WA3TTS posted a question/observation about the measured swr/impedance of a MFJ1793 ant as used on 160m.
His question/observation concerned the measurement of swr( impedance) at the end of a 1/2 w coax line and its change with temperature. The MFJ 1793 antenna in his application made use of a L shunt feed or L type network to transform the impedance at the antenna.




My suggestion if you want to want to understand your losses and how they vary with temperature and other parameters, is to make the measurement of the real part of the impedance at the ant terminals before any shunt elements (L shunt).  
If you measure where you did in front of the shunt L, any changes in the ant reactance(like a change in center freq), causes the real part of the input impedance in front of the shunt L to change( and the change may be magnified, that is a small percent change in C causes a larger percent change in the input real part impedance, to a point where you have no idea what is really happening.
    R at input to L network (at res)  =  Rant + 1/ (w^2 Cant^2 Rant )
 
Another interesting thing is that if you model the MFJ 1793 on 160m it is looks like  a "2.2 ohm" ant. That is with ideal current return the real part of the impedance is 2.2 ohms. Nothing wrong with that, except you need to very careful about the losses. There are a number of sources of losses which degrade its performance, the coil loss of the 45 uh base inductor ( for example a Q of 300 is equivalent to 1.7 ohms itself ), and then the top loading inductor also adds more loss, as does the fact that you most likely do not really have a ideal current return.
I would speculate that the majority of the effective current return is not via the 80m/40m/20m elevated radials, but I would guess is coming via a path from ground. You can easily see that by measuring the real part of the ant impedance with the radials flowing, and then connect the ground paths and remeasure the real part of the impedance. My experience with similar situations using elevated radials of 1/8 w, is that the real part of the impedance changes, and actually goes up, which means the gain or the field strength goes down. In order to avoid this effect, the return current flowing via ground must be blocked.
FS = 10 Log (k I^2)  =  10 Log ( k Pin/ R)               (per W7EL, ARRL ANT Handbook)
 
73s
Rob W1MK


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