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My Antenna Adventures - An OCF [Part 1]

A Communicator Reprise: May 2013

I recently purchased a Carolina Windom 80 antenna and, after it was erected, immediately experienced some puzzling problems, as it would not take power from the transmitter, confirmed by high and erratic SWR measurements using a hand-held meter.  The full story will be related over a couple of postings but my first step was to acquire an antenna analyzer with more sophisticated features than the popular instruments and use it to help me with the diagnosis.  

The AIM 4170C analyzer was recommended by Jim VE7FO as a suitable instrument, so I purchased one from Array Solutions.  The AIM can do many advanced analytical chores and plot graphs, which is an advantage if you want to see results without the need to make and record a number of spot readings. It allows selection of the desired parameter for the plots from the menu, any or all of which can be displayed simultaneously. It also allows successive re-scans so that changes can be seen but retains in memory the raw data only for the most recent scan.

The initial step was to get familiar with the instrument’s many features, which I did by making some measurements on my SteppIR beam in order to document its characteristics for future reference.   I ran scans of SWR versus frequency for the SteppIR set to resonate on each of the HF amateur bands (2 curves, one with the antenna set to the low end of the band, and the other to the high end).  The resultant graphs showed, as expected, a low SWR on all bands with the minima near the SteppIR operating setpoint.  In the interest of space only the 20 m curves are shown in Figure 1, but those for other bands displayed a similar pattern.

Then, I wanted to compare the AIM measurements with two other popular analyzers – an MFJ 359 and Comet CAA-500 – to see if the readings agreed, because my measurements on the Windom with different instruments did not agree.  However, the results this time (on the SteppIR) were very comforting, as shown in the table.  The SWR agreed very closely amongst the 3 instruments, and Rs and Xs also compared favourably between the MFJ and the AIM.  The Comet does not read Xs (reactance); rather it shows SWR and Z (impedance) so only the Comet’s SWR reading is comparable with the other instruments.  These results gave me reason to believe that at least with a “well-behaved” antenna system, the readings of the 3 instruments should be trusted. 

Figure 2, which shows plots of several other parameters,
illustrates some basic antenna/transmission line relationships.  

For example, the resonant frequency of the system is the frequency at which the reactance changes sign from positive (inductive) to negative (capacitive), i.e. the point at which the net reactance is zero.  The phase angle shows the same thing, where it crosses the X-axis.  Note that there are two resonant frequencies –the first where the phase angle goes negative at 14.135 and again where it turns positive at 14.399.  However, the second one is not only at a high SWR point, but is outside the 20m ham band.  At the resonant frequency the Z curve coincides with the Rs curve because the reactive component at this point is zero.  In a system where Rs does not equal 50 ohms and reactance is present (typically because the antenna is too long or too short), the frequency of minimum SWR will likely not be the same as the resonant frequency.  In this case because the antenna is very nearly the right length, the resonant frequency and frequency of minimum SWR are very close. 

Having completed this test with satisfying results, I then tried another feature of the AIM instrument.  It has a mode called TDR (time domain reflectometry).  I do not profess to have any understanding of the theory of TDR, but the results are easy to understand.  In order to use TDR you must enter the velocity factor of the coax in question.  Now the SteppIR beam is connected to the shack by 60 ft. of what is supposed to be LMR-400, connected directly to an Alpha Delta surge protector which is connected to about 20 ft. of RG213 or RG8 coax leading to the radio.  Both feedlines are rated at 50 ohms impedance.  The velocity factor for Belden LMR-400 and RG213/RG8 are 0.85 and 0.65, respectively .  However, my LMR-400 coax is a Chinese import for which I have no specs.  Despite having two different types of coax connected together, only one value can be entered in the TDR setup.  After entering the velocity factor of 0.85, I hit “scan” and waited for the graph to plot.  

The figure shows the result –impedance of the transmission line versus distance.  Where the line turns vertical is the point at which the feedline is connected to the antenna.  Now as you can see this graph for 0.85 velocity factor says my transmission line is 110 ft. long.  I know this is not true, as the total length is around 80 ft.  So?  Change the velocity factor to 0.65 and bingo, the length now shows around 80 ft.    

I suspect my “LMR-400” has the characteristics of RG213 or RG8 and perhaps the product I purchased is not what it was claimed to be. If another type of coax with difference characteristic impedance was connected in series or there was an impedance bump from a faulty connector or break in the line, the graph would show it.  TDR looks to be a very useful feature of the AIM analyzer.  However, having completed the preliminaries now I’m ready to do some diagnostics on the Carolina Windom. 

More to follow... 

~ John Brodie VA7XB

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