Here's an interesting mental experiment which I might actually do.
Follow me and see what you think.
Imagine a dipole antenna, center fed with balanced line and 20 feet
long overall. According to the classic 468/f formula, the resonant
frequency should be 23.4 MHz.
Now, in the center of each element, place a two foot diameter circle
of sheet metal. You break the wire and connect each side of the break
to the sheet, right in the center. No nuts or bolts, just solder or
weld the wires directly to the center of each side the sheet.
The question is: How does this change the electrical length of the
dipole. Does the RF have to flow from the center, out to the edge and
back down the other side, or does the RF flow straight through the
sheet? Or does the RF just stop dead at the disk and the far side of
the wire is out of the circuit?
If it flows straight through, the resonant frequency will not change
(except for the added capacitance). If the RF has to travel around the
metal, then two feet are added to each side to make the new overall
length 24 feet and the new resonant frequency 19.5 MHz. If the RF
stops dead, the length is now only 10 feet and the resonant frequency
is now 56.8 MHz.
As for the capacitance, I think that could be accounted for by
connecting the wire at one edge of the sheet instead of the center.
The capacitance should be the pretty much the same (I think), and you
could measure the effect on resonant frequency.
I might actually do this if I can find suitable sheet metal.
So what do you think would happen?
73, Bill W6WRT
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