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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


The Peddler...

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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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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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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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"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=freecell>freecell</A> at: 7/18/04 4:14 pm
 
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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"the length of the feedline will not change the swr it will change the apparent reading depending on what wave length you read the swr at. . just as adding a signifigant length of coax, the resultant loss will look like a lower swr."



the length of the feedline will change the swr and the apparent reading on the meter if the load is not matched to the line. as usual, you failed to indicate the condition of the load in your example. for a matched condition at least these 3 parameters must be satisfied.



the load must present a 50 ohm input resistance/impedance

the load must present 0 capacitive reactance

the load must present 0 inductive reactance



and you can add all the coax you want to in a line that has an swr of *1:1, it isn't going to get any lower. under these conditions* all that is being accomplished is increasing line loss.



as long as the resistance presented by the load is something other than 50 ohms and/or ANY Xc or Xl reactive components exist at the load, your meter will indicate variations as it is placed in different spots in the line.



"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) have a LOWER SWR by adding a significant amount of coax?



........


</p>Edited by: <A HREF=http://p067.ezboard.com/bworldwidecbradioclub.showUserPublicProfile?gid=freecell>freecell</A> at: 7/19/04 8:35 am
 
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and furthermore, any transmission line which is not terminated in its characteristic impedance will act as an impedance transformer. The impedance at the input will be a function of the impedance values (line and load) and the length of the line. By cleverly picking specific lengths of line, and lines of different impedance, it may be possible to arrive at a desired value, such as 50 + j 0 Ohms, a match to the rest of the feedline and radio.


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Hello Freecell:



Freecell said:

"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"



Never forget the bandwidth, as the bandwidth will tell you if the antenna is being matched properly, or if the impedance matching circuit is working properly. Never forget the bandwidth!!!! A narrow bandwidth will indicate impedance matching problems.



By just looking at the SWR only, you are not seeing the forest for the tree's. With todays antenna analyzers its just too easy to see where the SWR edges are, and note how much bandwidth you have. When I install a antenna thats the first thing I look for is the useable SWR bandwidth. I write down the 3.0 to 3.0 and 2.0 to 2.0 SWR frequencies, then I see how many frequencies have the lowest SWR, and a plot a chart showing the SWR Vs Frequency is made.



If a Antenna has a 1 to 1.0 SWR, or is said to be flat (Ionly read that in the books, no one has every told me his SWR was flat. Tires yeah!) And you add or subtract coax lengths sure the SWR will not change more than likly. But our ol buddy Mr. Bandwidth will!. Again you need to see what the bandwidth did, after doing this several times with the antennas you work with you will get a sense of how the antnna is working, or not working.



Many antennas have a SWR of just over a perfect match like a SWR of 1 to 1.2. This is measured at the antenna itself. With a length of coax the SWR will now show a SWR of say 1 to 1.0 , because of the loss in the coax. And so will the bandwidth change a little. Get to know the bandwidth measuremenats. The SWR didn't really change.



I measured 3 different SWR Vs Frequency measurements of my Crusader 10K, 5 Element Yagi Beam. The 3 diffeerent SWR Vs Frequency measurements used 50 foot, 100 Foot, and 150 Foot lengths of coax. And wrote down the SWR Vs Freqency on a graph, this indicates the Bandwidth also.



The 3 different SWR Vs Frequency measurements did indicate that the SWR did change a very small amount (insignificant amount) with the 3 different lengths of coax. But because I had the "BANDWIDTH" staring me in the face I knew by looking at the SWR Vs Frequency data, that the SWR in fact did not change, it was only slightly attenuated by the longer lengths of coax. This was indicated by the usable Bandwidth of the SWR Vs Frequency measurement. The 3 differnt SWR Vs Frequency measurements, where very close to each other, and the bandwidth got slightly wider with the loner coax lengths.



As the Coax was made longer, the SWR Vs Frequency Bandwidth got slightly wider. So the SWR at all the data points did not change it was only attenuated a very small amount, with the longer lengths of coax.



Now be advised this beam antenna has a Toroid Balun that will not allow energy to radiated on the outside of the Coax. A expensive Balun is used so that radiating coax will not throw off the beams impedance matching, or pattern. And this shows as the SWR did not change at any of the SWR Vs Frequency points. Again the Bandwidth measurement shows this, as just a SWR measurement would not.



If you are seeing different SWR with different coax lengths you still need to see what the bandwidrth is, as it maybe just a slight attenuation in the energy in the coax, or the coax is radiating. Due to a lack of grounding surface, bad antenna, or a bad installation. Or just the way it is at your frequency for a mobile installation.



By learning what the antennas do in bandwidth, by ploted measurement of SWR Vs Frequency, will show you how well the matching circuit is or is not working, or if the antenna is too close to other metal objects.



I have wanted to use a Coax Choke to reduce the coax radiation in mobile installations. NOT Coax wound in a coil, as I have tested these and was not impressed, but the coax wound around a Toroid core. I have made these to reduce engine noise in cheap radios and such and worked pretty well, if thats where the noise is coming in from. But this needs to be tested, and SWR Vs Frequency measurement need to be documented to show the before and after bandwidth.



"Does cable length effect SWR"



The question needs to be qualified as to what type antenna and how it is installed.



Experenced antenna guys (that arn't normanl) will have a SWR Vs Frequency Measurement plotted out, showing the antennas usable SWR Bandwidth, and be able to show the Field Strength Levels Vs SWR, from a known reference antenna, buts thats another story.



www.a1antennas.com



Jay in the Mojave


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forget it Jay, that was not the point i was making........



i was merely responding to this comment from you........



"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 ........"



add all the feedline you want to under perfectly matched source/load conditions. there is no such thing as an SWR, VSWR or ISWR below 1:1



i'm not talking about bandwidth here and neither are you.




</p>Edited by: <A HREF=http://p067.ezboard.com/bworldwidecbradioclub.showUserPublicProfile?gid=freecell>freecell</A> at: 7/21/04 1:31 pm
 
This thread and the other (Does Cable Length Affect SWR?) has gone to the point that one or the other has to have the last word. I feel both threads have gone far enough...JMHO.


The Peddler...

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This is what the forum is for, discussion of theory, no pun, just people with there own facts, this is what makes it interesting, even though the guy did not get a answer in plain " Engrish" LOL <img src=http://www.ezboard.com/intl/aenglish/images/emoticons/smokin.gif ALT=":smokin">


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