HF Antenna Analyzer

HF Antenna Analyzer

Test and Measurement Equipment Concepts

Cloud ∇ is my homebrew approach to automated test equipment that uses a computer cloud interface for control and display. Automated test instruments are better than people to perform tedious measurements and prepare the results in a user friendly format for display. The concept is an integrated package of instruments capable of automatically measuring, analyzing data as inputs, processes it according to instructions stored in its memory before providing the results to a web browser using the same communication standards used on the Internet. Communicating like the internet means you can use any computer with your favorite browser (MAC, PC, Laptop or Tablet) to control Cloud ∇ equipment.

This is a different approach to a classic problem, how to make a control panel and display for a do-it-yourself electronics project. In this case I wanted a Vector Antenna Analyzer for antenna testing with high quality data graphics and data file exports. This many seem too extreme, but just about every radio made today is controlled by a computer (microprocessor) and the user pushes buttons for control and a liquid crystal display provides the status. Even the big frequency knob everyone has is digital encoder wheel just like a computer mouse! Some radios take this another step with an internet ready interface, after all the radio doesn’t know if you are in the same room or another country. Remote operation via the internet is now the norm for many business applications, however for most people it is totally new technology, the “Cloud”. Over the past few years has the Cloud become the standard term for the general public. The problem is that most people do not know what the Cloud is unless you are an information technology professional. You probably have no idea what the Cloud means or how to use it, but you actually use it every day.

Cloud ∇ Vector Antenna Analyzer Model 60

What Is the Cloud?

Cloud technology is when the computer performing your requested task is some other place. The computer you are sitting at is only a control panel and display. Think about every time you have used your computer to find a product and order it online, you are already using Cloud technology. Yes it is that simple. You are controlling a process at another location and getting the results sent back to you using your computer. The software you need is free, it is a web browser. All you need to know is how to point and click.

My approach was to build a Vector Antenna Analyzer for antenna testing with high quality data graphics; sweeps, frequency plots, Smith charts, alerts and data file exports using a web browser on any platform; MAC, PC, Tablet and even a cell phone. The key element is the analyzer controls and display are a simple website sever to communicate with any browser. I selected HTML 5 to support graphics displays which is compatible with; Internet Explorer 9, Chrome 3, Firefox 3, Safari 3, Opera 10 and Android 3. What this means is anyone who purchased their computer after 2009 is fine. The XP OS users need to load Chrome or Firefox.

What is Cloud ∇ Vector Antenna Analyzer?

A Vector Network Analyzer 60, (VNA 60) is an instrument used for antenna testing applications. Based on a variation of measuring RF impedance by means of the three volt method using a Wheatstone bridge that can determine both amplitude and phase of reflected signals.

Therefore the name Cloud ∇ is a combination of the “Cloud “ for communication with any browser and the symbol ∇ (Nabla) used in mathematics to refer to a connection in differential geometry. The “60” represents the maximum operating frequency of 60 MHz.

Wheatstone Bridge and Detectors

The analyzer is based on a variation of measuring RF impedance by means of the three voltage method using a Wheatstone bridge that can determine amplitude and indirectly phase of reflected signals. The bridge has two matched value 47 Ohm resistor at the left and two 100 Ohm 1% resistors to get close to a 50 Ohm reference. There are just three detector diode / capacitors as shown below to find the RF voltage peak value at three points. The CW signal used to sweep the antenna must be clean, minus 30 – 35 dBc for all unwanted energy as the detector measures Vp from everything. After many tries at building a clean and wideband DDS (AD9850, AD9851) and amplifier the best I could do was -29 dBc.

The problem being 1 to 60 MHz and clean! Cutoff filters normally used for transmitters were not the answer as they are not useful for a wideband application. I started looking for how others had solved this problem and found the AM QRP DDS60. The DDS60 gives 10 dBm output with a clean -40 dBc from 1 to 60 MHz, well done! Several times my shield connection to my antenna was intermittent with both SO-239 and N connectors. I used lock and star washers, ½ copper braid jumpers, but still had problems until I drilled and soldered to the chassis flange of the N connector.

Detector Comparator Filters

I learn to keep everything compact for bridge and filter, my best results required everything on a PCB about the size of a penny. The Mega A/D input impedance is approximately 400K Ohms based on my tests. I needed a pull down resistor to prevent the input from float around three volts. Using a 1 Meg resistor pull down gave a settling time of 10 mSec for 10 MHz frequency changes and 2 mSec for 100 KHz frequency changes. This delay was addressed by a software fix adding ten readings on this first frequency before starting each sweep. The filter PCB is shown here.

How do you find the resistance, reactance and impedances?

Warning do not read this section if you must have zero error. What is important is it works well for this application. The math errors do not become a factor until the VSWR is great than 20 to 1. The VSWR range under 10 to 1 provide results that are very accurate.

The simplified equations used in this antenna analyzer are approximations commonly used in Amateur Radio projects and work fine 99.9% of the time, the other 0.1% is called error margin. One percent error would be great, but the biggest error comes from the diode detector. I will discuss the diode error later, for now let us look at how to find the resistance, reactance and impedances.


Z = Unknown Antenna Impedance - Finding the value for Z is determined by a voltage divider circuit. Just like 15 VDC dividing between a 5K and 10K resistor shown in the figure below the V out is an unknown resistor, but the voltage to different parts of a circuit can be measured. Knowing the voltage ratio between the two resistors, you also known the Z ratio. Just multiple the Z ratio times the known resistance to find Z.

Unknown Antenna Impedance = Known Resistance X ( V antenna / V resistor )

Z = 50 X Va / Vr

Finding Unknown Voltage Standing Wave Ratio = VSWR is measured using the diode bridge circuit. No energy is reflected when the bridge is balanced (0 volts across the center) only when the antenna exactly matches the 50 Ohm impedance from the two 100 Ohm in parallel. When an antenna and 50 Ohm reference do not have matching impedances, the reflected power increases and is measured as V reflected.

VSWR = Unknown Antenna Voltage Standing Wave Ratio

The result VSWR is a ratio of the differences of forward and reverse voltages equal to:

VSWR = ( V forward + V reverse ) / ( V forward - V reverse )


Antenna Resistance = R

The VSWR resulting from a PURELY RESISTIVE impedance mismatch is equal to: VSWR = Z1 / Z2.

Example for a 100 Ohm resistive antenna match to a 50 Ohm resistive transmitter;

VSWR = Z1 / Z2 = 100 / 50 = 2 or 2:1 ratio

Example for a 25 Ohm resistive antenna match to a 50 Ohm resistive transmitter;

VSWR = Z2 / Z1 = 50/ 25 = 2 or 2:1 ratio

Looking at the Phasor diagram below if the antenna is purely resistive, you would see a horizontal line ( Z = R ). However with complex impedances of R + X the Z is equal to SQRT(R^2 + X^2) where R is the real part of the impedance and X is the reactive part.

The VAA 60 VSWR is referenced to 50 Ohms, so some of the variables can be replace with fixed values.

Doing some algebra these equations can be rewritten knowing Z and VSWR to find R equals;

R = (2500+Z^2) X VSWR/(50 X (VSWR^2 + 1))

Antenna Reactance = X

Knowing the values for Z and R it is possible to solve for by doing some algebra to the equation Z = SQRT [R^2 + X^2]. This is just some trigonometry you once had in school, for any right triangle the sum of the sides square equal the hypotenuse squared. In this case it can be rewritten knowing Z and R to find X equals:

X = SQRT (Z^2 – R^2)

This method does not determine if the reactance is inductive (+X) or capacitive (-X) and the bridge does not provide any parameter to find sign. The solution is to have the analyzer's software determine if the reactance it measures is actually capacitive or inductive by tracking changes in reactance and comparing them to change in frequency. This is the same method operators’ use on manual analyzers to find the sign of X. You find the sign by simply adjusting the VFO. If increasing frequency causes reactance to decrease, the load is capacitive (-X) because the reactance of a capacitor decreases with an increase in frequency.

Vectors or Phasors = Antenna Magnitude and Phase

Antenna Magnitude and Phase are based on the Phasor diagrams below and solving for the unknown values. The Phase is the arctangent of the ratio of X and R as illustrated by the far right vector diagram. This sound and looks difficult, but the magnitude is already known as the Z. The phase is determined by the ratio of R and X that is now known. This is just some trigonometry you once had in school, for any right triangle the ratio of the sides will determine the angle.

Homebrewing

Homebrew is an amateur radio slang term for home-built, noncommercial radio equipment. Design and construction of equipment from first principles is valued by amateur radio hobbyists for educational value, and to allow experimentation and development of techniques or levels of performance not readily available as commercial products. To build or to buy, that is the fundamental question for DIY projects. You will not be savings money and the time required build and test will detract from other activities. DIY a false economy so why build? Most things in life are store bought and you get what you get, custom tailored is no longer common place. It is all about rewards of meeting a self-determined goal, not money. Building is just one more facet of the hobby. DIY building offers some advantages, self-build gives you greater control over the functions or features. You can be as specific as you want about and set the whole thing up however you fancy. One unintended consequence with DIY projects, if they succeed is they are very educational.

Comparing features is never a simple yes or no and performance is more complicated quality because manufacturers specifications are not uniform. Looking at catalog and magazine advertisements is a waste of time as they never are a direct comparison. In the end I surveyed users’ reviews to develop a list of things I wanted and what to avoid. My Cloud approach includes the high quality data graphics; sweeps, frequency plots, Smith charts, alerts and data file exports using a web browser on any platform. My self-imposed design specification was 5% or +/- 1 Ohm whichever is greater.

An antenna analyzer used in amateur radio needs to be relative not absolute in accuracy for antenna alignments. Most tuning is looking for the point of lowest VSWR or zero j so your transmitter will work better. When it comes to matching circuit design a measurement needs to be both accurate and precise to handle fixed component values. In addition to accuracy and precision, measurements also have a measurement resolution, which is another design or feature choice. I elected to report VSWR to 1/100 and Z, R and X as integers. My reasoning is it is helpful to see small changes in VSWR during tuning and the Z, R and X factional values tend to jump around constantly and are just an annoyance.

With what seems like hundreds of antenna analyzers on the market there must be a standard way of specifying performance in the opening credits, right? Most industries have a de facto standard, right? It seems in the consumer world both common sense assumptions are wrong!. Except for the top end (above $5,000) commercial analyzers, most consumer analyzers I looked into are just selling with no rules. If you like it buy, if you don’t like it that’s fine. The only standard I can find is the measure of the market place based on the consistent testing of ARRL Labs and published in QST and some ARRL books. Using past reviews I did my best to make apples to apples measurements shown in the table below just to see how I compared. I was actually surprised on how well the VAA 60 looked or how many commercial analyzers do well in the market without accuracy. Either way this is the real world answer.

Comparison of VAA 60 to Commercial Antenna Analyzers

VAA 60 as built Performance Specification*

Frequency range: 1.7 to 60 MHz

VSWR measurable range: 1:1 to 10:1

SWR accuracy: better than 5%

Z Impedance range: 5 to 1000 Ohms

Impedance accuracy: better than 5%

X measurements including magnitude and sign

Phase measurements including sign

Frequency Plots displayed for VSWR, R, X, Z Phase

Smith Charts Plots displayed

PC Data Output of Plots or as CSV data (MS Excel Compatible)

External Power Supply 100-240 VAC 50/60Hz

Size (HWD): 5.6 × 8.8 × 1.6 inches, weight 1.5 lb.

Computer Interface: Ethernet RJ-45

* More truthful than most!


System Requirements:

Web Browser: Internet Explorer 7, Firefox 3.x, Chrome 4.x, Safari 3, or later browsers

OS: Windows 10, 8, 7, Vista, XP or Mac OS X 10.6 or later operating system

Using a Web Browser has practical exceptions

Remember my approach was to build a Vector Antenna Analyzer for antenna testing using a web browser on any platform. There are always a few exceptions like the On-Off switch and power indicator, see the front panel picture below. Two more exceptions were added during development; 1) Stop Sweep button that allows the operator to exit a sweep without losing any data. Sometimes after a sweep is started you notice you forgot to connect the antenna, you hit the button and can repeat or modified the sweep without data loss. 2) The VSWR Beeper switch is a 3000 Hz tone that starts at 2:1 VSWR, increasing in loudness to full volume at 1:1. A tuning feature for people like sound and some do not like beeps, therefore the switch.

VAA 60 non-Web Browser Interfaces

Hardware Layout

The internal Hardware Layout and Parts list is shown below. Almost all off the shelf items glued together with software. The exception is the diode bridge and comparator filters inside the copper shield enclosure attached to the N connector at the bottom left.

Parts List

  • Main Processor Arduino Mega 2560
  • Ethernet W5100 Arduino Shield
  • DDS and amplifier Am QRP DDS 60 based on the AD9851
  • Stop Switch Red Push Button SPST
  • VSWR Tone 3KHz Pezio Buzzer 3-12 VDC
  • VSWR Tone On/Off mini SPDT
  • N RF Panel Connector
  • Power Connector to Mega Plug 2.1 / 5.5
  • Power Connector to front panel Jack 2.1 / 5.5
  • 100-240 VAC Wall Power Supply output 12 VDC
  • 7808 Serires Regulator on 2 X 3 heat sink
  • Power Switch Green Push Button
  • Power Indicator 10mm Grn LED
  • Large filter capacitor electrolytic 1000 uF @ 35V
  • PCB for detect / comparator filters
  • 1N270 Diodes for detector
  • Carbon Resistors 1/4w film
  • Headers and Perf Bd for jumpers
  • Hook Up Wire #24 AWG
  • Screws, Washers, Nuts, Standoffs
  • 10 Ft LAN Cable Ethernet
  • Case ABS 5.6 × 8.8 × 1.6 inches

Interconnection

There are just three header to header jumpers plus power as shown below. The pin numbers and function are listed in the same order on each end for clarity.

Homebrewing your own Code

The first thing to decide is microprocessor development family before addressing the software; Arduino, Raspberry Pi, PIC or others. Review the interfaces you will need and the tolls available, “development environment”. I went with the Arduino because it is well documented and a standard Ethernet interface was available. If I was starting today the Raspberry Pi could be a better solution, but I don’t have all the details to make a decision. Be careful each compiler treats some characters differently you will discover through experience strange problems caused only in your application. Examples are; quotation marks (single, double, left & right handed), brackets <([{}])> have special meanings, and spaces to name a few. Another issue are browsers, sorry to report not all browsers are equal in displaying your carefully formatted page. I tested using IE, Firefox and Chrome to weed out special cases that I later found are known exceptions. Firefox Hangs sometimes in combination with some antivirus programs, the virus scan that Firefox starts after files are downloaded may cause Firefox to hang. You need to disable virus scan in Firefox preferences. Chrome requests favicon on every connection which looks like a second unexplained client connect. Internet Explorer does not support all HTML tags (format information) and some styles. A specific Arduino issue is the W5100 Ethernet card has a buffer overflow issue that has a work around and I have read there is a fix with newer compilers. When developing software keep in mind modes & states and do some serious planning before you code. I know flow charts (see mine below) are not used today but they are a great tool to think out functions before you code.

I will make this an official conclusion, building your own antenna analyzer is NOT RECOMMENDED! If you know nothing about building RF hardware, microprocessors or HTML, this is not where you start. Start with another analyzer or well defined kit get it working, then modify it. The same one step at a time applies to software. The following examples of Arduino web server code will run without modifications or inputs are demonstrations on the Arduino Mega 2560 and a W5100 Ethernet Shield, expect Example #4. You will need to connect to your computer and web browser using an IP address ( 192.168.1.88 ). There many ways to accomplish this IP connection, click here for some basic methods. Later when you are more experienced you access your micro sever over the internet using port forwarding. These examples were selected so you do your own customization later on as the code is commented.

Example Arduino Code #1 “micro Web Server Message Board”

This is a very simple web server that generates a message board to anyone connecting. It does not require inputs or have controls. It was an electronic bulletin board for a club Field Day activities. It could be modified to report remote repeater battery and room temperature by adding text followed by reading and printing analog values.

Screen print of Displays in PDF

Code in text format

Example Arduino Code #2 “Simple Web Server controlling Lights by Nitin William VU3GAO”

This is a very simple web server that controls four pins to anyone connecting. Inputs are made by mouse clicks on buttons with text name functions. When a button is clicked once pin output is updated (on or off). The display does not change, although a page refresh occurs with each click.

Screen print of Displays in PDF

Code in text format

Example Arduino Code #3 “micro Web Server Control and Display Demonstration”

This is a single page web server that primarily demonstrates many possible controls formats and styles to anyone connecting. The displays are input feedback; range limits, pattern or filed checks with operator help balloons. Radio Buttons (click on the choice from a list), fill in the blank fields, and ICON buttons are used. (HTML RADIO BUTTONS for preset values) All inputs are used to control pins or enter values of numbers or text fields. A selection with and without display status updating are demonstrated.

Screen print of Displays in PDF

Code in text format

Example Arduino Code #4 “SWR Sweeper by Alan Biocca W6AKB”

This is a sweeping VSWR meter with LCD graphic display for the ARRL book “Ham Radio for Arduino and PICAXE” The book is about project-building for radio amateurs using many platforms including the Arduino.

Project Description in PDF

Code in text format


Example Arduino Code #5 is “micro Web Server VSWR Plotter”

This is a simple web server that generates a VSWR plot vs. Frequency to anyone connecting. This server will run without inputs as a demonstration. The code includes the calculations to Plot VSWR values from inputs of; VSWR, sweep start frequency, sweep stop frequency, sweep step size.

Screen print of Displays in PDF

Code in text format

Example Arduino Code #6 “micro Web Server Smith Chart + Plotter”

This is a simple web server that generates a Smith Chart to anyone connecting. This server will run without inputs as a demonstration plotting an arc. The code includes the calculations to Plot Smith Chart positions from inputs of; Resistance, Reactance and lead or lag (sign +/-).

Screen print of Displays in PDF

Code in text format

Example Arduino Code #7 “micro Web Server Bar Chart”

This is a simple web server that generates bar graphs for antenna analyzer values to anyone connecting displaying; Voltage Standing Wave Ratio, resistance, reactance and complex impedance (magnitude and phase). This server will run without inputs as a demonstration displaying dynamic bar movement and color changes with respective numerical values. The code includes the calculations to display color changing bar graphs and text values from inputs of; Voltage Standing Wave Ratio, Resistance, Reactance, and Complex impedance (Magnitude, Phase, and lead or lag)

Screen print of Displays in PDF

Code in text format


Sensor Design Continued to Develop over time

AD8307 replaced diodes on Wheatstone Bridge

Over a period of a year many attempts with the same Wheatstone bridge modified by using AD8307’s instead of diodes, as shown above, were tested. The Wheatstone bridge approach ultimately failed to measure more accurately over 400 Ohm impedances due to the bridge load imbalance from the AD8307, about 1K, placed on the bridge. At first glance a software based correction should have been a clean solution to back out the AD8307 load, however the results were still inconsistent. The issue was likely coupling in the PCB, variations in components, or generic measurement errors that could not be isolated. Later attempts to mitigate the extra load by changing the bridge resistor network values caused greater errors likely due to component variations.

The Very Successful & Final Sensor Design

Now all the redesigns have now passed and the parts are now in the scrap bin, I ultimately reaching the range and accuracy results desired. The solution was to replace diode based Wheatstone Bridge with a Stockton directional coupler for VSWR, plus a series resistor divider form impedance as shown below. I was on the correct path with the AD8307 but the sensor design was wrong for a low impedance measurement device.

AD8307 based Stockton directional coupler with series impedance comparator

The difference between the 1.05 : 1 and 10 : 1 (5 or 500 Ohms vs. 49 Ohms Resistive) is over 30 dB is no longer a problem for the AD8307 logarithmic amplifier that has the maximum 92 dB range. The 10 bits Arduino analog to digital converter accuracy is now the limiting factor in the design. I found that averaging multiple A/D samples addresses the sampling least significant bit errors and then get to at least 20:1 VSWR with 1/10 dB accuracy and useable 40 dB range. I cannot increase my range as the DDS noise floor becomes dominate in the measurements. This is a limitation of a wideband design is -50 dBc spurious noise is a problem but it exceeds my needs for antenna testing. In simple terms it works well. Very accurate under 10:1 VSWR, near 1% and 5% at 20:1 VSWR. Conclusion for my testing it is not important to distinguish between 23:1 and 25:1 VSWR, just increasing or decreasing.

Hardware Layout, inset showing AD8307 based Stockton directional coupler


I have tested with 5 to 500 Ohms test load and random complex loads from 1 to 62 MHz. Even though it is handmade prototype with discrete components it is accurate to 1%. The hardware is straigh forward; resistors are ¼ Watt @ 1% and the toroid is type 61. My RF and digital grounds are isolated and all AD8307 inputs are via 0.01 uF capacitors.

Final Words

I will make this an official conclusion, building your own antenna analyzer is NOT RECOMMENDED. I built mine because I wanted a challenge. I got my challenge plus the frustration of not having the proper test equipment. I was forced to work by trial and error with many squandered hours on false investigations. They say if it doesn’t kill you it makes you stronger. I say things always make sense after you figure out the cause of the problem. If you decide to build your own antenna analyzer use as many off the shelf components as possible and develop the minimum needed to get some functions working. Gradually add move functions after you have something working. Looking at my block diagram you can see in the end I only developed the detector and comparator the rest was software, lots of software. A good starting point is the antenna analyzer in the ARRL “Ham Radio for Arduino and PICAXE”, SWR Scanner and modify it to your needs after its working.

73

KN9B