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[KX3] SWR Reading Instability Follow-up

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Gary Hawkins

[KX3] SWR Reading Instability Follow-up

Firstly thanks to everyone for your input on potential causes of SWR
reading instability with my KX3. Since there were so many possibilities
(antenna, heating, radio, etc.) I knew I needed to go back to basics and
initially test the radio with a dummy load on the bench. Well, I
managed to recreate the issue and have documented my findings in a short
4 min video, http://youtu.be/cwxk4E01IfM

In short the problem appears to be associated with the KX3 reporting of
SWR in SSB mode. My radio is configured as follows: uC 01.79, dSP
01.27. As this issue has only recently developed (last month or so),
and while I can't be 100% sure, I think it may have occurred after I
upgraded to this firmware version.

I'll be contacting the folks at Elecraft to get their opinion on this.

73's Gary K6YOA
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VaibhaV Sharma

Re: [KX3] SWR Reading Instability Follow-up

I have noticed the same thing on my KX3 as well.

Can you redo the 1-2-1-2 test with FM?

You did the SSB test only with 1-2-1-2 but the FM test with XMIT full power
and no variation in power with speech.

--
VaibhaV

On Thu, Apr 10, 2014 at 11:06 AM, Gary Hawkins <[hidden email]> wrote:

> Firstly thanks to everyone for your input on potential causes of SWR
> reading instability with my KX3. Since there were so many possibilities
> (antenna, heating, radio, etc.) I knew I needed to go back to basics and
> initially test the radio with a dummy load on the bench. Well, I managed
> to recreate the issue and have documented my findings in a short 4 min
> video, http://youtu.be/cwxk4E01IfM
>
> In short the problem appears to be associated with the KX3 reporting of
> SWR in SSB mode. My radio is configured as follows: uC 01.79, dSP 01.27.
> As this issue has only recently developed (last month or so), and while I
> can't be 100% sure, I think it may have occurred after I upgraded to this
> firmware version.
>
> I'll be contacting the folks at Elecraft to get their opinion on this.
>
> 73's Gary K6YOA
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
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>

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Phil Wheeler-2

Re: [KX3] SWR Reading Instability Follow-up

I never watch SWR in SSB, but CMP and ALC.

73, Phil W7OX

On 4/10/14, 11:51 AM, VaibhaV Sharma wrote:
> I have noticed the same thing on my KX3 as well.
>
> Can you redo the 1-2-1-2 test with FM?
>
> You did the SSB test only with 1-2-1-2 but the FM test with XMIT full power
> and no variation in power with speech.
>

On Thu, Apr 10, 2014 at 11:06 AM, Gary Hawkins
<[hidden email]> wrote:

KE6BB

Re: [KX3] SWR Reading Instability Follow-up

In reply to this post by VaibhaV Sharma

Re: Can you redo the 1-2-1-2 test with FM?

I just duplicated the original test with my KX3 (FW 1.94/1.28) and got the same results. I repeated the "1-2-1-2" voice test and the SWR remained stable as it did in AM as well.

It looks almost like the KX3 is calculating the SWR using a good current value for forward power (dropping off to zero as the voice trails off between syllables or words) but using a stale value (not updated often enough) for reflected power. When the forward power goes to zero or near zero as it does on SSB between syllables, it doesn't take much reflected power to give you the appearance of high SWR if you are using a stale reflected power value. In AM and FM, the carries remains relatively high stabilizing the calculation by never letting forward power drop to, or near, zero between voice syllables.

Of course, it could also be related to the time constants in the hardware, but now I am speculating too much...and being a hardware guy, I love to blame things on the software guys! ;-)

Mark
KE6BB
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KE6BB

Re: [KX3] SWR Reading Instability Follow-up

The second sentence, first paragraph should have read:

"I repeated the "1-2-1-2" voice test ON FM and the SWR remained stable as it did in AM as well."

Mark
KE6BB
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Nr4c

Re: [KX3] SWR Reading Instability Follow-up

In reply to this post by KE6BB

I suspect that VSWR tests should be done with constant carrier CW, and not with a fluctuating signal as SSB.

Sent from my iPhone
...nr4c. bill

> On Apr 10, 2014, at 4:26 PM, Mark Petiford <[hidden email]> wrote:
>
> Re: Can you redo the 1-2-1-2 test with FM?
>
> I just duplicated the original test with my KX3 (FW 1.94/1.28) and got the same results. I repeated the "1-2-1-2" voice test and the SWR remained stable as it did in AM as well.
>
> It looks almost like the KX3 is calculating the SWR using a good current value for forward power (dropping off to zero as the voice trails off between syllables or words) but using a stale value (not updated often enough) for reflected power. When the forward power goes to zero or near zero as it does on SSB between syllables, it doesn't take much reflected power to give you the appearance of high SWR if you are using a stale reflected power value. In AM and FM, the carries remains relatively high stabilizing the calculation by never letting forward power drop to, or near, zero between voice syllables.
>
> Of course, it could also be related to the time constants in the hardware, but now I am speculating too much...and being a hardware guy, I love to blame things on the software guys! ;-)
>
> Mark
> KE6BB
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VaibhaV Sharma

Re: [KX3] SWR Reading Instability Follow-up

In reply to this post by KE6BB

Thanks for repeating the tests.

I started paying close attention to the SWR meter after I got the Alex
Loop. The antenna has a very narrow window where the SWR is low (receive
noise high). I try not to use the ATU with the loop. Even touching the loop
capacitor dial changes its behavior and it is often hard to calibrate it
for a low SWR while watching the SWR meter.

The KX3 SWR indication hopefully is only a UI issue. Is there a way to
stream SWR and other data off the radio so one can watch what the radio
sees?

--
VaibhaV Sharma
W7VAI

On Thu, Apr 10, 2014 at 1:26 PM, Mark Petiford <[hidden email]> wrote:

> Re: Can you redo the 1-2-1-2 test with FM?
>
> I just duplicated the original test with my KX3 (FW 1.94/1.28) and got the
> same results. I repeated the "1-2-1-2" voice test and the SWR remained
> stable as it did in AM as well.
>
> It looks almost like the KX3 is calculating the SWR using a good current
> value for forward power (dropping off to zero as the voice trails off
> between syllables or words) but using a stale value (not updated often
> enough) for reflected power. When the forward power goes to zero or near
> zero as it does on SSB between syllables, it doesn't take much reflected
> power to give you the appearance of high SWR if you are using a stale
> reflected power value. In AM and FM, the carries remains relatively high
> stabilizing the calculation by never letting forward power drop to, or
> near, zero between voice syllables.
>
> Of course, it could also be related to the time constants in the hardware,
> but now I am speculating too much...and being a hardware guy, I love to
> blame things on the software guys! ;-)
>
> Mark
> KE6BB
>
>

k6dgw

Re: [KX3] SWR Reading Instability Follow-up

With magnetic loops becoming more popular, it probably bears commenting:
The AlexLoop [and clones] is a resonant transformer and *must* be at
resonance to work. This means No ATU's, ever. At resonance, the
coupling between primary and secondary is very high and mine works very
well. Off resonance, it becomes an MDL [magnetic dummy load]. A friend
couldn't get his to work and when I saw him set it up, he was adjusting
quickly for a moderate SWR and then letting the ATU match it.

Yes, mine is a bit touchy on 40 meters.

73,

Fred K6DGW
- Northern California Contest Club
- CU in the 2014 Cal QSO Party 4-5 Oct 2014
- www.cqp.org

On 4/10/2014 1:56 PM, VaibhaV Sharma wrote:

> I started paying close attention to the SWR meter after I got the Alex
> Loop. The antenna has a very narrow window where the SWR is low (receive
> noise high). I try not to use the ATU with the loop.

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Don Wilhelm-4

Re: [KX3] SWR Reading Instability Follow-up

In reply to this post by VaibhaV Sharma

Mark's analysis was 'right on'. Due to the varying nature of the SSB RF
waveform and the workings of digital sampling, it is impossible for the
forward power and reverse power to be sampled at exactly the same
instant (one of the limitations of digital sampling intervals).
So, if the reverse power happens to be sampled at a time the RF is high,
and the forward power is sampled when the RF is low, a higher SWR than
actually exists will be computed (and displayed).

The solution is to check the SWR when in TUNE (set TUN PWR to a sane
value) and adjust the Alexloop using that input and SWR indication. Then
when operating SSB, either ignore the SWR indication or switch to
display ALC and Compression which should be more meaningful.

73,
Don W3FPR

On 4/10/2014 4:56 PM, VaibhaV Sharma wrote:

> Thanks for repeating the tests.
>
> I started paying close attention to the SWR meter after I got the Alex
> Loop. The antenna has a very narrow window where the SWR is low (receive
> noise high). I try not to use the ATU with the loop. Even touching the loop
> capacitor dial changes its behavior and it is often hard to calibrate it
> for a low SWR while watching the SWR meter.
>
> The KX3 SWR indication hopefully is only a UI issue. Is there a way to
> stream SWR and other data off the radio so one can watch what the radio
> sees?
>

Lynn W. Taylor, WB6UUT

Re: [KX3] SWR Reading Instability Follow-up

In reply to this post by VaibhaV Sharma

My only question here is WHY?

I think Mark is on the right track. It could also be that between
syllables, and during "quieter" times in speech we're simply not seeing
enough RF to get off the bottom bits of the A/D converter.

Either way, looking at a wildly varying transmitter output and trying to
somehow "track" true SWR probably lies beyond what the electronics is
meant to do reliably.

I don't think this is poor design either: I think it fails to accurately
measure SWR below some very low power level and we should not expect it
to do so.

Trying to capture a stream of inaccurate data is not going to produce a
useful result.

73 -- Lynn

On 4/10/2014 1:56 PM, VaibhaV Sharma wrote:
> Is there a way to
> stream SWR and other data off the radio so one can watch what the radio
> sees?
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KE6BB

Re: [KX3] SWR Reading Instability Follow-up

In reply to this post by Gary Hawkins

This really looks like a data latency of a stale data issue, as I discussed in an earlier posting and Don expanded on in his post. To me, this is a minor curiosity, but I am sure that there will be a few who will say their radio is worthless because of it. MarkKE6BB
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