[Towertalk] Shunt Feed Tower

Bill Tippett btippett@alum.mit.edu
Wed, 23 Oct 2002 07:30:09 -0400


KC4HW wrote"

>Can anyone suggestion something to improve this situation?

        Jim, if you have access to an instrument like an MFJ-259,
read the resistive and reactive parts of the impedance at the
base with no capacitor in circuit.  Tap height affects Zo only 
and the C tunes out the reactive part.  You want to move the tap up 
or down (my guess is down) a small amount (3' is plenty) and determine
whether Zo goes up or down and how much it changes.  Once you know 
that, you can estimate where to move your tap so that Zo = 50 ohms 
(assuming that is your desired feed impedance).  Once you've found 
the 50 ohm point, tune your capacitor for zero reactance on the MFJ.

        If all you have is an SWR bridge, this may require a few
more trips up the tower since you are really just guessing...but the 
exercise will probably do you good!  :-)

                                                73,  Bill  W4ZV