I'm hoping someone can help me understand what I'm seeing. Setup is a Rohn
25 tower with 3 yagis -- a short 40 at 105 ft, a C-3E at 97 ft and another
C-3E at 69 feet. I tapped the tower at about 30 feet with a #14 shunt wire
spaced ~24 inches.
With about 400 pf of series capacitance, reactance minimum is 10 ohms at
1865 Khz, as measured on an MFJ 259B. However, at that frequency the
measured R is only 3 ohms, where I was expecting more on the order of 12-15
ohms, based on modeling and other people's experience.
There's no sign of any BC-station-induced freakiness, and the nearest AM
station is a 500-watter over 5 miles away. Does this measurement suggest
that the tap should be moved up in search of a higher resistance point?
Any ideas roughly how far? I'm fully prepared to use a UNUN to convert the
feedpoint impedance to the 50-ohm area if I can get it to a reasonable
amount, or else set the tap anywhere up to 68 feet or so if that would get
me closer to 50 ohms.
Any advice to minimize trips up the tower would be much appreciated.
73, Pete N4ZR
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