a 40m to 15m 3rd harmonic stub filter: http://www.k1ttt.net/technote/techref.html#filters A fellow ham sent me an email suggesting a use for a shorted 1/4WL stub that had not occurred to me before. Many hams have static electricity problems with dipoles since there is not a DC path between the poles. For a single-frequency dipole, a shorted 1/4WL stub could be installed at the feedpoint to drain off the static electricity without affecting the performance much on the fundamental frequency. It would also aid in the suppression of even harmonics. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
Henry Kolesnik wrote:
Dave Nice site, I like the "white paper" approach as I prefer the info without the glitter. I've only read a few items and I quote: "This is a plot of the attenuation provided by the stub. You can see that it provides about 32db of attenuation at 28.25Mhz. " I've noticed that the literature I've purused indicates that stubs either attenuate or reject. None say reflect! I don't want to get into a discussion of word definitions becasue reflect and feject are close but attenuate is not in the same class. Comments... They indeed do attenuate or reject. Reflections is the method by which stubs accomplish the attenuation or rejection. If one is only interested in implementing stubs, and not interested in how they work, one need not deal with reflections. What one must realize, however, that the SWR inside a resonant stub is as high as it can be. All the reflections take place at the shorted or open end of a stub, i.e. at the physical discontinuity point where rho = |1.0|. I don't have any argument with people who say, "I only want to use stubs. I don't care how or why they work." My argument is with people who falsely describe how they work, e.g. "Since the impedance looking into a shorted 1/4WL lossless stub is infinite, no current flows into or out of the stub." If that were really true, one could remove the stub without changing anything. The net superposed current may be zero but the forward and reflected currents exist there and can be very high values. In fact, I have melted the insulating material at the end of a shorted coaxial stub due to high I^2*R losses. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
yes, that works very nicely also. it does provide a dc ground for otherwise
ungrounded antennas like verticals or dipoles providing a way for static build up to drain safely. "Cecil Moore" wrote in message ... Dave wrote: stubs work very nicely. you can get practical stub information at my web site, including how to build a 40m to 15m 3rd harmonic stub filter: http://www.k1ttt.net/technote/techref.html#filters A fellow ham sent me an email suggesting a use for a shorted 1/4WL stub that had not occurred to me before. Many hams have static electricity problems with dipoles since there is not a DC path between the poles. For a single-frequency dipole, a shorted 1/4WL stub could be installed at the feedpoint to drain off the static electricity without affecting the performance much on the fundamental frequency. It would also aid in the suppression of even harmonics. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
the harmonic is 'attenuated' in that the magnitude of it is reduced when the
stub is in line. i look at it like you could replace the stub with a lumped filter at the same point so the term attenuation makes more sense than reflections or rejections... i don't really care where the harmonic goes, i want to know how much it is attenuated by so i can compare with other types of filters. "Henry Kolesnik" wrote in message m... Dave Nice site, I like the "white paper" approach as I prefer the info without the glitter. I've only read a few items and I quote: "This is a plot of the attenuation provided by the stub. You can see that it provides about 32db of attenuation at 28.25Mhz. " I've noticed that the literature I've purused indicates that stubs either attenuate or reject. None say reflect! I don't want to get into a discussion of word definitions becasue reflect and feject are close but attenuate is not in the same class. Comments... -- 73 Hank WD5JFR "Dave" wrote in message ... "Henry Kolesnik" wrote in message ... I know that a shorted 1/4 wave stub exhibits a very high impedance. But for the 2nd harmonic it's a 1/2 wave stub and exhibits a very low impedance or a short. There are claims that this can be used to filter the even harmonics. Shorts can't diisipate power and must reflect, so how does a stub work? stubs work very nicely. you can get practical stub information at my web site, including how to build a 40m to 15m 3rd harmonic stub filter: http://www.k1ttt.net/technote/techref.html#filters as you may have noticed by now you have kicked the proverbial hornets nest. reflections are a touchy word in this group, usually attracting the endless argument that travels from thread to thread. in time this will deteriorate into name calling and endless argument over reflections, interference, virtual impedances, and a few other topics. |
A fellow ham sent me an email suggesting a use for a shorted 1/4WL stub that had not occurred to me before. Many hams have static electricity problems with dipoles since there is not a DC path between the poles. For a single-frequency dipole, a shorted 1/4WL stub could be installed at the feedpoint to drain off the static electricity without affecting the performance much on the fundamental frequency. It would also aid in the suppression of even harmonics. -- 73, Cecil http://www.qsl.net/w5dxp Very effective way for monoband antennas. The improved version of this is the "Bazooka Balun" which besides above mentioned benefits acts as a balun. I used it on all my monoband antennas. Provides DC path to ground, harmonic supression, RF choke function and balanced to unbalanced feed point conversion. Yuri, K3BU.us www.computeradio.us home of Dream Radio One |
Dave wrote:
the harmonic is 'attenuated' in that the magnitude of it is reduced when the stub is in line. i look at it like you could replace the stub with a lumped filter at the same point so the term attenuation makes more sense than reflections or rejections... i don't really care where the harmonic goes, i want to know how much it is attenuated by so i can compare with other types of filters. Where is the bulk of the attenuation taking place, at/in the stub, or as a V^2/R loss back upstream? (Not a trick question) -- 73, Cecil http://www.qsl.net/w5dxp |
I know they work! One of my reasons for asking the question is I've not
found any mention in the literature of where the "attenuation/rejection/reflection/filter residue" goes. -- 73 Hank WD5JFR "Dave" wrote in message ... the harmonic is 'attenuated' in that the magnitude of it is reduced when the stub is in line. i look at it like you could replace the stub with a lumped filter at the same point so the term attenuation makes more sense than reflections or rejections... i don't really care where the harmonic goes, i want to know how much it is attenuated by so i can compare with other types of filters. "Henry Kolesnik" wrote in message m... Dave Nice site, I like the "white paper" approach as I prefer the info without the glitter. I've only read a few items and I quote: "This is a plot of the attenuation provided by the stub. You can see that it provides about 32db of attenuation at 28.25Mhz. " I've noticed that the literature I've purused indicates that stubs either attenuate or reject. None say reflect! I don't want to get into a discussion of word definitions becasue reflect and feject are close but attenuate is not in the same class. Comments... -- 73 Hank WD5JFR "Dave" wrote in message ... "Henry Kolesnik" wrote in message ... I know that a shorted 1/4 wave stub exhibits a very high impedance. But for the 2nd harmonic it's a 1/2 wave stub and exhibits a very low impedance or a short. There are claims that this can be used to filter the even harmonics. Shorts can't diisipate power and must reflect, so how does a stub work? stubs work very nicely. you can get practical stub information at my web site, including how to build a 40m to 15m 3rd harmonic stub filter: http://www.k1ttt.net/technote/techref.html#filters as you may have noticed by now you have kicked the proverbial hornets nest. reflections are a touchy word in this group, usually attracting the endless argument that travels from thread to thread. in time this will deteriorate into name calling and endless argument over reflections, interference, virtual impedances, and a few other topics. |
Yuri Blanarovich wrote:
Very effective way for monoband antennas. The improved version of this is the "Bazooka Balun" which besides above mentioned benefits acts as a balun. I used it on all my monoband antennas. Provides DC path to ground, harmonic supression, RF choke function and balanced to unbalanced feed point conversion. A 4:1 toroidal balun will also do the same thing - provide a DC path between conductors. -- 73, Cecil http://www.qsl.net/w5dxp |
you can look at it various ways.
1. the wave traveling wave analysis... the harmonic goes down the stub, reflects back, and when it gets back to the transmission line it is 180 degress out of phase with the next cycle so it cancels it at the junction... therefore there is no harmonic to propagate down the line past the junction. this of course will raise the hackles of the anti-reflectionists who will then say there is also no harmonic left to propagate down the stub which means there is a virtual short at the junction, but no way to generate it since nothing can be there to go down the stub. 2. the power analysis. power goes in, power comes out, it all reflects back and forth until the energy becomes infinite and the amp blows up... but of course energy is conserved and momentum must go somewhere so the stub probably walks across the table with each wave reflection. 3. the sinusoidal steady state analysis. this takes the stub and transforms the shorted impedance at the far end back to the junction and then does all calculations as if the real short existed at the junction.... this will of course annoy the reflectionists who will point out in never ending detail how you can't explain tv ghosts, radar, or other transient phenomena this way... of course by assuming the sinusoidal steady state at the start you exclude those systems from this type of analysis, but that won't stop the protests. 4. the s analysis... who know what this will say except cecil who will find some way to steer the discussion over to it. 5. the optical layer analysis... see above. "Henry Kolesnik" wrote in message ... I know they work! One of my reasons for asking the question is I've not found any mention in the literature of where the "attenuation/rejection/reflection/filter residue" goes. -- 73 Hank WD5JFR "Dave" wrote in message ... the harmonic is 'attenuated' in that the magnitude of it is reduced when the stub is in line. i look at it like you could replace the stub with a lumped filter at the same point so the term attenuation makes more sense than reflections or rejections... i don't really care where the harmonic goes, i want to know how much it is attenuated by so i can compare with other types of filters. "Henry Kolesnik" wrote in message m... Dave Nice site, I like the "white paper" approach as I prefer the info without the glitter. I've only read a few items and I quote: "This is a plot of the attenuation provided by the stub. You can see that it provides about 32db of attenuation at 28.25Mhz. " I've noticed that the literature I've purused indicates that stubs either attenuate or reject. None say reflect! I don't want to get into a discussion of word definitions becasue reflect and feject are close but attenuate is not in the same class. Comments... -- 73 Hank WD5JFR "Dave" wrote in message ... "Henry Kolesnik" wrote in message ... I know that a shorted 1/4 wave stub exhibits a very high impedance. But for the 2nd harmonic it's a 1/2 wave stub and exhibits a very low impedance or a short. There are claims that this can be used to filter the even harmonics. Shorts can't diisipate power and must reflect, so how does a stub work? stubs work very nicely. you can get practical stub information at my web site, including how to build a 40m to 15m 3rd harmonic stub filter: http://www.k1ttt.net/technote/techref.html#filters as you may have noticed by now you have kicked the proverbial hornets nest. reflections are a touchy word in this group, usually attracting the endless argument that travels from thread to thread. in time this will deteriorate into name calling and endless argument over reflections, interference, virtual impedances, and a few other topics. |
Henry Kolesnik wrote:
I know they work! One of my reasons for asking the question is I've not found any mention in the literature of where the "attenuation/rejection/reflection/filter residue" goes. An open 1/4WL stub detours all the associated frequency energy into the stub and then reflects it back to the source. Consider the following: XMTR---------feedline-------+----load | | 1/4WL | open | stub The stub causes complete destructive interference between the forward voltage and reflected voltage at the mouth of the stub. Since the net voltage at the mouth of the stub is zero, no current flows into the load. All the current in the feedline is flowing in the stub and is reflected at the open end. The reflected waves head back to the source. Therefore, for lossless feedlines, all the attenuation/interference occurs in the source. Of course, for real-world feedlines, I^2*R losses will occur because of the high SWR at the associated frequency. The effect of the above stub is much like shorting out the load. Shorting out the load increases the dissipation in the source. -- 73, Cecil http://www.qsl.net/w5dxp |
"Cecil Moore" wrote in message ... Henry Kolesnik wrote: I know they work! One of my reasons for asking the question is I've not found any mention in the literature of where the "attenuation/rejection/reflection/filter residue" goes. An open 1/4WL stub detours all the associated frequency energy into the stub and then reflects it back to the source. a new theory! now i'll have to add the traffic theory of waves to the list. just who posts this detour sign? so does all the energy go down to the end of the stub and have to make a u-turn to get back? are you sure they aren't issued a ticket for doing that?? and then again at the junction they have to be directed back to the source, sounds like a job for robo-cop! got to be fast to direct all those waves back and forth! The stub causes complete destructive interference between the forward voltage and reflected voltage at the mouth of the stub. Since the net voltage at the mouth of the stub is zero, no current flows into the load oh, oh! i know this one!! but if the voltage at the mouth of the stub is zero, how does any wave flow down into the stub? there is no voltage there to drive it?? how does the incoming wave know to not flow past the stub connection and get reflected instead?? doesn't that make it a virtual short and just reflect back to the source from there?? but wait, how does the stub get hot then?? where does the current at the end of the stub come from? |
Dave wrote:
"Cecil Moore" wrote: An open 1/4WL stub detours all the associated frequency energy into the stub and then reflects it back to the source. a new theory! Not a new theory. I learned that at Texas A&M back in the 50's. I had a pretty smart RF prof. It's the same thing that happens without a load. XMTR---------feedline---------------open so does all the energy go down to the end of the stub and have to make a u-turn to get back? Energy is normally completely reflected at a physical open circuit in a piece of transmission line. Stubs are no exception to transmission line theory. oh, oh! i know this one!! but if the voltage at the mouth of the stub is zero, how does any wave flow down into the stub? If the standing-wave minimum voltage is zero, why is the current at that exact point at its maximum value. Maybe you should review "Plumber's Delight" beam construction rules and standing-waves. there is no voltage there to drive it?? how does the incoming wave know to not flow past the stub connection and get reflected instead?? Because reflections only occur at a physical impedance discontinuity. There is no physical impedance discontinuity until the open is reached. XMTR---one wavelength feedline---+---1/4WL---open | load Note the load is connected at a "Plumber's Delight" V=0 point. The voltage 1/4WL from the open end is zero. What do you think the current is 1/4WL from the open end? What do you think the voltage is at the open. Sounds like you believe that there is no current or voltage on the entire length of the above since it is an odd number of 1/4WL's long. doesn't that make it a virtual short and just reflect back to the source from there?? Nope, in the above example, there is a virtual short at the XMTR output. Does that mean to you that there's no voltage or current on the entire feedline? Virtual impedances don't cause reflections. On a 50 ohm feedline with an SWR of 2:1, an impedance of 100 ohms repeats every 1/2WL but it doesn't cause any reflections at the 100 ohm points because there is not a physical impedance discontinuity there. -- 73, Cecil http://www.qsl.net/w5dxp |
What if the source doesn't or can't dissipate?
-- 73 Hank WD5JFR "Cecil Moore" wrote in message ... Henry Kolesnik wrote: I know they work! One of my reasons for asking the question is I've not found any mention in the literature of where the "attenuation/rejection/reflection/filter residue" goes. An open 1/4WL stub detours all the associated frequency energy into the stub and then reflects it back to the source. Consider the following: XMTR---------feedline-------+----load | | 1/4WL | open | stub The stub causes complete destructive interference between the forward voltage and reflected voltage at the mouth of the stub. Since the net voltage at the mouth of the stub is zero, no current flows into the load. All the current in the feedline is flowing in the stub and is reflected at the open end. The reflected waves head back to the source. Therefore, for lossless feedlines, all the attenuation/interference occurs in the source. Of course, for real-world feedlines, I^2*R losses will occur because of the high SWR at the associated frequency. The effect of the above stub is much like shorting out the load. Shorting out the load increases the dissipation in the source. -- 73, Cecil http://www.qsl.net/w5dxp |
By how much does the length of the short-circuiting bar at the end of the
1/4-wave stub affect its resonant length? What open-circuit resonant frequency does the stub change to when the short circuiting bar is removed? ---- Reg, G4FGQ |
Yuri Blanarovich wrote:
Very effective way for monoband antennas. The improved version of this is the "Bazooka Balun" which besides above mentioned benefits acts as a balun. I used it on all my monoband antennas. Provides DC path to ground, harmonic supression, RF choke function and balanced to unbalanced feed point conversion. A 4:1 toroidal balun will also do the same thing - provide a DC path between conductors. -- 73, Cecil http://www.qsl.net/w5dxp Not the SAME (thing), but SOME. That is, minus harmonic suppresion. Jus' to be preeeesize :-) Yuri, K3BU.us God Bless President Reagan! RIP Thanks for freeing me and millions of others from behind the Iron Curtain!!! |
Henry Kolesnik wrote:
What if the source doesn't or can't dissipate? Heh, heh, you're on your own for that discussion. I personally believe it is possible for reflected energy to wind up decreasing the power consumption from the DC source that is supplying the finals, which winds up decreasing the load on the 60 Hz power grid, which decreases the oil required from the Middle East, which tends to decrease the possibility of world war, which bodes well for the human race, but that is another thread for another time. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
Reg Edwards wrote:
By how much does the length of the short-circuiting bar at the end of the 1/4-wave stub affect its resonant length? I just twist the ends together. I don't use a bar. What open-circuit resonant frequency does the stub change to when the short circuiting bar is removed? Same resonant frequency - virtual impedance at the input is reversed. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
Yuri Blanarovich wrote:
Thanks for freeing me and millions of others from behind the Iron Curtain!!! .... and from the IRS by lowering the maximum tax rate by 300%? :-) -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
Dave wrote:
"Henry Kolesnik" wrote in message ... I know that a shorted 1/4 wave stub exhibits a very high impedance. But for the 2nd harmonic it's a 1/2 wave stub and exhibits a very low impedance or a short. There are claims that this can be used to filter the even harmonics. Shorts can't diisipate power and must reflect, so how does a stub work? stubs work very nicely. you can get practical stub information at my web site, including how to build a 40m to 15m 3rd harmonic stub filter: http://www.k1ttt.net/technote/techref.html#filters as you may have noticed by now you have kicked the proverbial hornets nest. reflections are a touchy word in this group, usually attracting the endless argument that travels from thread to thread. in time this will deteriorate into name calling and endless argument over reflections, interference, virtual impedances, and a few other topics. We bounce between the Physics of Operation and the Practicalities of Engineering. We have experts on all sides of the issue grin. And, typically, engineers and physicists both use English but can't communicate well grin, again grin But your basic question is how does one answer your question: "Shorts can't dissipate power and must reflect, so how does a stub work?" In practical terms, you have answered your own question! Reflections. In Physics terms, you have to deal with the wave inside the stub and the [V^2(theta) + I^2(theta)] energy storage as a function of position within the stub and the corresponding sources of loss also a function of V^2 and I^2 inside the stub and finally the reflections that exist within the stub caused by the intentional physical short circuit [or open circuit]. So, are you pursuing Physics or Applications type knowledge? |
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