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Why adding coax changes swr

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  #9  
Old 04-21-2011, 06:48 PM
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Quote:
Originally Posted by hookedon6 View Post
well, ya got part of it correct,............................ the only thing changing is the impedance.
Here is my example of what happens when I added 6'+ in the form of a choke to my Sigma4. As I said the frequency changed and went down due to the antenna being slightly reactive. The affect is as we would expect when adding length to the antenna and we know that with a reactive antenna the feed line becomes part of the radiating system.

So hookedon6 does the frequency change or not?

Maybe Colt45 can check his rig out and see if his frequency drops when he adds the jumpers.

Affect of adding feedline to antenna.pdf
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Last edited by Marconi; 04-21-2011 at 07:00 PM.

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Old 04-21-2011, 07:42 PM
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The SWR meter does not measure the SWR of your antenna, but measures the entire system from the meter down.

All vSWR is, is just a ratio of power output of the radio to power reflected back to the radio. Of forward to reflected power.

For something that causes power to be reflected back, its often refered to as a reflection. Bad connectors, a kink in the coax, water intrusion, or an antenna thats electrical length is not tuned to the operating frequency of the transmitter, all cause a reflection.

If you want to test the coax, connectors, amp,etc by themselves you need a 50 or 52 ohm load, aka dummy load or cantenna. use it in place of the antenna, in theory that paint bucket with the connector on it is the perfect antenna, should give a 1:1 SWR on any freq from 300khz to 30Ghz.

Also there is going to be some loss and reflection at the amp. Shoulnt hurt the radio so long as it is below a 3:1.

Tune the antenna without the amp, use a female to female adapter in its place but with same jumpers and all. Is SWR good?

You might also have a problem with lack of ground plane, seeing how most RV's are fiberglass shells. The metal body acts as other half your antenna, so without metal body the coax acts as the other half, where part of the trimming coax myths come from, cut the coax and you actually shorten the antenna.

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Old 04-22-2011, 06:06 AM
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Quote:
Originally Posted by Marconi View Post
... I added 6'+ in the form of a choke to my Sigma4...we know that with a reactive antenna the feed line becomes part of the radiating system.

So hookedon6 does the frequency change or not?
what "frequency" are you refering to?

lets see,.... you added a "choke coil" @ the feed point in an attempt to uncouple the line from the antenna. obviously that isn't effective. there ARE better methods available to uncouple a feedline.

put the VSWR meter AT the FEEDPOINT (where it belongs) and you will see NO change.

let me ask you a question,....... has any radio ever been damaged by a "high" VSWR????

ever?

the answer is no,....... because radios don't "see" VSWR, they are not in any way, shape or form affected by VSWR.

VSWR doesn't actually exist,... it's just a math ratio.

the ONLY thing a radio "sees" is impedance.

you really should brush up on Smith Charts and you will find out that the "VSWR circle" is really an elipse.

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Old 04-22-2011, 06:48 AM
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LOL!

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  #13  
Old 04-22-2011, 06:58 AM
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Quote:
Originally Posted by hookedon6 View Post
let me ask you a question,....... has any radio ever been damaged by a "high" VSWR????

ever?

the answer is no,....... because radios don't "see" VSWR, they are not in any way, shape or form affected by VSWR.


so finals cant be blown by high vswr ????????????

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Old 04-22-2011, 09:09 AM
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Quote:
Originally Posted by hookedon6 View Post
what "frequency" are you refering to?
hookedon6, in this case I suggest you look at the graph represented by the data from my SWR meter, and my analyzer, to see the low point for SWR noted. Also note in the data fields at the top of the graph, where the analyzer data is recorded, that the impedance within a modest range of resonance (or lowest SWR) did not change as you have suggested. For me it is obvious that the frequency did change somewhat dramatically, from about 27.405 down to about 27.105. IMO, this is the reason Colt45 saw a change in SWR for the system.

Again, I would claim that the SWR measured at the feed point of a reactive antenna, if measure before and after, would remain similar, while showing a change at the TX'er end of the feed line WON'T, it will appear to change due to transformation of the feed line.

The only real way to change the SWR for an antenna is to make a physical change to the antenna, tuner, or the environment around it. This is also why I recommend to always, when possible, tune at the feed point and at installed height.

Quote:
Originally Posted by hookedon6 View Post
lets see,.... you added a "choke coil" @ the feed point in an attempt to uncouple the line from the antenna. obviously that isn't effective. there ARE better methods available to uncouple a feedline.
I did add the choke to check to see if it would help to decouple the feed point from the feed line and it did seem to work in lowering the noise, but I admit that perception can be purely subjective in its determination. Otherwise I noted no other positive responses. I did note the change in resonance as noted above, and had I constructed the choke using the existing feed line instead, I doubt I would have seen the change in frequency.

Quote:
Originally Posted by hookedon6 View Post
put the VSWR meter AT the FEEDPOINT (where it belongs) and you will see NO change.
I agree and that is my point in a nutshell.

Quote:
Originally Posted by hookedon6 View Post
let me ask you a question,....... has any radio ever been damaged by a "high" VSWR????

ever?

the answer is no,....... because radios don't "see" VSWR, they are not in any way, shape or form affected by VSWR.

VSWR doesn't actually exist,... it's just a math ratio.

the ONLY thing a radio "sees" is impedance.
IMO, your last statement is a typical straw man argument, to use the words never, no way, shape or form does SWR ever affect the operations or the lack, with your transmitter. What about the cut back circuit that attempts to control the power under a high SWR condition?

Quote:
Originally Posted by hookedon6 View Post
you really should brush up on Smith Charts and you will find out that the "VSWR circle" is really an elipse.
I thought you said VSWR did not exists, but now it appears on the Smith Chart? That is interesting. Why don't you do a Smith Chart indicating the addition of more feed line to a vertical 1/4 wave antenna that is matched perfectly and one that is not. That should be easy to do, and interesting to see what it is that you see in doing so. I gave you an example of what I see, so how about it?
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Last edited by Marconi; 04-22-2011 at 10:21 AM.

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  #15  
Old 04-22-2011, 09:19 AM
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Last night I removed amp and swr meter and juast started adding coax insted and each short length raised the swr to with three 3 ft jumpers swr was 3.2 to 1 .This antenna mounted on a heavy steel chrom mirror bolted directly to windshiel steel frame post with 40 foot of steel chassis and framing around coach so I caint understand why it would not have enough ground to work correctly. Would a antenna tuner work to tweak this problem or would performance just suffer while meters look good? Without fancy meters how can an antenna be checked if swr is 1.5 to one across the channel 1 to 40 with 18 foot belton low loss coax? I'm thinking just take it all off cept 13 inch no ground antenna on the roof and cobra 29 naked and only taly 3 miles and forget about it!

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Old 04-22-2011, 09:46 AM
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Quote:
Originally Posted by colt45 View Post
This antenna mounted on a heavy steel chrom mirror bolted directly to windshiel steel frame post with 40 foot of steel chassis and framing around coach so I caint understand why it would not have enough ground to work correctly.
What is the body made of? Is it sheet aluminum or fiberglass?
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