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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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