This might be a dumb question, but I wanted to know if you can use a nanoVNA to test a transmitter built from a PCB kit (like instead of a dummy load). I realize VNAs are usually used for antenna applications, but I was just curious.

The short answer is NO you still need a dummy load or an attenuator that can handle the power from the transmitter. Then VNAs are not really designed to do spectral analysis of transmitter output. VNAs measure the characteristics of antennas, filters, and with the correct attenuator, amplifiers. They need to control the source frequency being used to measure the characteristic. A Spectrum Analyzer (SA) can measure the output of a transmitter for signal characteristics by scanning the frequencies that the receiver in the SA measures. An inexpensive SA would be the TinySA.

So a VNA is measuring the network being tested by adjusting the stimulus feeding the network and measuring the result. A spectrum analyzer scans the frequencies in question and measures what it sees regardless of what the device is sending.

Both have limits on how much power can be applied to the input before overload and damage occurs. That is why you need a dummy load and/or the proper attenuator.

You can look up NanoVNA and TinySA on the internet to find out more about these instruments.

73
Evan
AC9TU

On Sat, Feb 13, 2021 at 07:48 PM, Cierra wrote:

use a nanoVNA to test a transmitter

To over simplify a bit, the nanovna is a transmitter. It also has a sensitive meter circuit inside, which would probably be destroyed by power from an external transmitter such as your kit. A poor, rough comparison would be like using an ohmmeter to check your house power wiring while the power is still on.

But you CAN use it to check parts and circuits inside the transmitter while the the transmitter is not turned on (not connected to a power supply of any sort). Is that what you meant?

--
Doug, K8RFT

I use my nanoVNA to test the input swr. I found that the 70 watt kit on eBay has a high SWR at the input on some bands. I am in the process of optimizing that now.

On Sun, Feb 14, 2021 at 05:04 PM, Howard Fidel wrote:

I found that the 70 watt kit on eBay

Hello Howard,
Do you have this one bought?
https://www.ebay.de/itm/70W-SSB-linear-HF-Power-Amplifier-For-YAESU-FT-817-KX3-DIY-Kits/272629674094?hash=item3f79feb06e:g:xvwAAOSwAANY73ML

I could not make it to work.

Instead I build this 45W kit:
https://www.rudiswiki.de/wiki9/AfuSW-PA-45W

73, Rudi DL5FA

Howard and Rudi,
I have gotten the SWR for the DIY 70watt HF Amp kit to work with a Hermes HL2. It took a 16ohm 5watt or higher resistor across the secondary of the input transformer. If you look closely at some of the minipa100 amp pictures that use the same basic circuit, you will see the 16ohm resistor. This makes sense since the input transformer is a 2:1 ratio or 4x impedance. There is some impedance from the rest of the gate input circuit, so the 16ohms makes sense.

Also, there is a webpage with details on the build and then test and optimization of the amp here:
http://pa-11019.blogspot.com/2016/11/diy-kits-70w-ssb-linear-hf-power.html

That reference shows 15ohms as the optimum which would be a reflected 60ohms IF there were no other impedances connected.

The other alternative is to put a Pi resistor pad on the input. This is a good solution for higher power rigs like the µBITX where you also need to cut the input power to 5 watts.

Be sure to add the low pass filter that is needed on the output of the amp. It is difficult to get the bias just right to not have some harmonics above the allowed levels.

Hope that helps.
73
Evan
AC9TU

Well, the radio is a QRP Pixie kit and it doesn't put out very much power; I just wanted to test if I soldered the connections well. I have an analog multimeter I could use as well. Eventually I plan to get (or make) a dummy load, but I'm still practicing my soldering for now.

I say go for it, 1.2W is nothing and the soldering lesson you will get
afterwards
will be a great learning experience.

I once loaned a (VERY CHEAP) analog multimeter to some technicians that used it to check the resistance of the power lines. When they brought it back, the inside of the case was copper clad.. They reimbursed me for my $3 multimeter.
Ed

Since you’re playing with transmitters, a dummy load is an indispensable
tool. If I were you, I would stop and make one before continuing with the
pixie.
It’s simple enough to make one if you have some basic parts on hand. If
not, there are very cheap kits available for low power dummy loads. For
example, qrpguys.com has one for $10 which also gives you a dc voltage
proportional to the RF power you feed it. This will give you some
additional soldering practice, but more importantly will give you a way to
safely test qrp transmitters and measure their output power with your
multimeter.
A little dummy load like that will come in handy again and again as you
progress in your hobby. You will not regret it. I built a similar one into
a box with a dc meter movement years ago. Even though I have plenty of
other test equipment, that’s what I reach for when it’s time to smoke test
a QRP transmitter.
Good luck with your project and have fun!

To put things very bluntly, the NANOs have no place around a transmitter,
even 1 watt. It's circuitry without power, yes, but not at power.

Dave - WØLEV

On Sun, Feb 14, 2021 at 6:46 PM Dragan Milivojevic <d.milivojevic@...>
wrote:

I say go for it, 1.2W is nothing and the soldering lesson you will get
afterwards
will be a great learning experience.

On Sun, 14 Feb 2021 at 19:00, Cierra <dubosec@...> wrote:

Well, the radio is a QRP Pixie kit and it doesn't put out very much

power;

I just wanted to test if I soldered the connections well. I have an

analog

multimeter I could use as well. Eventually I plan to get (or make) a

dummy

load, but I'm still practicing my soldering for now.

--
*Dave - WØLEV*
*Just Let Darwin Work*

Spoilsport 😉

Maybe, but even at the low price, I can't see someone blowing up the device
if we could prevent it.

However, as I've stated before, experience is the best teacher.

Dave - WØLEV

On Mon, Feb 15, 2021 at 1:00 AM Dragan Milivojevic <d.milivojevic@...>
wrote:

Spoilsport 😉

On Mon, 15 Feb 2021 at 01:31, David Eckhardt <davearea51a@...>
wrote:

To put things very bluntly, the NANOs have no place around a transmitter,
even 1 watt. It's circuitry without power, yes, but not at power.

Dave - WØLEV

--
*Dave - WØLEV*
*Just Let Darwin Work*

Here is a YouTube that shows how to use the NanoVNA as a spectrum analyzer.
https://www.youtube.com/watch?v=GX1qN0ywsSs

So it can be done, but the results are not usually good enough to ensure emissions compliance. It is not really designed for this use. You must also limit the power input to very low values. I do not know the specifics but would assume less than +20 dBm or 100milliwatts. Anything higher would cause damage.

73
Evan
AC9TU

Found a better YouTube video that describes the limits as a spectrum analyzer:
https://www.youtube.com/watch?v=xMQZTy3iV9o

73
Evan
AC9TU

So, in theory an attenuator like this connected between a 5W transmitter and TinySA would work? http://www.qrpkits.com/attenuator.html

Yes. 5W is +37 dBm. This attenuator can handle 5W continuously, and its 41 dB attenuation will reduce the transmitter signal to -4 dBm. The TinySA max input power is +10 dBm with its internal attenuator set to 0 dB, so you are ok. Using more attenuation in the TinySA might be prudent. Note the qrpkit attenuator upper frequency limit is 200 MHz. Its attenuation will probably be reduced at higher frequencies.
Dave

Cierra <dubosec@...> wrote:

So, in theory an attenuator like this connected between a 5W
transmitter and TinySA would work?
http://www.qrpkits.com/attenuator.html

Sent with mySecureMail.
http://www.mysecurephone.eu/

On 2/14/21 10:19 PM, David McQuate wrote:

Yes. 5W is +37 dBm. This attenuator can handle 5W continuously, and its 41 dB attenuation will reduce the transmitter signal to -4 dBm. The TinySA max input power is +10 dBm with its internal attenuator set to 0 dB, so you are ok. Using more attenuation in the TinySA might be prudent. Note the qrpkit attenuator upper frequency limit is 200 MHz. Its attenuation will probably be reduced at higher frequencies.
Dave

Cierra <dubosec@...> wrote:

they have some other attenuators that might be more suitable (and cheaper) - you might look at the single T attenuator or at the dummy load. (I worry about a switched attenuator - if you accidentally forget to switch it, poof goes your SA)

That dummy load, which has a diode detector, looks interesting. What I would do is take a 50k noninductive resistor in series with the SA input and hook it to the 50 ohm line on the input of the load (making a 1000:1 voltage divider with the input Z of the SA).  That will give you about 60 dB of attenuation, putting your 5W at -23 dBm

On 2/15/21 6:58 AM, Jim Lux wrote:

On 2/14/21 10:19 PM, David McQuate wrote:

Yes.  5W is +37 dBm.  This attenuator can handle 5W continuously, and its 41 dB attenuation will reduce the transmitter signal to -4 dBm.  The TinySA max input power is +10 dBm with its internal attenuator set to 0 dB, so you are ok.  Using more attenuation in the TinySA might be prudent. Note the qrpkit attenuator upper frequency limit  is 200 MHz. Its attenuation will probably be reduced at higher frequencies.
Dave

Cierra <dubosec@...> wrote:

they have some other attenuators that might be more suitable (and cheaper) - you might look at the single T attenuator or at the dummy load. (I worry about a switched attenuator - if you accidentally forget to switch it, poof goes your SA)

That dummy load, which has a diode detector, looks interesting. What I would do is take a 50k noninductive resistor in series with the SA input and hook it to the 50 ohm line on the input of the load (making a 1000:1 voltage divider with the input Z of the SA).  That will give you about 60 dB of attenuation, putting your 5W at -23 dBm

And now that I read this.. just get the dummy load. Put your Tiny SA with its whip antenna near the load - it won't have flat frequency response, but you'll see your signal

If it's of any use to anyone, here I described a simple 40 dB / 20W attenuator I once built
https://reflector.sota.org.uk/t/qcx-mini-part-2/24564/54
You can also convert a QRP Labs or QRP Guys dummyload into an attenuator that way, it may not be the "perfect" 50 Ohm IN/OUT attenuator, but surely good enough to test a QRP rig. Even with ordinary resistors, it will be useable in the HF range.

Another option is to build a "power sampler", see a link in the post mentioned above.

As for that "better" video on Youtube that Evan mentioned.
It shows the problems all right, but it does NOT explain it, the maker of that video cleary missed some points.
"... it goes up and down ... some weird filter ... " huh ?

The simple explanation is that the NanoVNA is limited to 101 points, so runs over the spectrum in too coarse steps, missing several signals if they are not on one of those 101 frequencies. Reducing the sweep range may show more signals, but still miss some.
And nothing weird about the IF filter, but the IF of the NanoVNA is at 5 kHz, so you will see an image signal at 10 kHz offset.

So the NanoVNA is not a spectrum analyzer, but yes, it can be used as a very crude signal monitor ... if you know what you're doing, and don't expect too much.

73,
Luc ON7DQ