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Does Cable Length Affect SWR?

nothing to do with having the last word. i can see by jays lengthy thread posted previously that he is having a problem understanding just exactly what it is that i was referring to. but if any of the rest of you know any situation where a perfectly matched load (antenna) exhibiting a pure resistance with no measurable reactance can produce an swr lower than 1:1 i'd sure like to hear about it. for some reason jay just wants to talk about bandwidth and i'm not sure if he is just trying to change the subject or his comprehension skills are failing him.



again, here's where it all started........



Jay posted........



"However if a significant amount of coax is added to a PERFECTLY or not perfectly MATCHED ANTENNA, then the SWR measurement will show a lower SWR ........"



and here's my question....i thought it was pretty clear....



let's forget about the bandwidth for a moment. HOW CAN A PERFECTLY MATCHED ANTENNA (1:1 or no standing waves) have a LOWER SWR by adding a significant amount of coax?



and what do you mean by this, 211?



even though the guy did not get a answer in plain " Engrish" LOL



i'm not familiar with the Engrish language, even so, fortunately for you, i believe that we all know what you meant.



here it is one more time, in the simplest terms possible, the answer to saints question........



his question was....



Does Cable Length Affect SWR?



here's the answer:



"Yes and No........"



"IN ANY GIVEN CIRCUMSTANCE WHERE THERE IS REFLECTION ON THE LINE DUE TO LOAD MISMATCH, THE LENGTH OF THE FEEDLINE WILL AFFECT OR CHANGE THE STANDING WAVE RATIO."



"IN ANY GIVEN CIRCUMSTANCE WHERE THERE IS NO REFLECTION ON THE LINE OR THE LOAD IS MATCHED, THE LENGTH OF THE FEEDLINE WILL NOT AFFECT OR CHANGE THE STANDING WAVE RATIO."



"THAT'S IT IN A NUTSHELL."



nothing there about bandwidth and it wasn't part of the question either. the answer to the question is not a simple yes or no nor can it be. the answer is all in english, and if the words are too big or their meanings are unfamiliar, i can provide a link to a glossary so that those who do not understand the terms being used can follow along. it's no wonder to me though, with all of the sites on the net claiming to debunk coax length myths and only succeeding in promoting more junk science, i may have to continue repeating this at least as many times as they have repeated their fairy tales until some begin to wake from their stupor.



if we can get beyond this point i'd like to demonstrate some simple and practical applications (including some software) where we can use this information to correct the problems associated with load mismatch and afford ourselves the ability to mitigate the loss while restoring full output from the generator. this also applies to the "setting swr between the driver and final box" thread as well.



i can always move all of this somewhere else, no problem.














</p>Edited by: <A HREF=http://p067.ezboard.com/bworldwidecbradioclub.showUserPublicProfile?gid=freecell>freecell</A> at: 7/21/04 6:57 pm
 
211 yeh whatever?



Do you have z problem or something? I mean, hey, get over it, this is not soemthing to be a smart @#%$ about to me, if you dont like how this place is run, then why are you around, im sick and tired of all these guys who think the forum should be a church and with borring conversations, i mean, as long as you are not fighting, why should a topic stop because a certain someone does not agree?

I guess i have not had much sleep lately and am very easy to be pressed now days. Peddler, you have never bee none to show respect to anyone, not sure why, but thats is your problem not mine. Take it easy and lighten up, this is just a discussion board.


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"Peddler, you have never bee none to show respect to anyone"



I don't see where you feel I'm disrespectful, a smart azz I can be. I just voiced my disapproval through discussion and I feel that's what this board is a discussion forum.



I come here to try to understand a little on radio and not to...

never mind I'll just be quiet and let the intellectuals ramble.<img src=http://www.ezboard.com/intl/aenglish/images/emoticons/frown.gif ALT=":(">


The Peddler...

Website</p>
 
Hello All:



Will I certainly hope this discussion is not being taken personal. Gee whiz its a coax length and swr stuff, not world politics.



It really shouldn't be taken personal.



It should be a place where we can learn, ping pong back and forth ideas and such.



One of the problems that I can see is that I need to post charts and measurements that I have taken to support my views, and I just don't know how to do that! I am sure the others would like to post their measurements also.



Thanks for the e-mail guys.



Jay in the Mojave


</p>
 
"Note that the SWR is constant, but the impedance presented to the generator changes with varying cable length."



i'm with you 100% all the way to this point. however, even though the swr measured at the feedpoint of the load may not indicate any changes, the swr measured at multiple locations (say every 1/8 wavelength) along the line clearly shows that values of e-max/e-min are anything but consistent. this is clearly demonstrated by software programs such as tranalv.exe and other similar applications. it may be that in regard to the smith chart function that the constant swr is that of the load as opposed to the changing swr values exhibited by the line itself under the mismatched load condition. i understand that once the conjugate match has been created at the source that the standing waves can no longer see the source past the match point. this is key to allowing the source to once again continue to deliver maximum power to the load, regardless of the mismatch presented to the line at the opposite end. furthermore, whether or not the input impedance is corrected to allow this is a moot point because in either event the load ultimately absorbs and radiates all power delivered to it by the source.



the question then becomes whether or not we want to disregard small amounts of load-mismatch reflection and ignore the correlating reductions in source output levels or if we want to correct for them to offset the same.



all of this does however explain why it is that when the swr meter is placed in multiple points in any load-mismatched line that different readings are observed. the meter is not being "fooled" as so many are quick to explain, it is merely demonstrating that the swr (e-max/e-min) all along the line is constantly changing under the aforementioned load mismatched conditions.


</p>Edited by: <A HREF=http://p067.ezboard.com/bworldwidecbradioclub.showUserPublicProfile?gid=freecell>freecell</A> at: 7/22/04 1:27 pm
 
Please point us to an online source for info on the dynamic impedance of coaxial cable other than something from your site at firecommunications.Google comes up with tons of stuff about the dynamic impedance of solid state devices but nothing about the dynamic impedance of coaxial cable.I would have thought there would be at least something unless of course Google is not a good enough search engine.<img src=http://www.ezboard.com/intl/aenglish/images/emoticons/happy.gif ALT=":D">


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Garth 9VE01 PE993 CDX993 Learn from others mistakes.You can not live long enough to make them all yourself.</p>
 
Cut and pasted from the following link.



www.epanorama.net/documen...dance.html







How does coaxial cable chacteristics define the impedance ?



The length has nothing to do with a coaxial cable impedance. Characteristic impedance is determined by the

size and spacing of the conductors and the type of dielectric used between them. For ordinary coaxial cable

used at reasonable frequency, the characteristic impedance depends on the dimensions of the inner and

outer conductors, and on the characteristics of the dielectric material between the inner and outer

conductors.



The following formula can be used for calculating the characteristic impedance of the coaxial cable: ( formula

taken from Reference Data for Radio Engineers book published by Howard W. Sams & Co. 1975, page

24-21)



impedance = (138 / e^(1/2)) * log (D/d)



Where:



log = logarithm of 10

d = diameter of center conductor

D = inner diameter of cable shield

e = dielectric constant (= 1 for air)



In a nut shell the characteristic impedance of a coax cable is the square root of (the per unit length

inductance divide by the per unit length capacitance). For coaxial cables the characteristic impedance will be

typically between 20 and 150 ohms. The length of the cable makes no difference whatsoever in regard to

the characteristic impedance.





Please note the referance to the formula which does not contain any value for length.



The length of coax DOES NOT affect its impedance nor does it change the impedance of the antenna which it is connected to.It is part of the overall system and therefore the length will affect what impedance is transposed at the tx end but the impedance of the cable remains constant as does the impedance that it is connected to (antenna).Either this is true or Freecell is right and the rest of the worlds engineers and physicists are wrong.



Jeeeesshh! Such a simple question but what a drawn out reply.




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Garth 9VE01 PE993 CDX993 Learn from others mistakes.You can not live long enough to make them all yourself.</p>Edited by: <A HREF=http://p067.ezboard.com/bworldwidecbradioclub.showUserPublicProfile?gid=qrn>QRN</A> 
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at: 8/21/04 4:23 pm
 
The impedance of the cable does not change but the input impedance to the cable can and does change.There is the differance.


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Garth 9VE01 PE993 CDX993 Learn from others mistakes.You can not live long enough to make them all yourself.</p>
 
I think that the definition of the term "Characteristic Impedence" needs to be looked at more closely.



"The characteristic impedance (Z0) of a transmission line is the resistance it would exhibit if it were infinite in length...Characteristic impedance is purely a function of the capacitance and inductance distributed along the line's length, and would exist even if the dielectric were perfect (infinite parallel resistance) and the wires superconducting (zero series resistance). " Also by definition, the term Charactistic Impedance of Coax applies ONLY to the coax and not to other factors introduced to the coax such as an RF signal. By definition, the characteristic impedance of coax is THEORETICAL because it implies an infinite length of coax.



In a system, if the termination impedance does not match the source and cable's characteristic impedence, we all know that standing waves are introduced. When used at 1/4 wave multiples, the coax can function as an impedence transformer, which can allow you to match mismatched impedances. However using coax as an impedance transformer does not mean the coax's characteristic impedance has changed....it can't change becuase it's a measurement that is made without any source or termination. The dynamics only exist when describing a whole system.



I suggest a read of the following text. Make sure and read forward until the chapter ends.



www.allaboutcircuits.com/vol_2/chpt_13/3.html



Moleculo






</p>
 
We're getting hung up on symantics because you're using the terms incorrectly. By definition, nothing changes about the characteristic impedance of the coax whether it has a load or not. That is the main point that everyone is disagreeing with you about. We all agree that the source may "see" a different impedance than the characteristic impedance of the coax when presented with a mismatched load, but that doesn't mean the characteristic impedance of the coax has changed. The charactistic impedance of coax is not dynamic. The impedance that the source "sees" is dynamic when there's a mismatch. Yes, it's symantics.



Moleculo


</p>
 
Freecell :you're wrong qrn, not only has the value changed at the input to the feedline, the voltage (E) to current (I) vector relationship is changed along the entire length of the line. E / I phase vector relationships don't simply and abruptly alter and disfigure themselves just because they've come to the end of the line.



and if either of you had ever actually made the necessary measurements you would know that. as a matter of fact, if either of you would have ever bothered to make the required measurements then you wouldn't be in here arguing with me.



Your right.I guess I must have slept through Big Rig CB radio and antenna installation class.I had to settle on commercial broadcast systems with paultry multi-tower phased AM arrays and feed systems.Not nearly as much cable theory in that.I'm done beating a dead horse.See Y'all in another thread.


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Garth 9VE01 PE993 CDX993 Learn from others mistakes.You can not live long enough to make them all yourself.</p>
 
Freecell, we understand how to solve your word problems. But doing so doesn't prove your point. You said it the same way I did, which is that there will be a range of values "seen" on the feedline. What is "seen", and what "is" the characteristic impedance of the line can be two different things. You're making the leap that nobody else makes which is you're beliveing that because the source "sees" a different impedance, then obviously the characteristic impedance has changed.



At any rate, everybody's just going 'round in circles now...kind of like a Christian trying to convince a Jewish fellow that Jesus is the Messiah and the Jewish guy trying to convince the Christian that Jesus was just a man. Never gonna happen...





Moleculo


</p>
 

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