[OKDXA] Shunt-Feed Tower for 160 m

[Bert Aaron]

Kim,

You can usually determine the type of reactance by adjusting frequency. If
frequency is increased and reactance (X on the display or Impedance on the
meter) decreases, the load is capacitive at the measurement frequency. If
frequency is reduced and reactance decreases, the load is inductive at the
measurement frequency.

73,
Bert  K2BA

-----Original Message-----
From: okdxa-bounces at mailman.qth.net [mailto:okdxa-bounces at mailman.qth.net]
On Behalf Of Kim Elmore
Sent: Saturday, March 01, 2008 10:39 AM
To: okdxa at mailman.qth.net>
Subject: [OKDXA] Shunt-Feed Tower for 160 m

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 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 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 N5OP

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