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Does Cable Length Affect SWR?

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  #1  
Old 07-15-2004, 10:20 AM
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Default Re: Does Cable Length Affect SWR?


related discussion is already in progress........



p067.ezboard.com/fworldwi...=1&stop=20


</p>Edited by: <A HREF=http://p067.ezboard.com/bworldwidecbradioclub.showUserPublicProfile?gid=fr eecell>freecell</A> at: 7/15/04 12:11 pm


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  #2  
Old 07-15-2004, 04:33 PM
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Default Re: Does Cable Length Affect SWR?

I,m no tech but I will say that if the equipment is 50 ohm and the antenna is 50 ohm then the cable length makes no difference. I have run length from 5 ft to 25 ft in an odd assortment of length with very little or no SWR change.



I will get lambasted for this but Oh Well.



Jim


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Old 07-15-2004, 04:38 PM
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Default Re: Does Cable Length Affect SWR?

your main concern is to have the antenna matched down low with a MFJ meter, this is the best way to get it perfect. Different length coax is kinda like fine tuning when you try different length's in line, you may see a 1.5 with 12 and then maybe a 1.1 with a 18, you never know what your vehicle and antenna will like untill you play around with it enough. Also, when adding a antenna you want the most solid ground you can get on the vehicle. hopes this helps a tad.




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Old 07-15-2004, 04:59 PM
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Default Re: Does Cable Length Affect SWR?

Hello Saint7ds:



These are really good questions.



A few things have too be qualified to be able to answer your questions, since there is a lot of varialbles to all this coax length stuf.



If the antenna is matched to the coax, then it is said to be flat or have no SWR. In this case adding or subtracting coax is not a big deal.



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 a wider bandwidth. The reason the SWR will show a lower value, and the bandwidth will show to be wider, is the added loss in the coax. As the coax is made significantly longer, then less energy will gets to the antenna and less energy will return as Reflected energy or SWR. This is a old trick to make antennas look like they have a very good SWR and wide bandwidth.



This can be proven with a antenna analyzer or SWR Meter, by measuring the SWR and bandwidth right at the antennas connector. Then taking say 50 or 100 Ft of coax and measureing the SWR and Bandwidth again. You will see a differance. And this is a normal thing that happens with all antennas.



When a length of coax is slightly made longer or shortened, and diffferent SWR is measured, this indiactes that the coax is now radiating energy from the outside of the shield. So in this case there will be a length of coax that will trick or indicate the SWR meter into showing a lower SWR. But this also indicates that the antenna end coax is not properly grounded, somehow. Or even the design of the antenna will cause such radiation on the coax shield.



I think you have a good handle on it all. But I have NOT read any one book or heard any one person tell me how all the different variables and coax lengths are supposed to work and why. Welcome to the club.



If you have a antenna analyzer, its great to go out and experment with all the different antennas and such. Adding coax lengths and seeing what happens will be a some fun. But document your measurements and list the antennas configurations, so that you won't have to repeate measurements. And you will learn a lot.



Obviously I have not covered it all, but neither has any one else!



Jay in the Mojave



www.a1antennas.com


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Old 07-15-2004, 08:23 PM
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Default Re: Does Cable Length Affect SWR?

from: Antennas>Does Cable Length Affect SWR?



saint7ds,



"I still haven't been given that one definite answer."



there isn't one. unless of course there's only one possible matching condition that can exist between the line and the load. but we all know that's not the case.



either one of two conditions exists between the line and the load, either the line is matched to the load or it isn't. noting the latter of the two, if the line is not matched to the load then reflection or standing waves exist on the line. as long as the line exists in this condition, changing the length of the line will alter the input impedance looking into the transmitter end of the line. this being the case, we can alter the length of the line and by doing so we can change the input impedance looking into the line at the transmitter end (by taking advantage of the wide range of varying impedances presented in a line with reflection / standing waves) and if we can present a value of impedance at this point which more closely matches the source or generator output impedance then the reduction in generator output due to the mismatch reflection created at the other end of the line can be eliminated. full power output is restored. this same function can be performed by inserting an lc network between the output of the source / generator and the input end of the line.



then again, that is all that coaxial cable actually is, just an ongoing series of lumped lc circuits. when the line is flat then the characteristic impedance of the line is solely determined by the physical properties of the conductors, the spacing between the conductors and the dielectric material used to separate them.



when reflection exists on the line, upsetting the normal voltage-to-current E/I vector relationship, the physical properties of the line alone are no longer sufficient to maintain a constant impedance all along the line. not only does the impedance of the line change, it presents a wide range of complex impedance values all along the line. the line now assumes the dynamic properties inherent in any line that is not terminated in its characteristic impedance.


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  #6  
Old 07-18-2004, 08:21 AM
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Default Re: Does Cable Length Affect SWR?

This is some interesting reading,

www.loudcrowd.com/loudcro...s/coax.htm


The Peddler...

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  #7  
Old 07-18-2004, 04:13 PM
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Default Re: Does Cable Length Affect SWR?

"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 ........"



let's forget about the bandwidth for a moment. how can a perfectly matched antenna (1:1 or no standing waves) reflect a LOWER SWR by adding a significant amount of coax?



LOL



this was the original question:



"Does Cable Length Affect SWR?"



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.


</p>Edited by: <A HREF=http://p067.ezboard.com/bworldwidecbradioclub.showUserPublicProfile?gid=fr eecell>freecell</A> at: 7/18/04 4:14 pm

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Old 07-18-2004, 09:38 PM
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Default Re: Does Cable Length Affect SWR?

Jeez Freecell...gettin a little nit-picky on the sentence structure and grammar there. If you wanna play that game...yours wasn't much better. You said, "how can a perfectly matched antenna (1:1 or no standing waves) reflect a LOWER SWR..". An antenna doesn't "reflect ...SWR" ; SWR or VSWR is a measurement that tells you how much reflection exists. LOL!!



OK, I'm pokin' a little fun at ya Freecell, but my main point is sometimes we get a little too narrowly focused on picky words and overlook the intent of another's message.





Moleculo


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