QCX Mini Measurements

 

This is a follow-up on my previous post on building the QCX Mini

Measurements

As always, click on the pictures to see them in a better resolution.

I don’t have a professional lab, so don’t expect a full report à la “Rob Sherwood” (see http://www.sherweng.com/table.html ) … maybe someone can send Rob a QCX Mini to have it tested.

But I do have a good Spectrum Analyzer with an interesting function to measure harmonics.

First I measured the total attenuation of my attenuators, a 30 dB 1kW Bird attenuator, followed by a 15 dB Microwave Modules attenuator.  Total attenuation on 14 and 18 MHz, including all cables, was 46.60 dB (I will neglect the 0.02 dB difference between 14 and 18 MHz). This was entered in the Spectrum analyzer as a reference offset.

With a maximum power of 6W (to be safe), expected input to the Spectrum Analyzer (SA) or Power Meter (PM) was -8.82  dBm or less. A safe level for both the SA and PM that I use.

 

Interlude 3: a cheap but accurate Power Meter

If you have nothing to measure power, you can of course use the method that Hans uses in one of his videos, using an oscilloscope to measure the voltage over a dummy load. Then get out your old TI-30 calculator and start typing … and hope that you don’t make any errors.

Or … you can look up this little beauty  https://nl.aliexpress.com/item/32990010852.html (if the link doesn’t work, search for “RF Power Meter -75~16 dBm 1-600MHz” on Ali, Banggood, eBay, etc.)

I have this cheap Chinese RF Wattmeter, but the name is misleading TWICE, since it only measures in dBm, not in Watt or milliWatt. I built mine into a mint tin, with two small Li-Ion cells (from cordless telephone origin) UNDER the piece of copper clad. As you can see, I put a FUSE in line with the two cells.

 

And the “cheap Chinese” might imply that it is a piece of junk, but wrong again!
Actually it is quite accurate!

The circuit uses the Analog Devices AD 8307 chip, and when I compare it to my HP Generator, the deviation is never more than 1 dB. Very good for that low price.

Maximum input is +16 dBm, so make sure to use sufficient attenuators. QCX output can easily be more than 5W, depending on band and supply voltage. 10W would be +40 dBm, so a minimum attenuation of 30 dB should be enough for QRP work.

The value of your total attenuation can be entered into the wattmeter, but only in 1 dB steps, so I didn’t use that here.

Now back to the measurements.

Vcc was 12V (minus the voltage drop over the idiot diode D33).

Test at room temperature of 20°C.

After tuning L4 and L3 for maximum power output (see build report), I obtained around 5.6 Watt on 17m, and 4.0 Watt  on 20m. I made a table of the power versus supply voltage (I will redo this from 7V up later if I find some time).

Vcc IN(V)	Voltage after D33 (V)  (QCX DVM)	Pout 18 MHz (W)	Pout 14 MHz (W)
11	10.41	4.5	3.1
12	11.39	5.6	4.0
13	12.36	7.0	4.7
14	13.34	8.0	5.5

 

PA Temperature

I’m not sure what happens  if I use the QCX Mini on 20m, while it was really built for 17m use.

I get less output power, but maybe more heat is dissipated in the three BS170’s ?

So I set up an experiment, measuring the temperature at the washer on top of the finals, with an infrared thermometer. I measured after 30 seconds and after 60 seconds of continuous key down.

Vcc = 12V.

18 MHz: Pout = 5.6 W, supply current = 810 mA
 

Time (s)	Temperature (°C)
0	22
30	26
60	29

 14 MHz: Pout = 4 W, supply current = 510 mA

Time (s)	Temperature (°C)
0	22
30	22.8
60	24.0

 So the good news is: the output power may be lower, but so is the current consumption and the heat production, so nothing to worry about.

Harmonics!

This is what you have been waiting for, right ?

Ok, let’s see. The 17m rig has a 17m Low Pass Filter, that should be OK.
Ftx = 18.080 MHz, Vcc = 12V.

First screenshot below is a regular spectrum, with two markers.
Marker 1 is set for the main carrier, and the unit was set in  Watt.
Marker 2 was set on the second harmonic, and reading the difference from Marker 1 in dB.

Below is a screenshot from a special mode in this SA, the “Harmonic Analysis”. It is normally a paid option, but for the first 128 hours, I can use all options to try them out … nice!

Just one push of a button, and you get an automatic report about all the harmonics.

Harmonic suppression is about 60 dB or more below the carrier, so very good!

Then the same was done for 20m, again a regular spectrum, and then the Harmonic Analysis screen.

Oh my … the harmonic suppression is only some 39 dB here ... so is it legal to use the rig on 20m then?

I checked the Belgian regulation and even asked a friend who works at the Belgian PTT (BIPT).
The result of all my investigations is that there really is NO standard for homebrew rigs, as long as you don’t build them to be sold on the market. I don’t know for other countries of course …

It comes to this: if you have a HAREC license (I have!), you can build anything you like, and tell the PTT that you “suspect” it conforms to the standards (for commercial rigs).
As long as there are no complaints about interference from your rig, you can use it.

Of course, harmonic suppression is one thing, but in combination with an antenna, the harmonics may further be reduced, especially when using a tuned antenna like a magnetic loop, or a monoband 20m groundplane, which will not radiate well on 10m (high impedance).

To remove any doubt, adding an external 3-element LPF section for 20m would probably reduce this 2nd harmonic enough to bring it within specs (but also reducing output  power even more). Something to try when I find some time ...

Now to be sure, I did a few more tests.

First I wanted to know if my Icom IC-7300 was meeting the specs, it is a commercial rig, sold “on the market”, after all.

Running at 100W on 14 MHz, I measured the second harmonic, and here the screenshots as before.

It is clear that the 2^nd harmonic is very low, but here the third harmonic is the strongest. It is within spec though.

For a real life test, I asked a friend who lives 1 km from my house, to listen for the harmonics of both the IC-7300 and the QCX Mini, and was reassured when he couldn’t hear any harmonic from either rig.

RX Sensitivity

 I don’t have the means to make exact sensitivity measurements versus SINAD audio quality, but I wanted to see how much effect the input BPF had on reception on the 20m band, compared to that on 17m.

I have heard China and Thailand on 17m, so I’m sure the receiver is pretty sensitive!

I used my RF generator to generate a signal on 17m, and put it at a level to get a scope reading of 4V peak-to-peak, at the best tuning point, in the middle of the CW filter.

I then switched to 20m, and increased the generator level for the same audio output on my scope. 

Surprise, surprise … I had to increase the level only a mere 5 dB, so you will lose almost one S-point in receive on 20m. I guess it won’t work for the weakest of signals, but there should be plenty of strong stations to have a QSO with! 

Have fun with your QCX Mini!

[EDIT : I made another post on how to use the QCX Mini on two bands! Read that story here]

73,

Luc - ON7DQ

Some notes on the QCX Mini

Building the QRP Labs QCX Mini - 17m

This post is not about "unboxing" the QCX Mini, there is enough of that on YouTube. 

Also no details when they are clearly described in the manual. Only where I diverted from the manual, I will describe what I did. And I will add some of my tips & tricks that may be helpful in your construction adventure ... 

I used the manual version 1.05. If you read this (much) later, there may be a newer revision, so not all that is written here may still be valid, check the website.

Here a view of my workspace ... the kitchen table!

I have a row of strong LED lights above that table, and I can lower the whole thing very close to my workspace. Soldering iron is a Weller 25W with a fine tip, nothing fancy. 
Also on the table, but not in the picture: universal DVM, Chinese component tester, and nanoVNA.

All components sorted in trays (from a box of chocolates ... You never know what you gonna get! HI)

Of course I checked the contents of the package, and inspected the board with a loupe. The loupe I use  is very small, you can see the little round thing just left of my soldering iron. It was the lens from a laser printer. I have to keep it very close to the board but it gives a very good image, just like using a microscope. 

The kit seemed complete, and the board looked OK, all good so far.

WARNING ... SMD COMPONENTS

Several people reported broken SMD components.

Maybe I was lucky, but my board was perfectly soldered and showed no broken SMD's.

BUT ... while mounting the through hole components, I caught myself almost destroying one of those tiny SMD's. Holding my side cutter flat to the board to cut off a wire from a capacitor, I noticed that I almost had pinched an SMD between the back of the side cutter. Had I pressed firmly I would have broken it.

I'm not accusing anyone that they did break the SMD's, but be warned to work very carefully ...

Seperating the boards

I couldn't wait to see how the three boards really fit together .. so first I jumped to section 3.30 Break apart inner PCBs of display board.  I filed the sides until all looked perfect, and then went back to the beginning ... 

Winding T1

Like in my previous two QCX's (e.g. see my 20m QCX here) , I haven't wound T1 as described in the manual.
I wind T1 with all four windings seperate, but cutting the legs at slightly different lengths. 

That makes it easy to insert the legs one by one into the correct holes, then pulling them tight and slightly bending them to keep T1 in place.

I cut the wires really short,  then heating them long enough until the enamel burns off, and the wire can then be tinned an soldered in one go.

Checking continuity with the DVM, all was good. Then I continued with the other components, up to and including section 3.8 (C27/C28).

The LPF : measure it first !

At this stage, I jumped to section 3.25 Wind and install toroid L2
Also  wound and installed L1, L3 and the BNC connector (section 3.28) at this time.

The coils were wound with the windings spread out 180° on the core, so I could still extend or compress them. I used the nanoVNA to measure the inductance and found all three coils giving way too much inductance.

Interlude 1: how to measure components with the nanoVNA.

I built this component adapter, with matching SOL calibration kit. Sounds expensive, but is really very cheap!

A 4-pin female header is soldered to a female SMA chassis connector, two pins to the center, two pins to the ground. That way I can measure different sizes of components, inserting them in the header, or touching two of the four contact prongs. Using the two prongs I bent upward, I can also measure SMD components.

The CAL kit is just a SHORT, and a 49.9 Ohm SMD resistor (2 x 100 Ohm in parallel would also work), soldered to a male 4-pin header. The OPEN is just ... leaving the middle pins open.
Calibration is super quick, since (un)plugging the header is much quicker than mounting an SMA connector, and it works really good, even up to VHF. 

The nano VNA is put in CW mode on 50 kHz (or a higher frequency if you want to measure your component at the working frequency). Measuring S11 at port CH0 gives capacitance or inductance right on your display, and is quite accurate (I checked with some precision components).

If you want to know what else you can do with a nanoVNA, study the slides from my nanoVNA Presentation here.

When I removed ONE winding from L1 and L3, and TWO windings from L2, the inductance was very close to the values from the table in the manual. Of course, check your coils according to the band you're building it for.

Now, how to measure the LPF ?
On one side we have the BNC, so using a BNC-SMA adapter, that was easy.

For the other end, I mounted an SMA connector with two short wires to C29 and ground.
I used the ground pad of C30, but of course this filled the hole for C30 with some solder.
No problem, later on, holding C30 in place with one finger, I heated the pad and could push C30 into place, flush with the board. Re-soldered to be sure, and soldered the other pad of C30.

When I first measured the LPF, with the coils as I wound them, the cut-off frequency was around 20 MHz.

With just a little tuning of the coils, I came to this curve (not sure why the curve is a bit crooked ... but did not spend much time on it, I wanted this Mini to be finished , hi):

At 18.1 MHz I had a loss of  0.75 dB, and at 36 MHz, suppresion was 50 dB, so that looked OK for me !

Now, Hans has mentioned before that the output impedance of the Class-E amplifier is not necessarily 50 Ohm. So is it useful to tune this filter for best response between a 50 Ohm source and load ?
I think it is !

As we will see later, my output power without any further tuning was already 4.5 Watt (at 12V DC in), and I think that is not bad for a 17m model. But I may do some more tuning later.

For now, satisfied with my filter , I jumped back to section 3.9, and continued as described.

Class-E inductor L4

I wound L4 as instructed in the manual, and checked the inductance with the nanoVNA (see above).

It was a bit more than the value in the table, but I left it as it was, because I wanted to try if I could use my QCX-17m also on 20m, so it would be better if the tuning of L4 was rather on the low side.
As we will see later, it does work on  20m!

I did the same with my QCX-40m and QCX-20m, using them on the 1-lower band (so 60m and 30m), both resulting in some QSO's.

Battery indicator

The battery indicator is disabled by default, and how to use it is only described in the Firmware manual. So if you decide to switch on the battery indicator in menu 7.2, you will find out that it doesn't work ...?

Oh yes, you should connect the DVM input on the QCX Mini to the +12V pin at the four pin male header "DVM/Power Meter".

Well, the header is not described in the assembly section of the manual (but it is described in section 4.20 Test Equipment), and it is not included in the kit, you have to put one yourself. I didn't put the header, because I don't intend to use the built in Power Meter function, so I just bent a wire in a loop to connect DVM to +12V.

Just think about it BEFORE you put everything in that little case ...

Protection for the LCD module

When following the final assembly in the (optional) enclosure, the manual says to remove the plastic protection film from the LCD. So I did, but I didn't want to put the LCD unprotected in its housing.
So, what I did ... I cut out  a piece of transparency film (you know, from the time we didn't use a beamer, but a "slide projector"), slightly larger than the LCD frame.
With four small drops of superglue, I fixed the film on the LCD frame. It is hard to see in the picture but it's there!

The famous "C38" mod ... and a FUSE!

I was lucky to receive my package late enough to hear about the 5V regulator trouble.

So of course I did the C38 mod, but I wanted to do something more .. put a FUSE in line, for any other accident that was waiting to happen.

Interlude 2: Hans thinks I have a Fuse Fetish, but if you were as stupid as me to connect an expensive IC706MkIIG in reverse on a 12V/7Ah gel cell, WITHOUT ANY FUSE in line, you learn it the hard way ....

NOTE: in case you're worried, the 706 did survive, it has two 60A polarity protection diodes, so the 120A current melted my rather thin DC cable, and then the diodes killed themselves, but by then I had pulled the plug, and burned my fingers on the hot wire ... another lesson learned, hi.

I could replace the diodes and the rig was OK, I still use it today ... but NEVER EVER again will I connect anything without a fuse!

OK, so this is what I did. I found a small 1A polyfuse in my junk box, and first mounted the fuse as in the picture:

Then came the "idiot" diode D33, with the cathode lead bent over towards the fuse, and soldered to it. Then soldered the other pin of the diode.

Finally came the capacitor, I found a small 22µF/16V tantalum which fit the available space, without touching the volume potentiometer.

Here a view from the side, with the Controls board also mounted.

Of course one can always put an external fuse in the DC cable, and maybe make a dedicated cable for the QCX Mini. And while you're at it, put an ON/OFF switch in line as well, it saves plugging/unplugging the DC jack all the time when testing.

I thought to add a 5.1V/1W (or a 5.6V) zener at the output of the 5V regulator, so it would blow the fuse if the output goes any higher than 5V, but Hans assured us that is not needed, so I skipped the zener, and I hope Hans is right ... hi. 

Anyway, careful as I am, I powered the QCX Mini main board without the microcontroller first, if anything bad happened, I would at least have a good CPU.

After checking that the 5V was OK, I unplugged the power, and then mounted the CPU, and continued with the other boards.

Then I completed the assembly of the three boards, but before putting it in the enclosure, I did the adjustments ...
The BPF went as expected, no problem.

Adjustment of the trimmers

Several people reported that the adjustment of the IQ-Balance with R27 didn't do what was expected.

At first I had the same experience, but after turning the other two trimmers halfway, everything adjusted as described.

Since I think the trimmers are delivered all turned CCW, a suggestion is to first turn R14 and R24 some 5 turns CW, and then start tuning R27.

Make an extension cable

I saw a good idea on the SOTA Reflector, by Marcel, DM3FAM. See a picture of it here.

So I also made this extension cable, to be able to work on the QCX Mini, and still have access to the LCD display. Thanks for the idea Marcel!

NOTE: make sure you plug it in the right way! ;-)

Don't plug your KEY into the PTT jack ...

Very sorry to read that one OM did this, so I feared of doing the same when in a hurry ...
I looked for some rubber plugs, but didn't find any, so I came up with this simple and cheap solution.

I took a tiewrap, slightly wider than the jacks, cut a length of about 3.5 cm.

Then narrowed the ends with a sidecutter, bent it into a U-shape, and pushed it in both the PTT and CAT jacks. It sits firmly, and does no harm to the connections. 

It looks like a nice belt clip, but I wouldn't trust to hang my new toy on my belt like that ;-)

I don't like Hans' buttons ...

Just a personal preference, but I never liked the buttons that were shipped with the QCX kits, so this was no exception. I went through my button collection and found two small push-on buttons, no screws to fix. And tuning the rotary encoder is also quicker with the smaller diameter, I like my buttons! (HI)

Results and Measurements

RBN 

I received my package on Thursday dec 17th, started the construction on Friday, and by Sunday noon, it was all assembled, adjusted and built into the enclosure, ready for some real tests.

First I programmed some message memories.

A regular CQ message in memory 1, a special QRP CQ message in memory 2, and two lines for the custom splash screen (see top picture), in memories 11 and 12.

Then I let the rig auto-CQ for a while, resulting in … NIL replies, quite disappointing to say the least.

BUT … RBN showed that I was at least putting out some signal.

 

Some days later, I did another attempt, now calling as ON7DQ/QRP, that should attract some more attention you would think, but no … NIL replies. 

Of course, 17m is not the easiest band, it’s not open day and night like 40m (and even 40m can sound dead at times). But RBN gave me good hope, I even made it into the USA !

Not bad for 4.5 Watt and a simple random wire antenna (33m long).

 

(Note: the greyline is not correct, the screenshot was taken at another time)

QSO’s

RBN is one thing, but could I make some real QSO’s ?

Sure, but it takes some patience and good luck.

Tuning around, I noticed that the receiver was excellent, I heard a station from China (BA5AD), and on an other day I heard one from Thailand (E29TGW).

First 17m

First ? you may ask, isn’t the QCX Mini a monoband radio? Well, let’s wait and see …

My log on 17m so far:

Date       UTC      Call      RST received

Dec 21     0803     RW3PZ     559

Dec 21     0943     UT2LA/QRP 559 (OM Serge was using 5W and I gave him a 529)

Dec 21     1304     TA1PB     599 (YES that’s Turkey … but not Hans Summers hi)

Dec 23     1102     SX9A      599

And now … 20m !

I had used my other QCX rigs on the “one-lower” band before. So using the 20m model on 30m, and the 40m model on 60m, both resulting in several nice QSO’s. 

I hadn’t changed anything to the rigs, not T1, not L4, not the LPF.

The output power drops some 20%, and the RX sensitivity will of course be lower, but my experience was each time that it was good enough to be useable, and harmonic suppression was more than adequate.

Would this 17m QCX Mini also perform well on 20m? Let’s try that out.

TIP: to make a band change easy, I programmed 14.062, the most used SOTA frequency, as the default frequency for VFO B (see Menu 3.3). One press on the right button changes to VFO B, but two presses are needed to return to VFO A, or you will be operating SPLIT.
If you think you will use SPLIT a lot, it may be better to program a frequency for the other band in one of the Presets (see Menu 1).

 

I connected another antenna , a 20m monoband groundplane, which does perform a little better than my random endfed.

I tuned a bit closer to the QRP calling frequency 14060.50, and started CQ’ing … and to my surprise, after only three CQ’s , I got called by no less than YT2T/QRP !

We had a nice 2-way QRP QSO, OM Marko was using 5W and I gave him a 559, and got myself a 569, with only 3.5W output, amazing !  

Conclusion : in an emergency, you get a second band for free with the QCX rigs. Great fun !

Just one thought: since the L4/C30 network is not tuned for 20m, efficiency is probably lower, and some more power could be dissipated in the BS170’s … so use at your own risk.

Note to self: I should do some temperature measurements when I find the time …

[EDIT: no risk for the finals, see measurements here]

First DX!

On Christmas Eve, I did a few CQ’s on 17m , calling for SKCC contacts, using my One Piece Plastic Cootie key (see a short video of that key here).

To my surprise, I quickly got a reply from no less than W3NP, Dave in Fort Ashby, WV .

He had a very good signal of 579 with some QSB.
We had a nice QSO, and I got a 349 from Dave, also with QSB, but he got all my info OK.
So this was my first DX QSO with the QCX Mini.
I called some more on 17m, but got no more replies, so I decided to try my luck on 20m.

After a few CQ’s, I got called again by W3NP !

This time I got a 449 report, with only 3.5 Watt and a groundplane antenna, amazing!

Distance for this contact was 3833 miles, resulting in a 1095 miles per Watt contact, so this was also my first time I crossed the 1000 miles/Watt limit and this entitled me to the SKCC's QRP MPW Award.

Tuning the toroids

After I made an extension cable for the LCD (see above), I could fiddle a bit with the windings on the toroids, and still see on which band I was. I powered the QCX Mini with exactly 12V at the DC connector (and I have the idiot diode installed).

.
I only changed L4 and L3, and after a few tries, I could crank up the power to 5.6W on 17m, and 4W on 20m. This is my final result.

Measurements

Since this post is already quite long, I moved the measurements into a new post here

And check out my post on making the QCX Mini into a dual-bander here

73,

Luc ON7DQ