#902

Рет қаралды 10,867

Күн бұрын

Пікірлер: 46

@SparkyTom1 Ай бұрын

I just passed my extra ham exam and I knew that an open 1/4 wave acts like this but this really helps me understand why an open 1/4 wave stub acts like this. And that 1/4 notch filter video of yours with my NanoVNA further helped my understanding of transmission lines. Thank you!

@clems6989 17 күн бұрын

Transmission lines are quite fascinating!

@w2aew 3 жыл бұрын

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).

@subramanianr7206 3 жыл бұрын

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

@SpottedEagleOwls 3 жыл бұрын

On what distance can you place a short on a 60hz line. Please help

@w2aew 3 жыл бұрын

@@SpottedEagleOwls the line would be over 1000km long.

@SpottedEagleOwls 3 жыл бұрын

@@w2aew Thank you for your help. Can you please help me with a formula to calculate this

@88njtrigg88 Жыл бұрын

@@SpottedEagleOwls Frequency multiplied by the speed of light..

@Lastaii 3 жыл бұрын

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 :)

@embrykendrick4517 3 жыл бұрын

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.

@tunatuna6723 2 жыл бұрын

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?

@embrykendrick4517 2 жыл бұрын

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.

@marka7831 2 ай бұрын

really well done, I will do this for my Saturday morning ham class.

@IMSAIGuy 2 ай бұрын

you might like this one too: kzbin.info/www/bejne/javJh4qlg9Zmqposi=Xp97WDH5o6zhnlyk

@gammaleader96 3 жыл бұрын

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 !

@l_szabi 3 жыл бұрын

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.

@edieharo4186 3 жыл бұрын

Excelent demonstration for wave reflection, Nice

@karlmatthias2698 2 жыл бұрын

Such a good demonstration

@BobHolowenko 3 жыл бұрын

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...

@MrBanzoid 3 жыл бұрын

Great demo, thanks.

@electricerger Ай бұрын

Good explanation. You definitely explain the principle well and the demonstration adds some good motivation for learning it (maybe you could have highlighted that 0V first to help emphasize the motivation).

@HA7DN 3 жыл бұрын

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.

@MrMarkneilson 2 жыл бұрын

What a great explanation. Awesome video!

@jalajchaturvedi472 10 ай бұрын

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

@IMSAIGuy 10 ай бұрын

sure, it is bi-directional and in both directions one frequency will be attenuated

@jalajchaturvedi472 10 ай бұрын

@@IMSAIGuy ,thanks for clearing that up for me

@asunasposibol 3 жыл бұрын

So another way to create a TDR is by playing with frequencies until we find the 180 degrees and then do the math

@pyrokinetikrlz 3 жыл бұрын

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?

@w2aew 3 жыл бұрын

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.

@brucecorbettn9bh398 Жыл бұрын

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?

@moatacemaskar7313 Жыл бұрын

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

@IMSAIGuy Жыл бұрын

kzbin.info/www/bejne/javJh4qlg9Zmqposi=i-u6ew8oWD-8FXbu

@2001pulsar 3 жыл бұрын

What about the voltage and current at each point? Power transfer?

@HeyBirt 3 жыл бұрын

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 3 жыл бұрын

@arconeagain Жыл бұрын

@@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.

@mohamedabdalla6479 3 жыл бұрын

Thank you very much

@DEtchells 3 жыл бұрын

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 :-/)