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Puzzling SWR/Dummy load issue

Discussion in 'General Ham Radio Discussion' started by Moleculo, Jan 27, 2009.

  1. Moleculo

    Moleculo Administrator Staff Member

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    Here's a head scratcher that I came across last night that I just haven't been able to reason through. :blink: I know there must be a simple explanation, but I'm not seeing it.

    Last night I was moving around some of the cables on the station and when I was finished, I decided to do a cursory check to make sure I put it back together right. The check was simply flip the switch to the dummy load and then to each both HF antennas to make sure all was normal on the SWR meter.



    Well, the antennas were the same as they always are, but when I went to the dummy load I measured about a 1.3 SWR. Not a big deal, but odd for a dummy load so I proceeded to find out why. I messed around for a while (tried a different dummy load), bypassed various pieces of equipment and finally narrowed down the offending piece of coax that was between the coax switch and the dummy load. Sure enough, when I put this 6' jumper of RG/8 on the analyzer and a different dummy load by itself, it got the same reading. I used a multimeter to check for shorts (would expect a much higher SWR if that was the case) and found none. I touched up all of the solder points on the connectors on each end which did nothing. I inspected for a place whe coax could be smashed or kinked and didn't come up with anything.

    So the question is, what on earth would cause a coax jumper that was perfectly fine one minute to suddenly show an increased (although minor) SWR reading into a dummy load? I don't get it :unsure:
     
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  2. WR0220

    WR0220 Active Member

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    My thoughts would be that the center conductor 'migrated' a bit from center which changed the impedance. Are there any bends in the coax between switch and dummy load?
     
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  3. W5LZ

    W5LZ Crotchety Old Bastard

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    That's a good one! And there can be several possibilities. Was this jumper used in conjunction with something else, or several something else's? And just how much of a change was there? I have to think that a change from a 1:1 to a 1.2:1 just isn't that big a change really. Enough to make me wonder, but I'm not sure how much worry I'd put into it, sort of. And, yeah, I'd have to do some checking too to see if I could localize the 'change'. That migrating thingy happens with foam dielectric more often than with the solid stuff. But, unless you really had the thing in a 'bind', migration wouldn't be my first thought. Still a possibility though. When you figure it out, let us know. (Hows that for a 'pass the buck' thingy?)
    - 'Doc
     
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  4. Beetle

    Beetle Well-Known Member

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    What's the actual DC resistance of the dummy load?

    And when you say "1.3 SWR", I assume that's 1.3:1, right? There are quite a few who would probably read that as "1:3" or even "3:1".
     
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  5. Marconi

    Marconi Supporting Member

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    The typical dummy load we would use is less than perfect and will not show ZERO reactance and/or 50 ohms of resistance consistantly accross a range of frequencies. They may be close, but they're not perfect. Add this to the lack of perfection in coax line characteristics and you have measurable but less than conquencial differences.

    Short jumpers of varying lengths, diameter, and make, that are connected directly from a radio to a dummy load may well show variations in readings on an analyzer. In fact you will often see these reactive differences even at a single frequency just by changing the length or the sample---even when derived from a common batch of coax.

    This situtation may not show up as obvious when using a SWR meter to test this, but you can see some rather dramatic changes in the values of R, X, and SWR on your analyzer while using a dummy load to test your jumpers. As the coax run under test gets longer the combination of all reactance/resistance point values along the line will improve and come closer to the actual claimed value, which is the average. Manufactures use the term "Characteristic Impedance" to describe this value for their line impedance.

    I don't think you saw a change from one moment to the next, Moleculo, I think you just realized that variations such as this do occur. Just run some more comparisons using an analyser and see.

    Aren't we lucky, even with this lack of perfection.
     
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    Last edited: Jan 27, 2009
  6. C2

    C2 Well-Known Member

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    Possible causes:

    You spillt beer on your coax and it weeped into the coax.

    You moved the coax and strained it a bit, and a stray piece of shield came close to the center pin, but did not touch.

    As Marconi said, it was always bad.

    What I'm assuming is that you have a piece of coax that is marginal. What could make it that way? MFG. or construction defect is about all I can think of if it is not damaged or got wet, etc...
     
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  7. Moleculo

    Moleculo Administrator Staff Member

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    I first noticed it on my inline cross needle meter. I was trying out a new coax inter-connection scheme I had been thinking about for a while. As it turned out, I decided I didn't like the new scheme and put everything back the way I had it to start with. The meter showed flat SWR into the dummy load before I started, and then showed about 1.3 after everything was put back. After I found the offending piece of coax, I put that piece of coax onto a different dummy load and into the analyzer and it showed the same thing. The variation may very well have always been there and just been masked, or as C2 said...maybe I just strained the coax in all the moving about?

    One other thing: It showed the same SWR on 80m, 40m, 20m, 17m, and 10m.


    correct

    Now that I think about it, there was a slight bend on the dummy load side, but the RG/8U is the flexible kind. It certainly wasn't a sharp bend. But that makes me think: If you put 1KW-2KW into a dummy load, it can get pretty hot. I wonder if that heat on the PL-259 didn't heat up the foam dialectric and change the spacing of the center conductor to the shield over time right at the connector. You might not even notice it until you move the coax around later. Maybe I should use a better quality final coax jumper into the dummy load that can take the heat.
     
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  8. Marconi

    Marconi Supporting Member

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    I was thinking that at first you ruled out the likelyhood of any physical problem that would ordinarily produces problems on the order of a direct short. One little wire end from the shield touching the center conductor in a jumper assembly can cause just such a direct short, but the result would be much more obvious than a small increase in SWR. A migration problem could also produce a terrible results like a direct short, but such a defect might also produce an impedance bump that only marginaly affects the SWR and this might describe what you were seeing. Whoever mentioned migration might be right on point in this case.

    I now agree that you were most likely seeing a physical defect in the jumper that happened somehow during the switch around. Have you found out what was really going on with that jumper? Compression damage can be obvious in some cases and not in others. Look the coax over real close and feel it well, up and down its length, for irregularities. Since you might not trust the jumper in your setup anymore, it might be a good idea to carefully cut the cover off and check the coax inside-out. I'm sure this must happen often and may prove to be a good bit of information.

    This is just another example of the Old Rule which I have broken many times and states, "If it ain't broke, don't fix it."
     
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    Last edited: Jan 28, 2009
  9. Beetle

    Beetle Well-Known Member

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    Beetle's Corollary: "If it ain't broke, fix it 'til it is."
     
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  10. WX2MIG

    WX2MIG Still Alive & Well

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    Damn Beetle.....I'm begining to think you're my long lost little brother the Dingo stole.......:D :LOL: :D
     
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  11. 74IN

    74IN Well-Known Member

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    Check velocity factor and jumper length, make sure the sign waves aren't cut off in mid SWR's. LOL.

    Any who, good luck.(y)
     
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  12. W5LZ

    W5LZ Crotchety Old Bastard

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    Just a couple of things, not all that important, but sort of handy to know every now and then.
    These are all "of course it does" thingys.
    Migration. It typically takes longer than you might think, and requires the cable to be put under more stress than is normal, sometimes a lot more stress than 'normal'. That's why you don't want to coil it up too tightly. It isn't exactly the most common thing to find, but if you take a look at the 'end' of a spool of coax, that tube it's wound on is the smallest the manufacturer thinks the stuff can be bent safely. Different sizes for different coax types.
    Foam dialectic coax bends easier than the solid dielectic stuff does, but migrations also 'happens' easier with it, that foam just isn't as dense as the solid stuff.
    If you have to make a sharp 90 degree bend then make a loop to sort of 'smooth' that sharp bend out some. It pays in the long run.

    If applied power is enough to cause enough heat to affect coax to any extent at all, it's too much. It's a fairly safe bet that if that kind of heat is present it'll show up in other things around that coax, or connector. That dummy load is gonna start boiling, leaking stuff out the vent.

    Wearing out coax is more a mechanical thing than electrical. The mechanical thing takes a fair amount of time, sort of. If there's an electrical 'wearing out', it's gonna happen sort of quickly. You can fix a mechanical 'wear out' in some cases. You're stuck with whatever electrical 'wear out' happens, ain't no fixing it, sort of. I have a problem with thinking of wearing out in terms of electrical stuff. I've found that electrical stuff wears out very quickly, with a bang in some cases. One of those it either works or it don't thingys. That's a personal foible though, not exactly something to make a big deal about.

    And then there's the old "S _ _ _ Happens" thingy. No particular rhyme or reason to it, it just does. "I've used that thing for over 63 years, why'd it quit now?"... you know? Oh well, "S _ _ _ Happens"...

    - 'Doc
     
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  13. C2

    C2 Well-Known Member

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    How about if you yank and bend the coax while monitoring the SWR?
     
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  14. Moleculo

    Moleculo Administrator Staff Member

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    Tried that and the SWR didn't seem to jump around at all. Maybe some slight movement, but nothing drastic.
     
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  15. W5LZ

    W5LZ Crotchety Old Bastard

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    You can if you want to, you have my permission. 'Course, that and a quarter won't make a phone call anymore...
    - 'Doc

    And a little more seriously, if wiggling coax makes the SWR change, you know you got problems somewhere. Typically not where you are holding the coax though, probably much more likely at the 'end' of that coax. 'Yanking' coax is sort of like a mini-coax 'stretch'. Not good. Almost always will produce some kind of change, very seldom a good one. Also why it's a good idea to support a long coax run in a couple of places, not just let it droop or hang there.
    A small monkey might get away with swinging on coax, but Tarzan would bust his... when that '259 turned loose...
     
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: coax, dummy load, swr

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