#1482

Episode 1482
using a 1/4 wave stub.
previous video: • #902 What is a 1/4 Wav...
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[@johnwest7993]

Thanks for covering the things hams don't generally understand, but need to. They read about this stuff, but it doesn't register until they actually see it in action, because quite frankly, to anyone who doesn't really grasp RF yet, it's magic. But once they see this effect in action they can finally understand repeater cans, (duplexers,) and some notch filters. You do a great service. Thank you. I refer people to your channel. You have exactly the right degree of sophistication and demonstration in your presentations for the hobbyist to fully understand what the heck is going on, without them having to stare into the pages and chapters of a calc text book and then a Field Theory text. As you clearly know, that's kind of a difficult balancing act for a mentor. You do it well. You're a true Elmer.

[@willthecat3861]

An open stub acts like a series resonant circuit. (A shorted stub acts like a parallel resonant circuit.) ... so a resonator, and an anti-resonator. These are very fundamental physical phenomena... everyone who observes their natural environment understands them. ff you've picked up a guitar, or banged a pot lid, you intuitively understand them.

[@nestorkalambay3894]

You are a prolific video maker. I really like your content. I am learning so much. Thank you for your good work.

[@jamesmauer7398]

Cool demo - the frequency shift when shorted was my favorite part.

[@HA7DN]

Something to add - this filter has an "undexpected" instertion loss that could be ~1.5dB, and it's not all "that" narrow, but it's still quite useful sometimes.

[@lucgondim]

I love seen this using as decimals units. ;)

[@johnleclair663]

Curses to you - On a nice relaxing Saturday morning, I am finding myself googling dielectric Constants, permeability constants and coaxial cable equations and now have the HP 50G turned on on the sofa (after searching for new batteries) and a pencil and paper reliving General Physics 3. My wife is laughing at me. Thanks! 😊

[@johnwest7993]

Perfect!

[@rfburns5601]

@ 4:03 - Woohoo!! - Thanks for the shoutout! Signed: A Pedant

[@rickwise9910]

Excellent video! As John West pointed out, this is one of these things that you learn later. You start electronics, and you learn signals follow conductors, air is a gap, a wire or cable need not be considered part of a circuit. Then you learn all the corner cases, what really happens with high voltage, high current, high frequency, tiny signals, etc.

[@jj74qformerlyjailbreak3]

I think I understand about 1/4 of what you are saying. After a little tuning I ran another cycle at 2x speed and now I am receptive to the concept. Thank you for all that you do.

[@byronwatkins2565]

More explicitly wavelength has units m/cycle. Period has units s/cycle. We generally omit the 'cycle' because it is implied.

[@IMSAIGuy]

but don't it make life much easier if you keep those cycles in your head

[@agnelomascarenhas8990]

The inductance per unit length, capacitance per unit length are related to the Charateristic Impedance Z0 and also velocity in the transmission line. You could measure inductance with a short circuit and capacitance with an open ciruit to evaluate Z0, velocity.

[@HouseholdDog]

Love the maths. It essentialy ended up being unfalsifiable.

[@richardl6751]

You see multiples dips at odd multiples of a quarter wave: 1/4, 3/4, 5/4 (1-1/4), etc.

[@rfburns5601]

When I worked in broadcast, we used to get the Belden 8281 double-shielded cable by the thousand foot roll. As soon as it arrived, the director of engineering would have us sweep the cable with a video test signal generator. Video sweep is a displayable picture signal with sync; on the left side of the screen, the frequency is about 50 cycles, on the right side of the screen the frequency is about 8 megacycles. We would crimp an amphenol BNC on each end of the spool of cable. Then we teed one end to the scope input and the sweep signal, and terminated the other end with 75 ohms, and check the scope display for flatness. It was fun to pull the terminator off and watch the dips appear in the signal, then short the end of the cable with needlenose pliers and watch the dips move 1/2 wavelength like @ 10:30

[@blacksmock445]

When I was in the TV repair trade we applied a short coax stub to one of the engineers antenna connections to his bench. We could receive 3 UHF channels around 800 MHz, spaced about 24MHz apart. We cut the coax to notch out the middle one. The engineer concerned was most perplexed, so he went off to consult a colleague. While he was away we shorted the end of the cable, so that he could now receive the centre channel but the other two were now very weak. The next time he left the bench we removed the stub and everything returned to normal. We eventually owned up to what we had done.

[@johnwest7993]

@@blacksmock445, never admit anything. :)

[@IMSAIGuy]

love it

[@xenoxaos1]

My prof always said "show me the physics"

[@bobdoritique7347]

Interresting, merci.

[@aduedc]

Speed of EM wave in coax is different than speed of EM wave in free space. You have to divide speed light by SQRT(ur)

[@jeffreywyke368]

What happens if you terminate the coaxial stub into 50 ohms....you should have shown that. 😉

[@johnwest7993]

I think any resistor across the end of the coax acts as a lower Q shorted stub.

[@willthecat3861]

IMASI Guy did the in vid 905.

[@cowshittt]

You are making a 3dB attenuator basically. Although the input wont be matched in that case.

[@jj74qformerlyjailbreak3]

@@willthecat3861 Thought about making a Truth Table for this channel or indexing it for all my electronic needs. 😂

[@nickcaruso]

Ah, the short has a dip at DC and the open probably does something similar as f->values where the open end is a good radiator, right?

[@GuitarNewby]

It'd be nice to see you play with duplexer cavities 👍.

[@IMSAIGuy]

kzbin.info/www/bejne/aYuWmZWKisx1b6M kzbin.info/www/bejne/pnrHe5SgbN14mJI kzbin.info/www/bejne/pHnQqqxohdpmrqc

[@louiskatzclay]

meters per cycle. Dimensional analysis is a topic worth covering. I wonder if this is efficient enough that if you use three in a row if you could use it instead of cans.

[@IMSAIGuy]

you are right about dimensional analysis. It is often not taught well.

[@sammin5764]

🌟🌟🌟🌟🌟

[@Chiavaccio]

👏👏👍

[@jerrydaugherty3781]

What if you had a variable capacitor on the end?

[@willthecat3861]

For an open circuit stub (driven below resonance) acting as a capacitor... you would be able to variably increase the capacitance by adding the variable capacitor, at the end, in parallel. (It's probably not going to work exactly like one thinks; because, RF is probably going to be radiated.)

[@nickcaruso]

How do we explain the roll-off, the fact that it can't be a perfect notch?

[@johnwest7993]

The loss from the cable itself lowers the circuit Q by reflecting back to the coupling point a bit less than 100% of the signal.

[@jensschroder8214]

but this does not work in imperial units

[@jeffreywyke368]

​​@Ed Luke....... I said NEGLIGIBLE...not non-existant! 1/4 wave stub at 169.6 MHz would be negligible...

[@jeffreywyke368]

If you had used 299,792,458 meters/sec (ACTUAL speed of light), then your answer WOULD have come out to 441.9111 millimeters....almost exactly what you calculated. (even without considering velocity factor, which would be basically negligible in a short stub) Thanks for the teaching. I always enjoy them!

[@argcargv]

The velocity factor will effect short stubs the same as long ones. The velocity factor is just the speed of propagation in the coax.

[@jeffreywyke368]

​@@argcargv I said NEGLIGIBLE...not non-existant! 1/4 wave stub at 169.6 MHz would be negligible...very little "trimming" necessary.

[@argcargv]

@@jeffreywyke368 Ok, I guess you have a different definition of negligible, but if you are off by 30% I would not call that negligible.

[@IMSAIGuy]

you should thank Ed also for teaching, as he is right about velocity factor, you will be off 20-30%

[@johnnz4375]

Isn't cycles per second measured in Hz ?

[@cowshittt]

Cycles per sec is the old school way to say Hz. Fun fact they used to call pico fard micro micro farad :)

[@IMSAIGuy]

if you use Hertz then you probably need to run to a book and see the formula. if you have cycles/sec that is all you need for calculations, you can work the formulas yourself.

[@leetucker9938]

is this like TDR for cable length ?

[@tiftik]

Kinda, it's just not time domain. It's called an open stub filter.

[@IMSAIGuy]

here is a method: kzbin.info/www/bejne/moaspGSkq7asn9k

[@1shARyn3]

pendantics --- that's me 😀

[@bruceblosser384]

Hello! Great Video... I want to set up one antenna for my regular 2 meter antenna and XCEIVER, for use on Repeaters and simplex, and I want to set up another antenna and radio for use just on 144.900MHz, for use as an APRS Digipeater. The two antennas would not be very far apart, so I am thinking that if i used one of these 1/4 wave filter stubs, to get rid of digipeater signal, on the coax of the regular XCEIVER, that would filter out the 144.900MHZ on that receiver. Seems like a great idea! Would using 2 of these 1/4 wave stub notch filters, separated by 1/2 wave of coax work even better? I am thinking it would, but not sure! :) I have a nano VNA H4 now, so I can do some testing, but I thought i would see what you say? Thanks again!