Antenna Analysers How To Use Them By Zs1jhg

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Antenna Analysers - How to use them and what the readings mean by ZS1JHG

Antenna analysers are handy test instruments for finding out how well your antenna is performing. How to use them: Always short your antenna leads before connecting to your Antenna Analyser. The zero bias bridge diodes are easily damaged by static. You have been warned! First of all it is recommended that you always attach an RF choke balun at the analyser to ensure correct readings. Any common mode current reflected back down your transmission line will give you incorrect readings. So if you are getting an unusually low impedance (Z) or resistance (R) this can be caused by common mode currents. If you are still getting strange readings your analyser may need to have its calibration checked. Power your analyser from a quality power supply, no wall warts please, and if variable set to the best voltage for your analyser per the handbook specs. This ensures that during the test period your voltage stays constant. The built in batteries are for outdoor use for short periods and here only high output alkaline batteries should be used. Set the inductance range so that the antenna you are testing is mid range if your analyser has overlapping ranges. Adjust the tune controls to the frequency you want your antenna to operate best on for example a Dipole for 40m to operate best on 7.060 Mhz. Now look at the SWR, Z and phase angle, or R resistance and X reactance readings (depending on the type of analyser). For example our Dipole has the following readings Frequency 7.060Mhz SWR 1.7 R 54 X 28 (or Z 61 Phase angle 27 deg)

Page |2 Alter the Tune control to move the freq up or down to minimise the value of X i.e. closest to X=0 possible. This is the resonant freq of your antenna system. Make a note of the Frequency and all other readings i.e. R, X ,SWR (Or Z, phase angle) over a range of Frequencies , say 7.000,7.050,7.060,7.070,7.100. Record the time and weather conditions (as a damp and dusty transmission line can alter your readings ,300ohm ribbon being the most affected.) Making a record of your readings is VERY important to follow how adjustments made are affecting the antenna and for future reference. If you only wish to twiddle the tune control to get the lowest SWR readings, then read on you may learn a few things. You will need to use some maths and basic Smith Charts to know what the readings mean, so if this puts you off hang in with us. I am no maths guru so I will be using the KISS system.

Lets look at the SWR reading first as it is the simplest or is it? SWR is not at single point but a circle of SWR best seen and understood on a Smith Chart. Smith Charts look like spiders webs to me, I agree they are difficult to understand, stay with me. Time now to do some easy maths so download a copy of SuperSmith at www.TonneSoftware.com Enter your Freq of interest say 7.060 Mhz per our example, double click on the load section of the circuit and up will pop R and X enter 54 and 28, press accept and the load will appear in the circuit diagram. Now click analyse and up pops the Smith Chart with a blue dot showing the coordinates of our impedance. Hold down the left mouse button while hovering on the blue dot and you will see the coordinates pop up. Click on VSWR setup and enter the SWR per your readings, in the example 1.7. Click accept and a VSWR circle appears which shows you all SWR points for a 1.7 to 1 SWR reading. Look at the 0 reactance horizontal or 50 ohm base line where the 1.7 SWR circle crosses. On the left hand side crossing R =29 X=0. On the right hand side crossing R=85 X=0 Remember you hold down the left mouse button to view any coordinates on the Smith Chart At both these points your antenna system will be resonant i.e. X=0 or close to 0, but the 85 ohm pure resistance (no reactance as X=0) is simpler to match for example use 75 ohm coax feedline and your SWR will be 1.1. This is why it is recommended that you tune for resonance at the operating freq of your antenna system. To get your antenna resonant you need to trim the length. Move your tune control slightly up or down in freq till you get the lowest X reading. If this is lower the antenna is to long if higher in freq the antenna is too short. Bad news, so always err on the long side when first cutting an antenna.

Page |3 After adjustments take new readings and record them, so you can refer to your first set of readings to see if you are improving the antenna system. “My beam antenna has a low SWR of 1.2 on 20m only rising slightly at band edges.” What is this telling us? Take out your beam antenna manual or check the manufacturer’s website for a SWR plot at 20m. Notice the sharp Dip in SWR and narrow freq range at resonance. So your antenna is most likely being influenced by ground as it is not mounted high enough i.e. less than 30 ft. This effect can also be seen on verticals with low SWR and broad banded. Mount them higher and the SWR and impedance goes up. Your beam could also not be resonant i.e. X=0 or close to it ,due to the influence of other wire antennas detuning the beam or your coax is lossy and reducing your SWR readings. So you thought a low SWR was always good ! This is how you use your antenna analysers readings: you compare them with other data i.e. from the manufacturers manual and see if your antenna is performing correctly. Another good point of reference for most types of Antenna is www.cebik.com where you will find reliable data to check your antenna readings against. Next we look at Z Impedance and Phase Angle But my analyser only gives X and R i.e. rectangular coordinates, don’t worry you can convert to Polar Coordinates with maths or use an Excel Impedance Polar Conversion Calculator. Told you we would make the maths easy for you. Z Impedance is comprised of X reactance and R resistance, so only when X=0 will R and Z be equal i.e. pure resistance and no reactance. Remember the formula Z=R+- jX. The j in the formula tells you it is an imaginary number and to add the number you need to use vector addition. Hey Bud I thought you said the maths would be easy. Hold on OM I will show you a simple method of working out Z and the 0 phase angle (we will discuss this in a moment). In our example R=54 X=28, so take a sheet of paper or use graph paper and draw a horizontal axis and vertical axis. On the horizontal mark off 54 divisions for R. On the vertical mark off 28 divisions of the same scale for jX. Now intersect the two points with a vertical line from R=54 and the horizontal line from jX=28. Draw a line from this intersection point back to 0 on your axis. This represents the vector and the angle at the axis can be measured and length measured to give you Z.

Page |4 A diagram is shown below in which Z measures as 61 and phase angle as 27 deg. 28 Z 61

27

54

This agrees with the Maths formulas using the Excel conversion to polar co-ordinates calculator (details of how to set this up will be given in the appendix). So what does this tell me. A phase angle of >0 means the voltage in a sign wave is leading the current by this amount i.e. 27 deg ahead of the current. This will result in a reduction of power output due to a mismatch in phase. When the phase angle is close to 0 then Z is pure resistance i.e. no reactance. The Z impedance readings can be used to check out ferrites to see if usable on HF or not (see note in appendix) The more advanced analysers give R resistance and jX reactance and the sign of X Resistance R is easy the pure resistance part of impedance. Close to 50 ohms (R) is the aim with a small reactance (X). Per our example Freq 7.060 R= 54 X=28 SWR 1.7 an acceptable match. Reactance jX can be (-) capacitive or (+) inductive . This tells you the value of inductance (i.e. add a matching coil ) or capacitance ( add an rf capacitor) to be added to the antenna system to give a match to 50 ohms. (In practical terms an ATU is often used to supply the required values of inductance or capacitance). The sign of X can be determined if not shown on your analyser by using the following method : Tune SLIGHTLY higher in freq and if X increases then it is inductive Tune SLIGHTLY lower in freq and if X increases then it is capacitive. What about multiband antennas? How do we check them with our analysers. Download – An SWR-Feedline-Reactance Primer Part 1 and Part2 from www.cebik.com The above articles will give you a very good introduction to understanding how reactance,impedance and SWR react in an antenna system Part 1 Shows how a dipole antenna should perform and in, Part 2 How a beam and loop antennas should perform and how to match them. What’s this got to do with the readings I get from my analyser on my multiband antenna ? I thought you said you would be keeping it simple!

Page |5 Well if you want to learn more about using your antenna analyser you need to read up so you are better equipped to be able to understand and interpret what your readings are telling you. This article is aimed at assisting you to expand your thinking and show you where to get the MORE info you need to be able to make the most of your antenna analyser readings and adjust your Antenna for the best matching and enhanced performance. Draw up a graph of your readings from your notes taken over a range of frequencies above and below resonance. (An example is given below ) You will need to do this for each band that your antenna resonates on ie 40, 20 and 15 meters

3-Element Beam R

X

SWR

70

Ohms (Ω)

50

2.2

40

2

30

1.8

20

1.6

10 1.4

0 -10

1.2

-20

1 14

14.05

14.1

14.15

14.2

14.25

14.3

14.35

Frequency (MHz)

In the example of a three element 20m beam above you will note that the SWR dips sharply at resonance and the reactance (X) rises with frequency increase. The impedance (R) stays fairly flat around +-27 ohms dropping off a bit after the resonant frequency. This graph is typical of the relationship between SWR, R and X for a Yagi with a reflector and driven element.

VSWR

2.4

60

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Summary To get meaningful readings make sure : - Your batteries and or power supply are supplying the correct voltage and operating in the voltage range for best accuracy per your manual. - You have connected an RF choke between your antenna system and analyser . - The Analyser has been correctly calibrated or agrees with QST ,test report readings -Take your readings over a range of frequencies below and above resonance or your operating frequency . - For Multiband antennas you need to do the above for each band. - Record all readings so you can plot on a graph later and use as a check in the future. - Compare your readings to a known reference ie antenna manufacturers SWR curves or if homebrew look at www.cebik.com lots of antenna examples here and all supported by antenna modelling readings of SWR ,R and jX and graphs over a range of frequencies. - Make adjustments to your antenna system so that your readings are similar to the reference. - Check your antenna periodically with your analyser over the operating range of frequencies to see if your readings still line up with those recorded last. - Obtain a copy of an Antenna Handbook – Recommended Practical Antenna Handbook by J Carr

Appendix SuperSmith at www.TonneSoftware.com to download the Smith Chart program An SWR-Feedline –Reactance Primer Part1 and Part2 – Download from www.cebik.com www.cebik.com where you will find reliable data on antennas to check your antenna readings against. How to check your ferrites - Common-Mode Chokes Confidential draft for YCCC review, by W1HIS

Page |7 How to set up Excel for Impedance Polar Conversion ie Z and Phase angle and back to Rectangular

IMPEDANCE POLAR CONVERSION Rectangular: Polar: R X Z φ 54 28 60.828 27.408

by D J Green

Copy and paste the layout as above into the top LHS of an Excel sheet. In the Polar Z cell enter the following formula or copy and paste

=(B5^2+C5^2)^0.5

In the Polar Φ cell enter the following formula or copy and paste

=ATAN(C5/B5)*180/PI()

Impedance conversion to Rectangular Polar Z 61

Φ

Rectangular R

X

27

54.351398

27.69342

By ZS1JHG

Copy and paste the layout as above into top LHS of a new tab in your Excel file In the Rectangular R cell enter the following formula or copy and paste

=COS(C5*PI()/180)*B5

In the Rectangular X cell enter the following formula or copy and paste

=SIN(C5*PI()/180)*B5

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