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FEATUREIAN WADE, G3NRW EMAIL: G3NRW-RADIO@NTLWORLD.COMRADCOM ♦ JANUARY 2012

INTRODUCTION. Chris Lorek, G4HCL,reviewed the AIM4170 antenna analyser inthe July 2007 RadCom [1]. He wrote, “TheAIM4170 antenna analyser uses a small boxcontaining the actual hardware, together withyour shack or laptop PC for the user interface.The system is designed to measure compleximpedance (ie both magnitude and phase)across any selected range between 100kHzand 170MHz.” Since then, the hardware has changed a little – the AIM4170C nowcovers 5kHz-180MHz – but the software has changed a lot.

Enhancements include improved graphics(multiple scans, two rulers, smoothed curves,grid lines), frequency markers (easieridentification of spot frequencies on graphs),more robust crystal measurements, quarterwave tuning stub improvements, Smith chartimprovements and improved plotting of L andC versus frequency for discrete components.

In addition there is a new Time DomainReflectometry (TDR) function that is especiallyuseful in measuring theimpedance of transmissionlines. More on this later.

Chris Lorek describedthe basic operation of theAIM4170 in his 2007review. In this article I will concentrate on threeaspects that highlight someof the other features of theinstrument:• Measuring impedance

at the antennafeedpoint

• Time Domain Reflectometry• Remote Operation of the analyser.

MEASURING IMPEDANCE AT THEANTENNA FEEDPOINT.Most peoplemeasure the impedance of the antennasystem at the feeder feedpoint – that is, at the transmitter end of the feeder (Figure 1).After all, that’s what’s important for ensuringa good match of the antenna system to thetransmitter. However, it is sometimes useful toknow the impedance at the antenna feedpoint,so that the effects of any mismatch to thefeeder are more evident. This is particularlyuseful for comparing modelled performanceagainst actual performance.

The impedance measured at the transmitterend of the feeder will usually be different fromthe impedance at the antenna feedpointbecause the feeder acts as an impedancetransformer. Even if there appears to be a nearperfect 50Ω resistive match at the transmitter,the antenna feedpoint impedance may be

rather different – it may even be quite badly matched to the feeder. The degree of impedance transformation in the feeder will depend on its characteristics (Zo,capacitance, velocity factor, losses, etc), its length and the frequency of the transmitted signal.

The AIM4170 has a very useful feature, Custom Scan, that lets you measure the impedance at the antennafeedpoint without needing to physicallyconnect the analyser to the antenna itself. You connect the analyser as usual to the feeder feedpoint and measure theimpedance at this point. The AIM softwarethen calculates the antenna feedpointimpedance by cancelling out the feederimpedance transformation from the measured value.

Measuring the impedance at the antennafeedpoint is a two-step process: a CustomCalibration followed by a Custom Scan.

To perform a Custom Calibration, disconnectthe antenna, then specify a frequency rangethat is a little wider than the band of interest(for example, 6.9-7.3MHz for 40m). Next,under the direction of the AIM software,connect in turn a short circuit, open circuitand a resistive load to the far end of thefeeder. The resistive load can be between50Ω and 300Ω – the actual value isn'timportant, but it must be known accurately.The AIM software then computes a customcalibration curve that represents the impedancetransformation of the feeder over the specifiedfrequency range.

Once the feeder is calibrated, you canconnect the antenna to the feeder and carryout a Custom Scan. The AIM4170 measuresthe impedance at the transmitter end of thefeeder as usual but, using the custom

The AIM4170 Antenna Analyser Revisited A fresh look at this versatile instrument

PHOTO 1: The latest version, AIM4170C.

FIGURE 1: The AIM4170 is usually used to measurethe impedance at the feeder feedpoint, but with customcalibration of the feeder it can also be used to calculatethe impedance at the antenna feedpoint.

calibration curve, it cancels out theimpedance transformation of thefeeder to give the impedance at theantenna feedpoint. An example isshown in Figure 2, where a 40mantenna was tested at 7.1336MHz.The actual antenna impedance, ifmeasured at the antenna feedpoint(A) is 53.149 – j6.436 ohms, but theeffect of the particular feeder was totransform this to 48.804 – j0.002 ohmsat point (B). The AIM4170 software,however, can cancel out the effect of the feeder and determine the actual antenna impedance,

53.149 – j6.436 ohms (C).Custom Calibration of the

feeder is a once-only process that you carry out for eachfrequency band of interest before connecting the antenna.For example, for my HF antennafeeder I carried out separatecustom calibration runs for the160, 80, 60, 40, 30, 20, 18,15, 12 and 10m bands. 10mwas actually split into four subbands because of its width.

The scans in Figures 3 and 4show the differences between aNormal Scan of my 40m antennaand a Custom Scan of the sameantenna. The Normal Scan(Figure 3) indicates that thecomplete antenna system (that is, feeder plus antenna) resonatesat approximately 7.1336MHz,whereas the Custom Scan (Figure4) shows that the antenna itself is not actually resonant. In otherwords, it is the feeder that bringsthe whole antenna system intoresonance, even though theantenna itself is not resonant.

TIME DOMAINREFLECTOMETRY (TDR). TheAIM software now incorporates aTDR function that lets you measurethe impedance along the length ofyour antenna feeder. It provides agraphical representation of anydiscontinuities, breaks or shortcircuits. There isn’t room here to explain the inner workings ofTDR, but [2] and [3] are usefulstarting points.

In practice, all you do in theAIM software is select the TDRfunction, then click on the Scanbutton to display the impedance ofthe feeder along its length. Figure5 shows the scan of a feederconsisting of 100 feet of 70Ω coaxfollowed by 100 feet of 50Ω coax– the picture clearly shows thetransition between the two cables.

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FEATURE JANUARY 2012 ♦ RADCOM

FIGURE 3: Normal scan of a 40m antenna at the feeder feedpoint. At 7.1336MHz the feeder input impedanceRs + j Xs is 48.804 – j 0.002 ohms and the antenna system (feeder + antenna) is, to all intents and purposes,resonant at this frequency – resonance occurs at the frequency where the phase angle theta (the purple trace)passes through the zero axis.

FIGURE 4: Custom scan of the same 40m antenna, to obtain the impedance at the antenna feedpoint. Thisturns out to be 53.149 – j 6.436 ohms at 7.1336MHz and, surprisingly perhaps, the antenna itself is notactually at resonance – the phase angle trace does not cross the zero axis at any point within the scannedfrequency range. The apparent resonance at the feeder feedpoint in Figure 3 is achieved through theimpedance transformation in the feeder.

FIGURE 2: To measure theantenna feedpoint impedance

(at point A), you connect the AIM4170 to the feederfeedpoint (point B) as usual, then conduct a custom scan. At 7.1336MHz the measured

impedance at point B is 48.804– j0.002 ohms. The AIM4170software then uses the custom

calibration curve to cancel out thefeeder impedance transformationat this frequency, to calculate theantenna feedpoint impedance:

53.149 – j6.436 ohms (point C).

REMOTE OPERATION OF THE ANALYSER.When making antenna adjustments theprocess can be quite long-winded: scan the antenna, walk into the garden, make an adjustment to the antenna, walk back, re-scan the antenna, walk back to the garden,make another adjustment, and so on. But ifyou have a wireless network – see Figure 6 –and a spare netbook or laptop, you can controlall the AIM4170 functions remotely (as long as you're within range of the network).Everything that is displayed on the AIM PCscreen appears exactly the same on thenotebook screen; any keyboard or mouseinputs on the notebook computer go straightto the AIM software in the shack, as if theyhad been input locally from the AIM PC.

Now you canmake all theantennaadjustments in the gardenand immediatelysee the effectsof any changeswithout havingto set foot backinside the house.

To make thiswork, you needto install someremote serversoftware on the AIM PC to set up atunnel betweenthe notebookand PCconnected tothe AIM4170.There areseveral suitablepackagesavailable that will do the job, but my favourite is RealVNC[4]. RealVNC

has the advantage that it doesn’t need aninternet connection to operate – very usefulwhen giving demonstrations in locationswhere the internet is not available.

No special software is required on thenotebook, just a standard web browser thatsupports Java – in practice, almost any oftoday’s commonly available browsers. Thenotebook does not even have to run Windows– I use an Asus eeePC, which runs Linux. Toconnect to the tunnel server on the AIM PC, yousimply open a browser window on the notebook,enter the IP address and port number of theserver (for example, 192.168.1.100:5800)and you are connected.

All in all, remote operation can save ahuge amount of time and legwork. Tuning

a multi-band HF antenna can be achieved inminutes rather than hours – when resonatinga three-band trap dipole recently, I did thewhole job in well under an hour. One just has to avoid a few hazards – I used a lanyardto avoid dropping the notebook and put somecling film over the keypad to defend againstoverflying birds. I also have to run the screenat higher than usual brightness, resulting inhigher than usual battery consumption.

CONCLUSION. The AIM4170 is quiteexpensive, at around £449 [6], but with its continuing software improvements I think it's worth every penny – each software update just makes it better and better! I think it is still the only instrument in the amateur price range that can measure impedance accurately; it compares very favourably with commercialanalysers costing many times as much[7] [8] [9].

ACKNOWLEDGEMENTS. Special thanks go to Peter Dodd, G3LDO, Mike Hubbard,G3OVL and Steve Hunt, G3TXQ, for reviewing this article prior to submission for publication.

WEBSEARCH[1] The AIM4170 Antenna Analyser, by Chris Lorek,

G4HCL, RadCom July 2007, also available online athttp://homepage.ntlworld.com/wadei/aim4170.htm

[2] AIM4170 user manual and software: www.w5big.com/prog_update.htm

[3] Time Domain Reflectometer: http://en.wikipedia.org/wiki/Time-domain_reflectometer

[4] RealVNC server software:www.realvnc.com/products/free/4.1/winvnc.html

[5] AIM4170 discussion forum: http://aim4150.proboards.com/index.cgi

[6] UK distributor of AIM analysers: Vine Antennas Ltd, www.vinecom.co.uk

[7] Antenna Analyzer Comparison by Bob Clunn, W5BIG:www.w5big.com/TestResultCombined.htm

[8] AIM4170 review by Larry Benko, W0QE:www.bnk.com/w0qe/AIM4170_page1.html

[9] Vector Network Analyzer Comparisons by Rudy Severns, N6LF:www.antennasbyn6lf.com/files/vna_comparisons.pdf

[10] AIM4170 Resources Page:http://homepage.ntlworld.com/wadei/aim4170.htm

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FEATURERADCOM ♦ JANUARY 2012

FIGURE 5: A time domain reflectometry scan of 100 feet of 70Ω coax inseries with 100 feet of 50Ω coax.

FIGURE 6: Remote operation of the AIM4170. The shack PC is connected to the AIM4170 and a wireless router. The notebook in the the garden communicates with this PC via Wi-Fi, allowing full remote control of the analyser.