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Hi all, I have measured the harmonics of my QMX high band where I have sacrificed 11M for 30M. As I'm a newbie on tinySA I want to ask the experts (you) if the screenshots I'm linking to below are reasonable and in the ballpark.

Screenshots from tinySA -> tinySA-App v 1.1.20.1

QMX hig band rev 2 @ 12V
Firmware 1_00_14
RWTST transformer
LPF0 (cut and optimized for 20M)

30M shot:?
20M shot for comparison:

Thanks!!

Anders

How much attenuation between the TX and the TinySA??

Looks like TinySA input overload.
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Allison
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About an inch of attenuation. I pick up the signal with a loop close to the dummy load. I said I was a newbie ;)? Not very scientific!

go to 3 inches then...? Keep the input signal blow -10dbm.
Also loose coupling like that may hear stuff going on before the filters!

THe correct way is;

TRX>>> power attenuator (30db) >>> step attenuator >>> TinySA (actually any SA).

Attenuation as needed to keep the SA input from being fried.? ?HINT 30DB of attenuation
makes 1W in become 1 milliwatt (.001W) out so more than 40 DB is needed.

Reason is you want to see what actually goes to the antenna.

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Hi Anders,

I have never been successful in doing signal purity measurements with an antenna.? I only know how to use a through attenuator or tap with a dummy load rated 5 watts or better for a QRP transmitter.? There are too many signals in the typical home that will interfere with the measurement.

Also, it is best to keep the input under -25DBm to minimize mixer products in the TinySA.? A simple test for overload is to increase external attenuation and verify that the attenuation increase reduces all of the peaks.? If not, the TinySA is overloaded, and the measurements are inaccurate.??

A 5-watt signal is 37DBm, so to get the level down to where the TinySA can operate, you need 37+25 = 62 DB of attenuation.? The best way to do that is a fixed attenuator to handle the power, then a step attenuator to step down and use to verify readings.

You can build a tap like this:


And then get a kit like this for the step attenuator.:


This would be the low-cost way to tool up.

An alternative could be to buy a through attenuator if you do not have a dummy load.? That eliminates the need for a tap:


You still need the step attenuator, connectors, and jumper cables.? When all is said and done, you can invest more in the axillary stuff than in a basic TinySA (the Ultra is better for more money).

BTW, the scans you posted do not meet FCC criteria and shouldn't be put on the air.

73
Evan
AC9TU

Thanks to you experts! I will start collecting the gear for proper measuring now. But I'm happy that I made this shot in the dark to get me started. On my work table right now are three 820 and two 100 ohms resistors, a couple of bnc jacks and an Altoids case. Looking forward!

Anders,
In previous messages, particularly with a high-band QDX on 30M, it has been shown that the harmonics do not meet US requirements. An external 30M low-pass filter is required when operating on 30M.
Gary
W9TD

I'm not finished yet! And I will work from Uganda where I gather the FCC doesn't have any jurisdiction... (The last part was a joke since I have realized that this is a very serious matter in the US, and that I of course don't want to do any ham or other fellow citizen harm.)

Coupling loop probably works better (more pick up ) at higher frequencies but definitely not flat

?So accurate spectrum measurements are impossible


Use a high quality 40dB attenuator that can handle to tx power?

I don't know about high quality 40dB attenuator, but I quickly built the AE6PM power tap and additionally an attenuator of 20dB (68/270/68ohm). Total 60dB. And then I ran my 3 Watts through that setup. New shot is here:

Indeed it looks much nicer to the eye, but the verdict is about the same. 2nd harmonic about 40dB below the fundamental. That is not good enough, right? FCC says 43dB?

And of course I had to examine my QDX high band also with 30M added, see this one:

About 38dB below the fundamental.

Now, to make things realistic I added a Z-match inline with everything i.e. QMX 3W @ 30M > Z-match tuned to 50Ohms at 30M > 40dB power tap > 20dB attenuator > tinySA. And then I got this one:

52dBs difference between the fundamental and the worst harmonic. Is that good enough? I have two QDX:s and one QMX and each of them have their own dedicated Z-match. I never run them without.

If the Z-match will do the trick I'm happy! Should I be? Will FCC still come and knock on my door if I visit the US?

Anders

Anders,

Whether an LPF or a BPF (the Z-Match is a BPF), it does not matter.? As long as what goes to the antenna is within specification, you should be okay.

73
Evan
AC9TU

Since the second harmonic is very dependent on the symmetry of the PA signal the second harmonic will vary from unit to unit and may vary with supply voltage.??

The attenuation should be the same at all frequencies.? Some taps I've encountered?
are inconsistent with frequency.

So long as no one complains and you make a reasonable attempt to comply there would be
likely no issue.

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Allison
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