HF Antenna Analyzer

HF Antenna Analyzer

Test and Measurement Equipment Concepts

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.

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.

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.

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.

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.

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.