Also would be interesting for you to show that if you place a short at the end of the 1/4 line that it will appear as an open at the far end. (magic disappearing short circuit).
Hi, Alan, I remember there was an argument between you two over the property (ies) of a 1/4 lambda line about more than a year ago. Yes, 1. 1/4 lambda line shorted at the far end is an open line at the near end and it's a parallel tuned circuit 2. 1/4 lambda line with far end open is equivalent to a series tuned circuit apart from exhibiting a short at the near end 3. Doubling the frequency makes it a 1/2 lambda. 4. Sending a momentary pulse along a fairly long cable and measuring the time lapse (to and fro) to calculate the VF (velocity factor) would be the next video, I believe. De VU2RZA
Great demonstration. In the infancy of cable television, everything was analog. To prevent subscribers from watching channels they hadn't paid for, a trap(s) was inserted in the feed line, safely on a pole or locked in a pedestal on the ground. My job required me to test these traps, and the ones made of coax had a notch depth of nearly 80 dB and we're temperature stable from -30F to 130 F. These weren't 1/4 wave stubs but based on that idea. I believe a 1/4 wave stubs would ring throughout the spectrum? Thanks for the demonstration, I enjoyed it.
I never saw one as you described it, but it could have been. Many TVs of that Era had 300 ohm antenna connections. The coaxial cable coming into the home was 75 ohm. A balun was used to match the two impedances. The 1/4 stub was usually connected to the TV antenna connection along with the balun. This just hung down behind the TV.
Very nice and practiacal look. Quarter wave stubs are a very versatile and common thing, for example used to filter out a specific tone or as a DC feed if you don't want your RF there like in a power amplifier. Thanks !
They're also used as impedance transformers, for example in Wilkinson splitters and branchline couplers. Practically made out of microstrip, so it's simple to use different impedance transmission lines.
I would LOVE to see you sweep a stub and show how many DB of rejection it can produce....maybe create a LOW power UHF duplexer application as an example or something...maybe a notch to take a nearby FM or TV signal out or something to that effect...
Used a quarter-wave stub to supress strong FM broadcast signals while listening for NOAA satelites. I'm still not sure how good it is, but seemed to help. I'll measure it when I get my hands on a VNA.
yessss! RF goodness! It would have been nice to see the quarterwave stub terminated in short. That would be seen as an open circuit at the connection point. How about terminating it with a reactance?
A quarter wave line inverts the impedance around the line impedance. Consider a complex load Zload at the end of a transmission line with an impedance of Zline. The equation is: Zinput/Zline = Zline/Zload. Or, solving for Zinput = Zline*Zline/Zload.
Another great explanation! I wonder: could you make a broader notch by paralleling 1/4 wave stubs with slightly different lengths? But I guess then you’d have an impedance mismatch at the input port. (OTOH, we must already have an impedance mismatch, but I guess it’s somehow taken care of by the splitter? - I’m an EE, but completely ignorant of RF electronics :-/)
I’m pretty sure that both an open and a short reflect back all the signal, but 180 degrees out of phase from each other. Can you do the same thing with the far end shorted?
This reminded me of something my Dad used to do. He would cut the coax for his CB base station to 9 or 11 feet (I don't remember which.) As I kid I always wondered why this was of benefit. Eleven feet is about 5.2M, so about 1/2 of the 11 meter band. How much if any would this really help though?
@@IMSAIGuy to Mr No (surprisingly, the creator of this video) and @HeyBirt a half wavelength, considering the velocity factor, at the design frequency would help in that you could measure the actual impedance of the antenna. So this is an advantage when tuning an antenna by not having to hook up your analyser right at the feedpoint of the antenna to see it's impedance. Another advantage is you can also use whatever characteristic impedance coax you want, the system will ignore it. I have heard of operators of the day using 75 ohm coax because of availability. Thirdly, it will eliminate any descrepancies of the coaxial's impedance anyway, so you have your 50 ohm input, 50 ohm load, coax irrelevant (technically at one frequency), happy system, except coaxial losses/attenuation.
Hi, Thanks for the video, very explanatory. I have one question though (it might be a silly one but I am a beginner, so pardon me) : Suppose we have a T junction, with an open stub at the T and I/O ports at both ends, then circuit theory tells me that the reflected wave will travel both ways ? Is it true or am I getting it wrong somewhere? Thanks in advance
Also would be interesting for you to show that if you place a short at the end of the 1/4 line that it will appear as an open at the far end. (magic disappearing short circuit).
Hi, Alan, I remember there was an argument between you two over the property (ies) of a 1/4 lambda line about more than a year ago.
Yes,
1. 1/4 lambda line shorted at the far end is an open line at the near end and
it's a parallel tuned circuit
2. 1/4 lambda line with far end open is equivalent to a series tuned circuit apart from exhibiting a short at the near end
3. Doubling the frequency makes it a 1/2 lambda.
4. Sending a momentary pulse along a fairly long cable and measuring the time lapse (to and fro) to calculate the VF (velocity factor) would be the next video, I believe.
De VU2RZA
On what distance can you place a short on a 60hz line. Please help
@@SpottedEagleOwls the line would be over 1000km long.
@@w2aew Thank you for your help. Can you please help me with a formula to calculate this
@@SpottedEagleOwls Frequency multiplied by the speed of light..
That's a very helpful explanation. I've frequently come across 1/4 stubs in ham radio, and it was never clear what they were doing :)
Great demonstration. In the infancy of cable television, everything was analog. To prevent subscribers from watching channels they hadn't paid for, a trap(s) was inserted in the feed line, safely on a pole or locked in a pedestal on the ground. My job required me to test these traps, and the ones made of coax had a notch depth of nearly 80 dB and we're temperature stable from -30F to 130 F. These weren't 1/4 wave stubs but based on that idea. I believe a 1/4 wave stubs would ring throughout the spectrum? Thanks for the demonstration, I enjoyed it.
80s-era cable TV boxes had a 'u' of heavy coax on the back of the box, both ends disappearing into the box. Same type as your traps?
I never saw one as you described it, but it could have been. Many TVs of that Era had 300 ohm antenna connections. The coaxial cable coming into the home was 75 ohm. A balun was used to match the two impedances. The 1/4 stub was usually connected to the TV antenna connection along with the balun. This just hung down behind the TV.
Very nice and practiacal look.
Quarter wave stubs are a very versatile and common thing, for example used to filter out a specific tone or as a DC feed if you don't want your RF there like in a power amplifier.
Thanks !
Excelent demonstration for wave reflection, Nice
Great demo, thanks.
What a great explanation. Awesome video!
They're also used as impedance transformers, for example in Wilkinson splitters and branchline couplers. Practically made out of microstrip, so it's simple to use different impedance transmission lines.
Such a good demonstration
I would LOVE to see you sweep a stub and show how many DB of rejection it can produce....maybe create a LOW power UHF duplexer application as an example or something...maybe a notch to take a nearby FM or TV signal out or something to that effect...
Used a quarter-wave stub to supress strong FM broadcast signals while listening for NOAA satelites. I'm still not sure how good it is, but seemed to help. I'll measure it when I get my hands on a VNA.
Thank you very much
So another way to create a TDR is by playing with frequencies until we find the 180 degrees and then do the math
Awesome!
yessss! RF goodness! It would have been nice to see the quarterwave stub terminated in short. That would be seen as an open circuit at the connection point. How about terminating it with a reactance?
A quarter wave line inverts the impedance around the line impedance. Consider a complex load Zload at the end of a transmission line with an impedance of Zline. The equation is: Zinput/Zline = Zline/Zload. Or, solving for Zinput = Zline*Zline/Zload.
Another great explanation!
I wonder: could you make a broader notch by paralleling 1/4 wave stubs with slightly different lengths? But I guess then you’d have an impedance mismatch at the input port. (OTOH, we must already have an impedance mismatch, but I guess it’s somehow taken care of by the splitter? - I’m an EE, but completely ignorant of RF electronics :-/)
Thank you for the video but i have a question what is the bandwidth of the filter if we assume that shorted stub is acting as band pass filter
ua-cam.com/video/WzfVWoNq3t8/v-deo.htmlsi=i-u6ew8oWD-8FXbu
I’m pretty sure that both an open and a short reflect back all the signal, but 180 degrees out of phase from each other. Can you do the same thing with the far end shorted?
What about the voltage and current at each point? Power transfer?
This reminded me of something my Dad used to do. He would cut the coax for his CB base station to 9 or 11 feet (I don't remember which.) As I kid I always wondered why this was of benefit. Eleven feet is about 5.2M, so about 1/2 of the 11 meter band. How much if any would this really help though?
no
@@IMSAIGuy to Mr No (surprisingly, the creator of this video) and @HeyBirt a half wavelength, considering the velocity factor, at the design frequency would help in that you could measure the actual impedance of the antenna. So this is an advantage when tuning an antenna by not having to hook up your analyser right at the feedpoint of the antenna to see it's impedance. Another advantage is you can also use whatever characteristic impedance coax you want, the system will ignore it. I have heard of operators of the day using 75 ohm coax because of availability. Thirdly, it will eliminate any descrepancies of the coaxial's impedance anyway, so you have your 50 ohm input, 50 ohm load, coax irrelevant (technically at one frequency), happy system, except coaxial losses/attenuation.
Hi,
Thanks for the video, very explanatory.
I have one question though (it might be a silly one but I am a beginner, so pardon me) :
Suppose we have a T junction, with an open stub at the T and I/O ports at both ends, then circuit theory tells me that the reflected wave will travel both ways
?
Is it true or am I getting it wrong somewhere?
Thanks in advance
sure, it is bi-directional and in both directions one frequency will be attenuated
@@IMSAIGuy ,thanks for clearing that up for me
@IMSAI Guy, I got the Rigol MSO5000 yesterday. Can you point me to the site where I can "improve" its performance? Thanks!
ua-cam.com/video/eaoHYWYLRV0/v-deo.html in the description
@@IMSAIGuy awesome. Thank you.
@@IMSAIGuy All upgraded and recalibrated. Worked great!
1:36 spot the baofeng.