Topband: Misbehaving verticals

[Dennis Ashworth]

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