I have 4 identical short verticals (part of a broadside array) where EZNEC predicts a 12.5 ohm feed-point impedance. That seems reasonable, but my actual measurements indicate 25 ohms at resonance.
My first thought was something lossy in the ground system. The ground system consists of 32 copper wires, .3 wavelength long, laying on the ground. The ground connections are via a 3/8” copper tubing ring; all radials are soldered. Additionally, the FCC tables used for ground conductivity assumptions on MF frequencies shows the area to exhibit between 15 and 30 millimhos (or millisiemens) per meter … quite good. I just don’t believe I have 10 ohms of ground loss. The radiator(s) are built of 3” aluminum tubing, 1/8 wavelength tall, top loaded with two top hat wires sloping away at about 45 degrees (180 degrees apart). Top hat lengths adjusted for resonance. How do I know the 25 ohms is real? An SWR bridge at the base indicates a 2:1 SWR. Two MFJ analyzers read 25 ohms. As mentioned above, there are 4 identical verticals/ground systems. I measured each vertical impedance and found consistent results. Measurements were made with the other 3 verticals floating. No other antennas, power lines, fences are in the near-field. There are no RF sources nearby that could affect the MFJ readings. I am wracking my brains on this issue. The ground system should be fine as configured. Somewhere I'm burning up half my power in this unknown source of loss. Ideas? Thanks Dennis, K7FL _________________ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
