swr mismatch

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An odd one....Flex 6400 that shows good SWR on all bands, and near enough matches the readings from my in-line Daiwa DN-801, except on 30m!

On 30m the Flex reads 1.3:1 (hard to be exact due to poor scale, it needs numbers). However, the external DN-801 reads just over 2.1:1.

Without having to buy yet another meter, any ideas?
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Steven G1XOW

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Posted 1 month ago

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Paul

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Afternoon Steven. It's possible that on 30m the length of coax between the rig and the Daiwa is transforming the impedance seen by the Daiwa closer to 50 ohm at the rig. You could try a different length to test if this is happening. Although it could make matters worse rather than better. You may need to try various lengths to find the best solution on all bands. It will always be a compromise with a multi-band antenna though.
Hope this helps. 73, Paul
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Pat N6PAT

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Changing the coax length worked for me. I added a homebrew 160 mod to my Hustler 6BTV and the SWR was higher than I wanted - about 2:1.

After reading what others did to reduce SWR with different lengths of coax for various antennas  I decided to experiment by adding different lengths of coax and eventually dropped the SWR to 1:1

It does work
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Jim Gilliam

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SWR is NOT a function of transmission line length. The length of coax can act as an impedance transformer to a degree but has nothing to do with the impedance of the antenna which causes the line reflections. Unreliable SWR readings usually have something to do with RF flowing on the outside of the transmission line, and poor grounding.


Jim, K6QE
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James Charlton

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This is a problem with having too many test instruments.  The Flex is always right because it is sensitive to a high reflected power, the Daiwa isn't.  The real SWR doesn't matter as long as your Flex will drive the antenna to full power.

For the record, Jim, K6QE, is correct.  SWR is caused by a mismatch between the drive point impedance of the antenna and the characteristic impedance of the feedline.  Therefore, it has to be fixed at the antenna end of the feedline.

Jim AD0AB 
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Craig Williams

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Agree
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Paul

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That's true Jim if the system is matched. If not then the impedance transformation due to the line length can be significant. It's also true that on 30m, the length of a patch lead is likely to be a small fraction of a wavelength. None the less, impedance transformation cannot be discounted without closer examination.
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Paul

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James, it definitely does matter if there is a high vswr anywhere in the system. The rig may see 50 ohms and happily launch 100W into the antenna system, but a mismatch further down the line will prevent the full 100W reaching the antenna.

Or to look at it another way, the high vswr is due to the antenna not being 50 ohm at a particular frequency. The internal atu gives the rig a match. It doesn't change any aspect of the antenna impedance. The mismatch still exists between it and the external meter and its' patch leads. Hardly surprising the external and internal meters show different swrs'. However, if an external atu was used after the external meter, it's more likely that it and the rig would agree.
(Edited)
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HCampbell WB4IVF

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Agree with Jim.  If the radio has a problem finding a match, changing coax length can change the impedance (but not actual SWR) presented to the radio, and can result in the radio finding a match.  But the radio finding a match does not seem to be a problem in this case.  If changing coax length significantly changes the SWR reading, I’d suspect RF on the shield.

And keep in mind that a meters’ calibration (especially a modest one like the Daiwa) can vary significantly with frequency and power.  If I had to choose one I’d go with the Flex.

BTW, I notice similar behavior with the modest LDG M-1000 meter connected to my LDG-1000 tuner.  At times it indicates significantly different (usually higher) SWR than my Flex and LP-700.

 Howard


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Stu 2E0XXO

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Try taking the daiwa out of line and see what reading you get from the flex.
does it match the reading you were getting from the daiwa ?
As someone mentioned,the daiwa may need calibrating.

stu
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James Charlton

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Hi Steve and all,
As you might guess, I have nothing better to do than write responses to this post.  It's bad weather here and everything is covered with ice.

Pretty much everything said here is true, but does not necessarily address Steve's question.

Steve, 
To answer your question, don't buy another SWR meter, sell the Daiwa.  Then you will only have the Flex indicator and that's the only one that matters.

The only reason to be concerned about high SWR at all is the effect the reflected power has on the radio's output stage.  I do not know anything about the Flex output stage, but the general effect of high reflected power is to raise the voltages on the output transistors and raise the heat dissipation.  At some point the transistors burn out.

With a SWR of 1.3:1 as indicated on the Flex, that is not a problem in your case.

One response from Jim, K6QE mentions current on the outside of the coax shield as a possible cause of different instruments reading different SWR.  This is called common mode current and is a very likely cause of reading differences - it does not affect the actual reflected power the radio is seeing.

An easy test for CM current is to connect an antenna analyzer in place of your radio and set it to indicate the SWR at the frequency of interest.  If the SWR seems to change when you touch the analyzer or the connections with you hand, you have common mode current.

The ARRL Handbook lists several easy fixes for CM current and I have found them to be effective.

Good luck,
Jim  AD0AB    
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Johan / SE3X

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Have had several, probably at least 10, different analouge SWR/PWR meters through the years. Never really trused any of them, even the most expensive ones. Sold of and bough the LP-100 and never looked back  :)  If you gona add another meter to your system, buy LP-100. Expensive Yes, amazing piece of equipmen? Yes!

I second the opinions expressed that the meter in the Flex is very accurate. Matches the reading of the LP-100 very well.
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Paul

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Hi Jim.
Re. "The only reason to be concerned about high SWR at all is the effect the reflected power has on the radio's output stage."

With respect that's incorrect; a high vswr reduces the power reaching the antenna. Eg. a vswr of 3:1 causes 25% of your tx power to be reflected, ie. only 75W reaches the antenna, to me that does matter. The rig will self limit in the face of a mismatch so damage is a secondary issue, the return loss is more important.
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Luis del MOlino

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When there is a difference in readings of SWR on different points of the line, or SWR changes when changing the lentght of the line, there is only one reason then can cuase this efect: there is another RF current circulating outside on the outside of the braid of the coaxial cable . You have to use a current balun to avoid thiis current. You can be sure.

Luis EA3OG
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James Charlton

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Thanks Paul,
I don't want to throw away power either and losing 25 watts out of 100 w represents a 1.25 dB loss of signal strength at the far end excluding any cable loss.  And you are right, the transmitter will reduce its output to prevent damage due to excessive reflected power.  This "fold back" does not change the SWR, it just reduces incident power which in turn reduces reflected power in proportion.  Either way you look at it, the power received at the distant end is reduced.

But that was not my point.  Without knowing more detail of the Flex output stage, it's hard to say what happens to the reflected power.  It is entirely possible that most of it gets radiated.  

If the output impedance of the Flex is 50 ohms - a match to the coax - then that's the worst case.  The reflected power comes down the coax and is absorbed by the Flex output stage.  That's bad and that's why the Flex acts to reduce reflected power by reducing incident power.

On the other hand, if the output impedance of the transmitter is lower than the coax characteristic impedance, say 5 or 10 ohms, the reflected power sees it as a mismatch and much of it is re-reflected back towards the antenna.  In that case, most of the power eventually gets radiated.

A lot of considerations go into the design of an amplifier but in my case, I have always strived for a low output impedance.  To launch 100w up a 50 ohm coax, all you have to do is have an output stage capable of providing about 71 volts at 1.4 amps at the same time and you're good to go. The amp's output impedance is not all that important but, If it is low enough, you don't have to worry (much) about reflected power because it will be reflected back towards the antenna.

I'm not saying high SWR or more specifically high reflected power is good, it isn't but it is not always what it seems and it's not always a total loss.  In Steve's case, an SWR of 1.3:1 as measured by the Flex gives only about 0.2dB of signal loss if every bit of the reflected power is absorbed and never seen again. 

I agree with the other writers that every SWR meter (excluding lab instruments) seems to give a different value.  I guess you get s much precision as you are willing to pay for.    

Jim  AD0AB

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Stan - VA7NF

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Many people described parts of the problem, most were correct in part.
  • Nothing will correct the intrinsic impedance of the coax with the antenna impedance short of correcting the antenna, but that is not as important as originally thought.
  • No-one mentioned the other important item, being the loss per 100ft of the coax
  • The best place for impedance matching is at the antenna end of the coax, but that may not be the most convenient position for that matching box
  • A portion of the reflected power (wherever the matching box is located) will be re-reflected back towards the antenna, with additional associated coax power loss. And a portion of that re-reflected power will go out the antenna except for very little re-re-reflected power.
  • Impedance presented to the transmitter will be transformed by additional length of coax, or not transformed as much if a length is removed.  All depending on the wavelength being added/removed (also considering the velocity factor of the coax)
  • Reflected power (that passes through the matching box) back to the transmitter will be dissipated in the final amplifier as heat.  That is why high power linear amps are de-rated when SWR is high.
So, changing the coax length when there is a mismatch will change the apparent impedance (therefore SWR) at the coax feed point.  The suggestion to add some length is very valid but will not reduce feed line losses.
SWR meters use a short length of coax with an embedded extra wire that picks up a directional current.  That current will be affected by where the meter is positioned.
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James Charlton

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I agree with all of above and can only think of one thing to add to reduce a pesky SWR and that is hydroventillated coaxial cable.  The length of the cable determines the amount of SWR reduction you get and is largely wavelength independent.

This cable reduces SWR by absorbing the reflected power along its length.  Therefore, the longer the cable, the greater the SWR reduction.  
Many hams with older and well weathered antenna installations may already be enjoying the benefits without being aware of it.  Until recently, that included me.

If you can't find hydroventillated coax for sale, you can make it yourself.  Start with about 25 feet of the lowest cost coax (off-brands are ideal) you can find.  The length is not critical.  With a sewing needle, poke holes in the covering ever inch or so along its length.  Be sure the holes go all the way through to the braid.

Now, install this coax as part of your feedline, but BE SURE it is outside and not inside your house.  Leaving it coiled on the ground or strung in trees is good.  Running it in a rain gutter is ideal.  That's it.

Settle back and watch as over time, your hydroventillated cable gradually reduces your SWR.
No doubt about it, lossy coax reduces SWR every time it's tried.

Jim  AD0AB
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Steven G1XOW

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Just to wrap this up....

There is NO antenna mismatch!   The antenna is a 3 element steppir with 40m trombone section added. It has nearly perfect SWR on every band and re-tuned per 100kHz to track the TCVR frequency.  I have checked this fact with a MFJ antenna tuner to be correct on all bands including 30m.

The only differences that might explain the anomaly is just 12 inches of coax between the radio and the swr meter. After that point everything else is the same. 12 inches is way to short as a function of 30m band to be significant but I will replace it just to be sure. 

If the DN-801 meter where bad I'd expect it on all bands, or a worsening as I moved HF, which does not happen.

73 



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Stan - VA7NF

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Now that you mention the antenna - there will be SWR on 30M.
See http://www.steppir.com/wp-content/uploads/2014/11/XTG_30-40-Loop_20141120.pdf
short excerpt:
Unfortunately 30M does not loop around far enough to gain any of the matching advantage so the SWR will be around 1.8 to 2.5:1 depending on antenna height.
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Paul

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Hi, thanks for the summary Stephen. I have found this thread very interesting. Clearly there are lots of differing opinions on here and elsewhere. My particular favourite is this;


https://www.iz2uuf.net/wp/index.php/2...


Some respondents to your question may benefit from a thorough read. 73, Paul
(Edited)