I've been trying to determine the feed impedance of a relatively
short, shunt-fed tower for 160 m. As configured, the system appears
to be resonant at about 9.6 MHz.
I borrowed a friend's MFJ-259B analyzer to supplement my readings
from my noise bridge, as I didn't think those made sense. I kept
getting an inductive reactance at a frequency well below what I knew
to be the system's resonant frequency.
The MFJ-259B showed a reactance of about 360 ohms, but without sign
information. To remedy that, I put a T-connector on the port, put the
load on one side and piece of RG-8 coax about 19" long on the other
side. A 19" long piece of RG-8 should have about 48 pF of
capacitance. The 259B said it had about 60 pF, so that seemed close
enough, counting the connectors. Thus, I added about 60 pF in
parallel with the unknown reactance.
I figure that if the impedance is inductive, adding a capacitor will
increase the reactive component by cancelling out the inductive
impedance since I'm essentially approaching parallel resonance.
Alternatively, if the reactance decreases, then the load reactance
must be capacitive.
The added capacitance increased the reactance, so I conclude the
antenna impedance must be inductive. Does this sound right?
Kim Elmore, N5OP
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