WA3TTS posted a question/observation about the measured impedance of a MFJ1793 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 shunt feed to transform the impedance of the antenna. 
The antenna itself ( without the L shunt) looks like a series R ( Rant) and series C ( Cant).
at resonance the equations are
R = Rant   +  1/ ( w^2 Cant^2 Rant )       and       wL= X1 = (Rant^2) w Cant + 1 / ( wCant)
what WA3TTS wanted to determine was Rant and Cant from his measurements and thus how they varied with temperature.
since we know L, w and R (from his measurements)
Rant =  X1^2 R / ( R^2  +  X1^2)   and   Cant =  1 / ( w ( R-Rant)Rant)^.5)
the two cases from the WA3TTS data
f =  1806 KHz    R = 50    L= 3.9uh
f=1816 kHz        R=36   L=3.9uh
Rant is thus 21.7 to 21.9 ohms  (very little variation between the two cases)
Cant is 3555pf to 4980pf   
there is very little change in the real part of the ant impedance, most all of the change is due to the change in capacitance of the ant.
73s
Rob  W1MK



________________________________________________________________________
Interested in getting caught up on today's news?
Click here to checkout USA TODAY Headlines.
http://track.juno.com/s/lc?s=198954&u=http://www.usatoday.com/news/front.htm?csp=24