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QMX (1_00_020) SWR - Issue?

 

My QMX was delivered the other day but I have not been having a very good time with it so far. I am really hoping it is because I am just dumb!

It came shipped with the 1_00_019 firmware. I put 11.7v into it and when I press the CW paddle all I get is a 'S' next to the frequency. After upgrading to 1_00_020 and reading the manual it seems that this is due to high SWR (which is set in the configuration to not Tx above 3)

I have a little antenna (not great sure) but when I measure the SWR on my nanoVNA it shows about 1.2 on thye 20m band but when I move the antenna over to the QMX the swr is read at almost 5 resulting in nothing being transmitted. .

I have even tried connecting a putty terminal to the QMX and then sweeping the SWR (and let me tell you this is not always a simple exercise - the sweep often hangs at 75% and then you can only reboot the QMX and reconnect)

What I see on the terminal when it does work is the following:



Looks like a good dip at I am guessing 14Mhz

One last thing I have tried is to look at the terminal "Hardware Tests" as you can see in Hans'



Hans Hardware Test (After he has pressed the "T" key)


When I press the "T" key to tune Hans gets Power and SWR readings however I my Voltage goes to 9.8V (in Red) and 0.0W power with no SWR as you can see below:





My Hardware Test (After he has pressed the "T" key)

Sadly I have lost my Dummy Load so I cannot test it into that.

But ... Why is it so different from my NanoVNA? I am reluctant to turn off the Protection however I am not sure what else to do at this point. Any Ideas?


 

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If this were me I would first completely calibrate the VNA for a bandwidth of 13.8 MHz to 14.5Mhz to zoom in on the 20m band, so that you can accurately read the Return Loss across this narrow frequency band. A good reading is below -20dB and -25dB is better.

Once you know absolutely that the antenna is suitable at your operating frequency, which might be 14.074MHz, with lower than -20dB return loss, then you can start checking other possibilities, like flakey cable.

Dave

On Jul 12, 2024, at 17:09, IU1SWR via groups.io <sean.nelson.riley@...> wrote:

?

My QMX was delivered the other day but I have not been having a very good time with it so far. I am really hoping it is because I am just dumb!

It came shipped with the 1_00_019 firmware. I put 11.7v into it and when I press the CW paddle all I get is a 'S' next to the frequency. After upgrading to 1_00_020 and reading the manual it seems that this is due to high SWR (which is set in the configuration to not Tx above 3)

I have a little antenna (not great sure) but when I measure the SWR on my nanoVNA it shows about 1.2 on thye 20m band but when I move the antenna over to the QMX the swr is read at almost 5 resulting in nothing being transmitted. .

I have even tried connecting a putty terminal to the QMX and then sweeping the SWR (and let me tell you this is not always a simple exercise - the sweep often hangs at 75% and then you can only reboot the QMX and reconnect)

What I see on the terminal when it does work is the following:

<20240712_160702.jpg>


Looks like a good dip at I am guessing 14Mhz

One last thing I have tried is to look at the terminal "Hardware Tests" as you can see in Hans'


<HansHardwareTest.png>

Hans Hardware Test (After he has pressed the "T" key)


When I press the "T" key to tune Hans gets Power and SWR readings however I my Voltage goes to 9.8V (in Red) and 0.0W power with no SWR as you can see below:


<SeanHardwareTest.png>



My Hardware Test (After he has pressed the "T" key)

Sadly I have lost my Dummy Load so I cannot test it into that.

But ... Why is it so different from my NanoVNA? I am reluctant to turn off the Protection however I am not sure what else to do at this point. Any Ideas?


 

On Fri, Jul 12, 2024 at 06:16 PM, Dave VE3GSO wrote:
Return Loss
Thank you for your suggestion.

So I followed these 2 videos
I calibrated my VNA and set the following parameters:
Freg Range - Start: 13.8Mhz
?????????????????? - End: 14.5 Mhz?

I set 2 markers. One at 14.024 and another at 14.075 750

As you can see from the picture I have a very low SWR. I believe that I also have a Return Loss of -26.77dB at 14.024 MHz and -20.76dB at 14.074 MHz which I think is within the acceptable limits.



I unplugged the antenna from the VNA and plugged it straight into the QMX and as you can see when I run the Hardware Test SWR Sweep I get nearly 5.0 on the 14.074 MHz frequency.



Am I doing something wrong?
 

How does the QMX behave with a 50 ohm dummy load?? If the QMX reads a low VSWR with a dummy load, the QMX is working properly. In your previous post showing the Putty screen, VSWR was shown as 1.94 to 1, not 4.9 to 1 shown on your QMX screen.? It may be that you have a bad coax cable. If that's not the problem,? depending on your antenna, you may have a high common mode current on the coax feedline causing erroneous and/or variable readings. I'm not sure what your "little antenna" is. If this is the problem, it can be helped with a common mode choke at the rig. If you still get a bad VSWR with a 50 ohm dummy load, there may be a problem with a bad coax cable or a problem? VSWR circuitry in the QMX - perhaps a bad or intermittent solder connection, but this is unlikely
-Steve K1RF


------ Original Message ------
From "IU1SWR" <sean.nelson.riley@...>
Date 7/12/2024 2:11:22 PM
Subject Re: [QRPLabs] QMX (1_00_020) SWR - Issue?

On Fri, Jul 12, 2024 at 06:16 PM, Dave VE3GSO wrote:
Return Loss
Thank you for your suggestion.

So I followed these 2 videos
I calibrated my VNA and set the following parameters:
Freg Range - Start: 13.8Mhz
?????????????????? - End: 14.5 Mhz?

I set 2 markers. One at 14.024 and another at 14.075 750

As you can see from the picture I have a very low SWR. I believe that I also have a Return Loss of -26.77dB at 14.024 MHz and -20.76dB at 14.074 MHz which I think is within the acceptable limits.



I unplugged the antenna from the VNA and plugged it straight into the QMX and as you can see when I run the Hardware Test SWR Sweep I get nearly 5.0 on the 14.074 MHz frequency.



Am I doing something wrong?

 

  1. VNA comes with SMA and QMX with BNC. So you use at least one additional adapter. Could you tell how the antenna cable is connected in each case?
  2. BNC connectors are reliable. But in rare cases contacts maybe out of shape producing bad connections.
  3. I'm not sure about the frequency accuracy of the used VNA. Afaik you could use a VNA as a generator for single frequencies. So you may set the VNA on a frequency in the 20 m band and receive with your QMX. Is there a difference? Please don't connect both! Put each device away from the other. Don't connect an antenna to the QMX. An open BNC should be ok.
  4. You should really look for a dummy load.
73, Ludwig
 

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It is a great pity you have misplaced your dummy load, because your antenna appears faultless, and my next step would be to put a decent 50 Ohm load on the QMX and see what it thinks of it.

Perhaps you can fashion something out of resistors you have. 4 of 200 Ohm, or ten 1k resistors placed in parallel would do. ?Unfortunately your VNA reference load would be cooked quite quickly by the QMX’s output any longer than a few seconds. But it would only take a few seconds to see if it would show the expected 1:1.

If the QMX did think a 50 Ohm load looked like 5:1, then the bridge wiring would be suspect.

Dave

On Jul 12, 2024, at 19:11, IU1SWR via groups.io <sean.nelson.riley@...> wrote:

?On Fri, Jul 12, 2024 at 06:16 PM, Dave VE3GSO wrote:
Return Loss
Thank you for your suggestion.

So I followed these 2 videos
I calibrated my VNA and set the following parameters:
Freg Range - Start: 13.8Mhz
?????????????????? - End: 14.5 Mhz?

I set 2 markers. One at 14.024 and another at 14.075 750

As you can see from the picture I have a very low SWR. I believe that I also have a Return Loss of -26.77dB at 14.024 MHz and -20.76dB at 14.074 MHz which I think is within the acceptable limits.

<20240712_195836.jpg>


I unplugged the antenna from the VNA and plugged it straight into the QMX and as you can see when I run the Hardware Test SWR Sweep I get nearly 5.0 on the 14.074 MHz frequency.

<20240712_200031.jpg>


Am I doing something wrong?

 

Thanks Steve,

I think you may be right.

I found my dummy load (hooray). ?I measured it with my VNA and got 1.07. ?I then put that into my QMX and at first got a set reading if 10 but disconnected everything and tried again and now I am constantly getting a swr of 1.15 on my dummy load.

I am really happy about that.

So it seems that the antenna I have or indeed the coax is causing me issues. ?I will setup another one later that I know for a fact works with both my ICOM 7600 and my Yaesu FT857D and I will test with that.

Thanks again at least I have hope again that I will be able to activate some SOTA and some POTA here in Italy.
 

Thanks Ludwig,

I actually found my dummy load.

I believe it is the coax or possibly even the antenna that is causing my a problem. What was confusing was that the SWR and the Return Loss on the VNA for that same antenna was well within acceptable ranges using the setup I had including the converters I was using to test both QMX and nanoVNA.

The dummy load test as I expect it should so that is a relief. Later I will try an antenna that I know is tuned and works on my other rigs.

Thanks for the suggestions on what to looks for.

73
Sean
 

On Fri, Jul 12, 2024 at 04:38 PM, @Ludwig_DH8WN wrote:
I'm not sure about the frequency accuracy of the used VNA
It is typically well beyond what is needed to measure an antenna's impedance which varies slowly with frequency compared to, say a crystal filter.
73, Don N2VGU
 

You can test for intermittent contact by attaching your VNA and then flexing the wires, connections, etc. until you find the offending part.
73, Don N2VGU
 

On Sat, Jul 13, 2024 at 02:36 AM, Donald S Brant Jr wrote:
It is typically well beyond what is needed to measure an antenna's impedance which varies slowly with frequency compared to, say a crystal filter.
Don, usually yes, but the SWR figure from the VNA looks similar to the figure from the QMX, only shifted by some 100 kHz.

73, Ludwig

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