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Why adding coax changes swr

Discussion in 'CB Antennas' started by colt45, Apr 21, 2011.

  1. colt45

    colt45 Member

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    On my rv rig I have a good swr reading on a 102 whip 1.2 to 1 and on a 4 foot top load ant it's 1.2 to 1.4 or so why does swr go up when I add swr meter or amp with jumper or if I add amp and swr meter including 2 jumpers swr goes even higher...What would be the problem here ?antenna? or amp or ground plane? Tonight I'll add 3 jumpers 3 foot each with out swr meter or amp and see what that does. I'm reading swr also on the meter on the cobra 29 I just had a shop check it all out and both worked together on the bench so problem must be in antenna setup or rf feedback being close to antenna but that would'nt matter with just swr meter causing higher swr reading. I'm getting about ready to give up on this amp better antenna stuff, I get good swr with just 18ft coax and whip so does'nt that mean the antenna is all correct?


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  2. Simon004

    Simon004 Member

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    My first guess would be an insufficient ground plane. Forgive my ignorance, but it's mounted on your RV, which I think is a big mobile home thing, is this made of fiberglass by any chance? How and where is the antenna mounted?
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  3. Marconi

    Marconi Supporting Member

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    It is my opinion that the SWR for the antenna is not actually changing, but when you add some length to the system you are seeing the SWR change. It doesn't seem to make sense, but it does.

    What is happening is the frequency is changing due to the transformational affect which can happen in a system where the antenna is not set perfectly at the frequency of choice. Either or both the resistance or the reactance is off a bit, which means your system is a bit reactive.

    You are making the change and then checking out the SWR, but missing the fact that the lowest SWR reading has shifted in frequency a bit, maybe 10-20 channels when I see it happen. That is why you're often told to ignore it, that the meter is just being fooled or is lying to you, which is CB BS. If the meter is close to correct it is just showing what it's supposed to at the frequency where you take the reading, and where the meter is located in the feed line.

    If you bother to take a simple bandwidth reading, record the info, and then make a graph...you will be able to see the BW curves', best SWR, move around according to frequency.

    Click this video:
    YouTube - Marconi Demonstrates Antenna SWR Bandwidth Curve

    What causes this can be anything that makes your antenna tune off a bit, tune, construction, installation, ground, objects close by that can cause reflections.
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  4. "why does swr go up when I add swr meter or amp with jumper or if I add amp and swr meter including 2 jumpers swr goes even higher"

    your post reads like youre using the meter on the radio since your vswr is going up when you ADD YOUR METER , AMP AND COAX . you dont want any amps or meters or filters or anything else between the swr meter and your antenna besides coax . i also would not use a radios built in meter to tune swr . always use a external swr meter .
    #4
  5. hookedon6

    hookedon6 W9WDX Amateur Radio Club Member

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    well, ya got part of it correct,............................ the only thing changing is the impedance.:headbang
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  6. loosecannon

    loosecannon break on through

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    colt45,

    are you saying that you are using those double female barrel connectors and coax jumpers to add length to your coax?

    if so, they could well be the source of your problems.

    you need one uninterrupted length of coax going from the antenna to the output of whatever meter/amp you have in line.

    i cant say for sure that these things are messing you up, but if it were my setup, thats what i would do.
    LC
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  7. Beetle

    Beetle Well-Known Member

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    One uninterrupted length of coax is the ideal, but as long as the sections of coax have the same characteristic impedance, ± a few (say 5) ohms, there's no problem with one or two barrel splices. The length of the "discontinuity" is so short (relative to a wavelength at HF), that for all practical and measurable purposes, it isn't there. Just be sure to weather-seal each such splice properly.
    #7
  8. W5LZ

    W5LZ Crotchety Old Bastard

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    The total reactances of the feed line and the antenna are what make up SWR when measured at the transmitter end of the feed line. If you add/subtract coax it changes that total 'mix' to change the SWR. But that's only really gonna happen if the antenna isn't presenting a 50 ohms, non-reactive load, meaning it is really tuned incorrectly. That's NOT an uncommon thing to have a mis-tuned antenna, quite common if only an SWR meter is used to do that tuning. An SWR meter wouldn't know a reactance from a strawberry malt.
    - 'Doc
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  9. Marconi

    Marconi Supporting Member

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    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? (y)

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

    View attachment Affect of adding feedline to antenna.pdf
    Last edited: Apr 22, 2011
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  10. towerdog

    towerdog one-niner-seven

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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.
    #10
  11. hookedon6

    hookedon6 W9WDX Amateur Radio Club Member

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    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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  12. W5LZ

    W5LZ Crotchety Old Bastard

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    LOL!
    #12


  13. so finals cant be blown by high vswr ????????????
    #13
  14. Marconi

    Marconi Supporting Member

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    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.

    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.

    I agree and that is my point in a nutshell.

    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?

    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?
    Last edited: Apr 22, 2011
    #14
  15. colt45

    colt45 Member

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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!
    #15


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