[Antennas] Help with feedpoint impedance measurement

[John Lawson]

   I have a loop around my back fence (and rear eaves of the QTH) of about 
480' total length, rectangular in about a 4:5 ratio.  It is an average of 
5'5" from the ground, save for the house which is nearly 7'. Topography is 
high desert, ground is alluvial clay/sand - and quite dry most of the 
time. Dunno what the actual ground resistance is - but salt-marsh it 
ain't. (North-western Nevada, Carson City / Virginia City area)

   Currently I am running 450 ohm ladder-line from the feedpoint of the 
loop (the interface is a large bakelite barrier strip with approximately 
the correct 'spread') back about 30 feet to an Ameritron ATR-15 tuner - 
which my Johnson Valiant feeds - 160 is a bit touchy to tune, 75M can be 
gotten down to 1.1:1 across the fone segments, 40 tunes 1.3:1, 20 gives 
the same, it doesn't like 15, and 10 is tunable but also quite tricky.  I 
am primarily interested in 80, 40, and 20 - perhaps 160 too.

   I have a rescued broadcast transmitter that I am planning to use - it 
will put out full legal limit when locked into 'low power' mode... so my 
Plan is to use a home-brew automatic (or semi-automatic) tuner at the 
balanced loop feedpoint, and employ a coax balun there, and run 50-ohm 
line back to the transmitters: they will always see 50 ohms regardless of 
what the feedpoint impedance happens to be at that moment.

   I have the parts and overall design data for the tuner - but I am 
wondering what the advice would be to actually measure the impedance of 
the feedpoint over the range of my frequencies of interest - as opposed to 
using a modelling program (like EZNEC) to derive this information.  I have 
a good selection of 'high-end' test gear, and I can think of a couple of 
ways to do this - but I'm by no means an antenna expert - so I thought to 
ask before I leap...  ;} Once I know what the range of feedpoint 
impedances is, then the design of the tuner can proceed from there.  I 
will primarily be using a pair of large roller-inductors, with fixed 
capacitances.


   Thanks!!


John
KB6SCO
DM09fg