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You can find my Arduino lessons at my site M0IFA.

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Learning about the Nano VNA

Quite amazing is this Nano VNA, Vector Network Analyser. There are two basic capabilities, to measure a single impedance, like an antenna. Or to measure an input/output device, like a filter.

Here are the slides I made for presentation at our local club.

The Nano VNA is very low cost - around £30! and comes with three important SMA connectors, one a short circuit, one an open circuit and one of 50R load. These are used to calibrate the system.

Also available is a PCB with 18 demo circuits that can be used to learn about using the device. These include LCR circuits, capacitors and inductors, a Low or High Pass Filter and a couple of ceramic notch and peak filters.

It's important to know there is also a PC (Windows) interface which allows the device to be setup and used from the computer - which is a lots easier than using the on-screen menus.

The design is ingenious. Effectively it is three direct conversion SA612 receivers. Each fed with an RF input, from the CH0 & CH1 inputs and mixed from two outputs of the Si5351 directly. The clever bit is that the outputs CLK0 & 1 of the Si5351 are always separated by a fixed low (audio) frequency. Thus they output audio signals proportional to the RF inputs. The three SA612s feed a triple A/D TLV320AIC3204 convertor which output an I2S digital audio bus to the main computer the STM32F072C8T6. This computer chip is made by STMicroelectronics is based on the ARM architecture. It also programs the Si5351 and drives the colour TFT display.

There is a 50R resistor bridge for CH0 (S11) single port measurements outputting to the first two receivers the reference and reflected RF levels. CH0(S11) also outputs RF to a device under test. The CH1 (S21) input to the third receiver measures the output of a network (such as an LPF or BPF).

The core of the schematic shows the SI5351 using outputs CLK0 & CLK1, the bridge at CH0(S11) connection and to the mixer inputs (REFERENCE, REFLECTED and CH1), giving three audio outputs.

EXMAPLES

So let's have a look at three examples, first a Low Pass Filter, using a 40m kit from QRP-labs.

And this is the PC Python software display (nanovna-saver-0.2.2.exe) showing the LPF response (bottom right graph). A marker can be placed on the curve (inverted red triangle) and the data read in the middle area.

Next an antenna, this is a Wonder Loop which I use all the time for FT8 comms.

And the result at 40m is,

The top right graph shows the Reflection Coefficient, and the middle data shows the antenna VSWR and impedance. Interesting to note how narrow is the bandwidth of the antenna, meaning it must be re-tuned for any small change in frequency. When tuned accurately I get an SWR of 1.8 at the TX.

Lastly a notch filter on the experimenters board.

Here you can see the connection to the experiment board, the response. 

I have yet to understand and experiment with the Smith Charts and Polar Diagrams.

RF Sniffer / level indicator

In the last post I talked about a low power 1W amplifier that I was building for WSPR TX. This amplifier has a variable gain - a simple potentiometer at the input, nothing fancy. This will set the output level.

The Amp output will go through a 40m LPF, on to my VSWR/Power meter, then my auto ATU and small loop antenna.

Wouldn't be nice to have some indication of the level of RF output? 

So I have looked into this and have come up with a row of LEDs, four green, one yellow and one red. The idea being that if the red one is alight the amp is close to overload.

To implement this I found on eBay an Audio Sound Level Indicator . I wired this to a simple peak detector and bingo it lights up when it detects RF.

The diodes are germanium low drop.

Here's a mock up, driven from a +12V supply, RF is detected on the short yellow wire at the bottom, when my ELAD FDM-DUO is transmitting at 7W power into my loop on the window sill!!!

Nano VNA

There's a new kid on the block, and if you haven't noticed go buy one. It's the Nano VNA. A compact 50kHz to 900MHz network analyzer.

I have only just got to grips with this new toy, but already I can display my antenna SWR vs frequency. I get a best SWR of 1:2.8, so I must use my AutoATU, when I get < 1:1.5.

EXAMPLE

And here's an example using it to display the characteristics of 40m LPF I built from a kit.

The triangle marker is at the -3dB point, which shows top right as 8.3MHz for this filter.

MENUS

To get this display I did this

STIMULUS - START 2MHz, STOP 20MHz
DISPLAY - TRACE, turnoff all except Blue
FORMAT - LOGMAG
CHANNEL - CH1 THRU
SCALE - 10 x1

TFT display

There are some interesting TFT 3.5" displays on the market today. These plug directly into an Arduino, but if this is an UNO then they occupy all the I/O pins! But if an Arduino Mega is used then there are more than ample other pins to use - both digital and analog.

There is also a couple of libraries which make programming the display relatively easy

- Adafruit GFX
- MCUFRIEND_kbv (what a kinky name!)

These can be used natively to address the display.

TFT.h
But I have re-written my popular OLED header (Oled.h) which provides functions suitable for many Amateur Radio applications - Bar graphs, Frequency, Date & Time, messages...

This is called TFT.h

All the info you need to use this header file in on my site M0IFA.me. Here you will find the libraries needed and a TFT TEST sketch (under UTILITIES) so you can try it out and learn how to use it.

Update!!! ADF4351 GHz VFO

There has been some updates to the code for my ADF4351 VFO.

Arduino Sketch
Arduino library ADS4351

Try it out. It drives an I2C OLED screen.

RTL-SDR dongle and CubicSDR software on my MacBook.

A tuneable VFO version is also on my download site.

This implementation has some limitations because the Arduino Uno (Funduino) has only 32 bit maths, and at low frequencies 32bit division foxes the UNO and at high frequencies 32bit is not enough to calculate the parameters for programming the chip. It should be good for 100MHz to 3GHz or so.

Club ZEST - grow your club

ZEST

So you are new to amateur radio? You thought it might be an interesting hobby and exciting exploration of communication technology and design. After all you have an electronics technical background. Or maybe you simply would like to talk to the world?

But sadly, it doesn't happen. You get your licence then find that all it consists of is buying a multi-thousand pound black box (ICOM, YAESU, KENWOOD etc) and stringing up a piece of wire in your garden. And that's it. Now you find the amatuer radio bands are quiet and few people want a chat or even a contact.

This outcome has some activites though, you simply take your expensive blackbox into the nearest meadow (or distant island paradise), chuck your antenna up a tree and use it from there. Its called a field day or a contest as you swap call signs and 5 & 9's with as many people as you can.

And then there are radio clubs who do little else than occasionally activities, while spending the rest of their time chit-chatting, more often than not on other subjects but Amateur Radio.

But do not despair, there are a few activities, shrouded in mystery that are pioneering things, for eaxmple Amateur Digital TV or Satellite communicaitons, with strong but somewhat clique clubs that you can join. And there is a background of indivual world-wide bloggs of rather poor content for you to browse. There is also your own RSGB organisation, who have a boring magazine with few stimulating articles except constant reviews of commercial gear and the occasional shallow technical briefing.

At the Banbury Amateur Radio Society we do things differently. We supply ZEST. A deeper interest in what make our hobby tick. In learning about the electronics and signaling technicques of radio, of embedding competance in software coding and in buliding examples of equipment to use this knowledge. We call it "Learn - Code - Build". It has in the last 4 years doubled our meetings to four a month, is well attended and has covered many interesting projects.

For example,

1. Software Defined Radio, when SDRs were almost un heard of and noone had seen a waterfall. A project to design and build an SDR receiver, as an example for 40m. Using a digital sythesiser Si5351, an IQ generator 74HC74 and a Tayloe detector FT3253 & dual op amp. Giving IQ audio outputs (0-100kHz stereo audio) to a PC running free HDSDR software. The club designed the PCBs and sourced kits of components for 10-15 people to build the system. It worked very well and opened a lot of eyes.

2. A more integrated SDR covering the full 1-30MHz range, using again the Arduino UNO board, our own developed code but a commercial Elektor SDR module that plugs directly into the UNO. This was boxed up in an extruded aluminium box and uses a rotary encoder for tuning and tuning step and a 1.3" OLED frequency display. A really useful piece of shack equipment - an RX and +/-100kHz spectrum display across 1-30MHz.

3. An on-going Arduino coding course which is bringing new ideas and shining lights for members in this new world of coding, with a fundamental START session followed by 25 project coding activities using peripheral devices. An advanced course is availble for dedicated radio apps.

4. A QRP VSWR meter, based again on an Arduino board, this time the very low cost NANO, with an innovative RF front-end using a two hole ferrote toroid and two AD8307 log deectors. This allows VSWR to be measured from 0.1-10W and displays the power and SWR on a 1.3" OLED.

5. A major project was the build of three useful shack items. A three output digital RF signal generator (1MHz to 180MHz outputs), an RF power meter coverng the range -70dBM to +10dBm, and higher powers with external attenuators. And a Direct Conversion receiver aimed at receiving 40m FT8 signals. Kits were sourced PCBs Components and boxes for 15 members to build them.

It was followed up by experiments using the signal generator to output WSPR, QRSS, Beacon signals and together as instruments to measure antenna return loss and resonance of tuned circuits.

6. An audio signal generator based on a digital sythesiser. Actually capable of sythesising sine waves from 0-10MHz! Using the AD9833, with code written, again, for the Arduino UNO.

7. How does your blackbox generate SSB? we asked. Turns out few know how it does it (surprising isn't it?). So we designed and built a breadboard to show how it is done. With carrier and VFO RF inputs from our signal generator (#5 above) and audio input from our  audio generator (see #6). The SSB exciter was built using an audio-carrier mixer (11MHz input), followed by a xtal filter (4 stage 11MHz) and another VFO mixer (4 or 18MHz for USB of LSB output) an a final band pass filter and output stage. All stages used BF245 JFET devices.

8. The latest project underway is a GHz VFO digital freuency denerator based on the ADF4351 witha 3.3V Arduino UNO to output around 100MHz to 3GHz. The coding is on-going. (See above blog post)

10. And don't forget our summer "Pixie Challenge". A race to build the low cost RTX kit found on eBay and read a CW beacon transmission in the club house. Followed by a challenge to make a CW QSO with another Pixie. Surprising how rusty our CW was...

That's ZEST. And that's the way to stimulate more club membership and lots more interest in our hobby.

Find out more on this blog and at my download site M0IFA.me