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Just finished building my QMX Midband. Everything seems to be working fine except that the output on 15m is 3dB below that on the other bands. Here is output power by band and by voltage:

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The scans look more or less OK except for a few outliers:

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Image sweep on 60m is pretty poor:

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on the other bands (e.g., 20m) it looks OK:

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SWR sweep (into a dummy load) looks OK except for 15m:

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I checked my dummy load with my VNA. The SWR is practically 1 (return loss <-30dB) and flat from 0-50MHz. So not sure why the QMX SWR sweep on 15m is so bad.

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ADC I/Q test looks good:

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Haven't been able to test the QMX on the air except for two FT8 QSOs which were working fine.

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This is my second QMX. The first QMX I built was a Highband and while it was working for a while, the sweeps were inconsistent (the ADC I/Q test was horrible) and it suddenly stopped working. My second attempt of building a QMX (this Midband version) seems to have gone better.

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Any thoughts on the low output power at 15m and the poor image sweep and SWR sweep on 60m and 15m, respectively?

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73,

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Andreas, K6AKW

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I will try that!

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73,

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Andreas, K6AKW

On Tue, Jul 16, 2024 at 02:51 AM, Andreas K6AKW wrote:

Image sweep on 60m is pretty poor:

Andreas,

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I think it looks poor because of the very very low signal level of -80 dB(m).

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For 60 m and 40 m BPF3 is in use. How are the results for 40 m? If also such a low level the path between L401b 34t end (number L405 in the manual is a typo) across C402 to IC402 pin 13 is suspicious.

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73 Ludwig

Hi Ludwig,

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thanks for the reply! Results for 40m are excellent:

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73, Andreas

I also just noticed that the results are not always reproducible. I just did an image sweep on 60m and now it looks OK:

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I didn't make any changes. I just cycled through the different bands a few times. All of a sudden the results for 60m came out OK. So it's probably a software issue.

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73,

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Andreas

And now I am back to the bad results again:

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Again, no changes. Just ran a few sweeps (Audio filter, RF filter, etc.) then back to the Image sweep. So what we see in these sweeps can't really be taken at face value all the time. There seems to be a certain amount of randomness.

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73, Andreas

On Tue, Jul 16, 2024 at 05:42 PM, Andreas K6AKW wrote:

I didn't make any changes. I just cycled through the different bands a few times. All of a sudden the results for 60m came out OK. So it's probably a software issue.

Andreas, sometimes ago I had similar effects. For me after a Factory reset the sweeps were ok and after a while working with the QMX I had strange results again but no bad effects on the receiving. No real solution until now.

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73 Ludwig

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