WA3TTS posted a question/observation about the measured impedance of 
a MFJ1793 antenna 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 (3.9uh) 
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 for L shunt feed 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 are

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 varied from 3555pf to 4980pf for the two cases
   
There is very little change in the real part of the ant impedance
(Rant) with temperature at his location, most all of the effects he 
observes are due to the capacitance of the ant changing.

73s
Rob  W1MK


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