SWR not matching

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  • Updated 4 years ago
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Here's my issue. A 6300, and I'm using an MFJ 993B autotuner.

The SWR meter on the 6300 is reading differently from the MFJ. The MFJ does it's thing, and reads 1.0. Whereas the 6300 meter reads 1.3. It's never folded back on me, but is  this normal or abnormal?
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Michael Coslo

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Posted 4 years ago

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Jim Gilliam

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I would use a Bird Wattmeter or something similar before I would guess which is more correct.

Jim, K6QE

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Richard Fusinhski

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Official Response
You will almost always get a different reading on the tuner meter Vs the flex meter or any other rig meter. The flex is seeing the match to the tuner and the tuner is seeing the match to the antenna, Rarely will they read the same.

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George Molnar, KF2T, Elmer

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Even a small length of coax can lead to different readings. I'd -suspect- the Flex metering is more trustworthy, given the quality control process and the sampling being done at the rig. But, the difference between 1:1 and 1.3:1 at HF is trivial and can be safely ignored.
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Ken - WA8JXM

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FWIW, at the same 40m frequency, my Elecraft K3 shows 1.1:1, the Flex 6300 shows 1.4:1 and my Palstar AT2K tuner shows 1.7:1 (in bypass mode).  As long as the rig is happy, it doesn't really matter.  Yes, I know it's frustrating but a 1.3 SWR is a decent match.  If my math is right, that's what you would get with a 65 ohm load.
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K2CB Eric Dobrowansky

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I've also seen discrepancies between my 6500's SWR meter and my LP-100As.

For example, when driving an 8877 based amplifier, I use two separate LP-100As, one on the input and one on the output. I do this to monitor amplifier gain, as well as other parameters.

In effect, the input LP-100A is seeing exactly what the Flex is seeing, load wise, other than a 3ft RG8X coax jumper and radio coax switch between the radio and LP-100A coupler. While the LP-100A shows a low input SWR into the amp, the radio's internal meter will show abnormally high SWR readings on some bands. For example, the LP-100A will show an amplifier input SWR of 1.28:1, while the Flex meter will show an input SWR of 2.20:1.

It appears the radio SWR meter circuit is sensitive to variances in the load impedance components (reactance, capacitance). I suspect the input tuning circuit and tube may be causing the 6500's meter circuit to read incorrectly at times, depending on frequency and tuning of the amplifier (Alpha 9500).

For what it is worth, when I see the higher reading discrepancy on the Flex, I can simply flip my station's radio coax switch over to my IC-7800 or TS-990, and their internal SWR meters will be very close to the LP-100A readings, so there is definitely an issue somewhere in the a Flex metering circuit. It is usually not a big deal, but I can see where this could be an issue once you approach an SWR of 2.5:1, at which point the output power will start to fold back.

For what it is worth, I have also observed similar results with the internal SWR metering of the ANAN radios. You can download the schematic of their PA board and review their metering circuit, which may reveal why it also suffers the same anomaly.

FWIW, it would be nice if there was a schematic of the Flex's metering circuit for review as well. I suspect these internal metering circuits are quite simple, and therefore may not have been designed as well as they could have been to compensate for varying load impedances.


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Dave KD5FX

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Try changing the jumper between the two units, use a different length jumper. Does that change the values? I recently made some changes to my station, using some un-tested jumpers. I found that one of the jumpers was bad.
73, Dave
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Luis del MOlino

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All of you should know that when there is a discrepancy in SWR at different points of a coaxial line and the SWR readings changes, these discrepancies are always diue to the fact that there is an external Rf corrent circulating by the outside of the cosaxial cable that is propagating from the antenna until your local common earth.

You can check this with this test: if the SWR changes with the lenght of the coaxial line, try adding a small short lenght of coaxial cable then if the reading changes, there es RF outside the coaxial cable and the lenght of the coaxial cable affects all the antenna system. Then you need to place a tensión balun at your antenna or a corrent balun on your feedline.

Otherwise, when no RF circulates outside the cable, the SWR does not changes at any point of the coaxial cable, not changes with changes in lenght and you will find always the same Reading anyplace.

73 Luis EA3OG  

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Dave KD5FX

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This is very true but a bad jumper can do the same thing! I have baluns on all my HF antennas but was showing higher SWRs than normal after re-wiring my shack and using some un-tested jumpers. Turned out one of the jumpers was bad and was causing the problem. Replaced it and all was back to normal SWRs.
73, Dave
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Michael Coslo

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I'm going to  change out jumpers and see if there is any change.