Transformers 2.0

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     After a very cold Winter Field Day, the weather got mild fast, and it rained hard for a day, so our snow is almost gone, and the temperatures are back above freezing.  As promised, this means that I am now revisiting the unun that I recently installed in my vehicle, to do more antenna tuning.  In a previous post I explained the slight lack of clarity in the directions that came with the unit, and talked about my hunch about a better way to go about the process.  
     I basically decided to "start from scratch" so that I could talk you through what I did, what measurements I took, etc.  With that being said here we go.......

Step 1?

     The directions that come with the unit I purchased tell you start by tuning for lowest SWR without the unun in line.  Since I was working with my 40 meter Hamstick, I decided to target the center of the voice portion of the band (I don't plan on doing digital modes and such while driving!)
     The analyzer I have is the RigExpert AA-54, so I exported the charts to my computer after taking my readings - it was my first time playing around with this feature.  I didn't realize how powerful it was!  As I tune the rest of the Hamsticks I might take my laptop along out and use it in the "live" mode.  After tuning for lowest SWR at 7.212 this is what I had:

    Some interesting notes on this.  The range of the graph covers just slightly more than the 40 meter band.  The SWR at its lowest is 2.05:1 which will work, but isn't great.  The 857d doesn't like SWR levels more than 3:1 per it's manual, so that puts the 3:1 frequency range at 7.138 to 7.282 which is a bit short of covering the whole voice portion available to Extra's.  
     Also, take note of the subset box.  You can see that at the lowest SWR the impedance of the anteanna is 58.5+j38.6.  In my mind this seems like a funny place to start matching from.  My gut (Elmer's jump in if I'm wrong!) tells me that I should be starting from a point where the antenna impedance is pure resistance, i.e. starting from a resonant point, instead of a low SWR point.  I actually had two of these points "relatively" close to the low SWR point.  One of the neat things with loading the data from the analyzer onto the computer is that I can show you those two points on the same graph:

     The first resonant point, above,  is actually just below the lowest frequency on the scale.  At the very bottom of the scale, we're almost there at 7.7 +j0.2 Ohms.  This would be a 6.48 SWR at resonance.  
     The second resonant point, below, is up at about 7.261MHz (thats as close as the resolution of this graph will show anyway). At that point we've got an impedance of 125.1 +j2.2 ohms for an SWR of 2.5:1 which isn't horrible.  

    So where should I tune my antenna to, before starting my matching?  

Step 2

     I decided to go with the point where the impedance is 7.7 +j0.2 Ohms, primarily because the unun I bought assumes your antenna side is less than 50 ohms, and matches from there.  It would work the other way just by reversing the input and output, but this means that every time I switch to the 40 meter antenna, I would not only have to change the unun setting, but I would also have to disconnect the coax from both sides, and swap them (or build a switch to accomplish the same thing.)  It seemed easier to just tune to the point where I don't have to re-wire my antenna just to change bands....
     After re-tuning so that resonance was at 7.212 my chart looks like this:

     From an SWR standpoint - awful, but I'm still hanging my hat on the fact that it's better to start at resonance and then match from there.  This however, is where the directions that come with the unun could leave you wanting.  At this point they say just click through the setting and use the setting with the lowest SWR.  this makes it sound like one of these setting ought to get you close to to 1:1.  This isn't the case.  I ended up re-graphing on each setting, and choosing the setting that had the lowest dip, the closest to 7.212.  This was actually relatively far from where I was.  I had to graph with a really wide frequency range to find the setting with the lowest dip:

     So after doing these steps, with the unun in line, the lowest SR is 1.13:1, but all the way down at 6.972 MHz.  Time for the final Step.

Step 3

     So, for the final re-tuning of the antenna - re-tuning to get that 1.13:1 SWR point up to 7.212 MHz, which meant shortening the antenna a bit.  After a number of consecutive shortening and re-measuring activities, I ended up here:

     So after all that, was my thought process sound, and is my final result a good one, or have I unknowingly made this way more complicated than it needed to be, and screwed something up without realizing it?  Hello?  Is anybody out there?
     Oh - on a final note, I re-tuned my 75 meter Hamstick too.  Resonance was very close to low SWR on that one, so the adjustment's weren't nearly as extreme, or as exiting, so I didn't bother sharing the graphs of those.