Video: QMX+ receiver sensitivity comparison with KX2, FT-891

Receiver sensitivity of the QRP Labs QMX+ is currently being discussed on the QRP Labs forum with at least one member of the group is disappointed that the MDS value of the QMX+ isn't published.

With the QMX+ being a kit, any published value of MDS would have to be a range of fluid values since receiver characteristics will depend, not only on component tolerance deviations, but of builder's methods in soldering and the winding of toroids & transformers.

I decided to compare my QMX+ receiver sensitivity with the KX2 and FT-891, using a TinySA Ultra and appropriate attenuation.

As can be seen in the video below, the QMX+ does quite well.

After the video, I "measured" (by ear) the point at which a signal becomes too weak to copy on the other bands. The CP1 directional coupler does not cover 6 meters but I measured -113 dBm on that band, which is probably inaccurate. I will sweep the coupler on 6m with my VNA and post a corrected value soon.

Directly on YouTube

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The utility of Elecraft's CP1 bi-directional coupler with the tinySA

Set-up for measuring spectral output of 100W rig. Note the 100W dummy load behind the tinySA

This is an example of where the availability of a new item adds value to something that's been around a while. The new item is the tinySA - many hams now have one of these small spectrum analyzers/signal generators - and the old item is Elecraft's CP1 bidirectional coupler.

Questions I get whenever I post something related to the tinySA are:

• Can I use it with non-QRP equipment?, and
• How do I measure a transmitter's output to make sure I don't exceed the tinySA's 6dBm (4mW) max input?

I decided to use my tinySA ULTRA to measure the spectral output of my Yaesu FT-891 when operated at full power: 100 watts.

tinySA ULTRA in Spectrum Analyzer mode, with Index QRP+

I'm really liking the new ULTRA and am finally using it as a spectrum analyzer instead of just a sig-gen.

The new Index QRP+ has been daring me to measure its spectral output - and the ULTRA's latest firmware supports storage of the displayed traces as a bitmap file, saved to an included 32GB MicroSD card.

A combination of Elecraft's bi-directional coupler and my step attenuator knocked its 10W output down by 40dB, to 1mW.

For some reason, I saw no harmonics on 160, 80 or 40 meters so I brought out the big (comparatively) Rigol. Same thing - and something I need to investigate. Surely, there are harmonics being generated on these bands...

Incidentally, I looked at the other bands on the Rigol as well - the displayed traces were practically identical to those of the tinySA ULTRA.

So why buy a Rigol instead of the tinySA?

Video: Checking (and comparing) receiver sensitivity with tinySA ULTRA

Soon after the original tinySA was offered, I made a video showing how to make an MDS measurement of a receiver by using the tinySA in 'Signal Generator' mode. But with the tinySA's minimum output being -76dBm (almost S9), a lot of external attenuation was required to get the signal within range of typical MDS values.

The new tinySA ULTRA has a minimum output of -123dBm, making it much easier and less expensive to make the same test. And with less clutter since less external attenuation is needed - actually, none, other than the step attenuator you'll need anyway.

You can see the original video here.

A former co-worker of mine verified the output level accuracy of my tinySA ULTRA with an HP power meter and found it to be within 1dB of its indicated value on HF down to -100dBm. After that, inaccuracy increases to 3dB at -123dBm.

So, with the output set to -100dBm, and a 0-62dB step attenuator, I did a quick by-ear comparison of my new Index Labs QRP+ to my IC-705.

Doping toroids

Back in the olden days, when kits were plentiful and freshly-made products from Small Wonder Labs, OHR, Norcal and KD1JV filled the air, a common question was "Should I dope the toroids?"

By which they mean, should the coils be secured to the board - and/or the windings to the core - with a small dab of clear fingernail polish?

Even then, before POTA, many QRPers enjoyed taking their rigs on hikes or backpacking trips and were concerned about the possiblity of jostling a toroid out of whack (a technical term). After all, they are a fairly significant mass to be held in place with two tiny (at times) 26g connections.

Answers varied but were mostly in favor of not doping them. I don't blame the kit vendors for offering this advice - can you imagine the headache of a kit builder, doping a coil into place and then learning that it contained one winding too many? Better to not dope them and retain the ease of removal/adjustment.

The stated reason for not doping them was that the fingernail polish would change the characteristic of the coil?

Some of my commercial gear does have doped toroids - the FT-891, for example. Meant to be used in a mobile environment, it makes sense. Of course, Yaesu is certainly compensating for any possible change in a coil's inductance by taking the effect into account with the coil's initial construction.

With a few redundant LP filters on hand - and a possible bicycle ride over bumpy terrain this spring, QRP rig stowed - I decided to see what the effect would be.

Tale of two spectrum analyzers: tinySA vs. Rigol DSA-815, looking at IC-705 IMD

After yesterday's IMD measurements of the (tr)uSDX, I decided to have a look at the IC-705's IMD characteristics with two spectrum analyzers: a Rigol DSA-815 and the tinySA.

More than anything else, this comparison seems to help justify the purchase of the upcoming tinySA ULTRA - not that I need much help in that department.

As expected, third order IMD of the IC-705 was about 30dB down from the injected frequencies on both SA's, but...

...with the lowest resolution BW of the tinySA being only 3 kHz, it was unable to distinguish the RF representation of the two audio frequencies, seeing them only as one signal rather than two signals 1200 Hz apart (700 & 1900 Hz).

Finally - a decent step attenuator kit

I've gone through several step attenuators in the last two years, both kits and built units. I was unhappy with them all for one reason or another and got rid of them.

Most weren't in a metal housing - a must for such a device when used at RF - and weren't worth the trouble to install into such a housing due, usually, to cheap, flaky buttons that created more (or less) attenuation from one press to the next.

Thanks to a recent review in QST Magazine for an unrelated kit, I've finally found a good quality step attenuator.

With a metal housing.

And toggle switches.

 

The review in QST was for a two-tone generator from HecKits. I have no need for a 2TG but a look at HecKit's website turned up the attenuator I'd been looking for.

1 to 4 GHz: nanoVNA 2 Plus4 vs. Keysight FieldFox

After a three week transit, my nanoVNA 2 Plus4 arrived in my mailbox. It was around $170 and I would not have spent that if the prior nanoVNA's hadn't given me the confidence to do so. But their performance has been incredibly accurate, so I ordered and had high expectations of the new VNA.

Today I finally had the opportunity to put the diminutive piece of test equipment on the bench to get a real idea of its performance.

The nanoVNA 2 Plus4 differs from the previous versions in several ways. It has:

• A metal case
• An internal 18650 Li-Ion battery (not included due to shipping restrictions)
• A charge status indicator for the battery & charging from a computer's USB jack
• Beefier controls for slewing the cursor and markers
• A 4.4 GHz upper freq limit
• 20dB more dynamic range than earlier nanoVNA's

It is primarily due to the increased dynamic range that I bought this VNA. Like most hams, I don't need to measure anything at 4 GHz but I do want the increased accuracy (hopefully!) that comes from increased dynamic range.

Measuring MDS with TinySA, RMS multimeter and attenuators

The popularity and low price of the nanoVNA and TinySA devices allow hams and other radio hobbyists to make measurements that were out of reach just a few years ago due to the expensive equipment required.

For those who have a TinySA, you not only have a spectrum analyzer - you also have a surprisingly accurate signal generator.

With a series of attenuators, a TinySA, RMS multimeter and some cabling, I had all I needed to make an uncalibrated MDS measurement of my radios. Maybe you do too. Here's how to do it.

Harmonic content of mcHF's output

From time to time I've read comments on various forums that mcHF transceivers should not be used with amplifiers because of the high harmonic content on the output.

I almost always use an amp with mine when operating from home so I decided to measure my  mcHF. My amp, a Hardrock-50, contains its own lowpass filters on the output so I was never concerned with whether or not the mcHF was "dirty".

Here are the results, made with a recently (July 2020) calibrated HP-8563A spectrum analyzer.

Each horizontal line represents 10dB. So the top line, where the fundamental is placed, is the reference or 0 dB. The next line down from that is -10dB, then -20dB, etc.

I've put a delta marker on the first harmonic and it's easy enough to count down from the top of the display to see the attenuation of subsequent harmonics, keeping the 10dB per line in mind.