Ok guys I will do the experiment. I believe I know the answer but
validation is always a good thing.
Here is what I will do:
1.) Start with a #10 or #12 self supporting copper wire dipole that is
about 10" each side of the center (Fres ~ 280MHz)
2.) I will solder a 2" dia. thin copper disk to each element at the 1/3
point out from the center on each side and remeasure Fres.
3.) Then I will move the disks to the 2/3 point from the center and
remeasure Fres.
Is this reasonable? I will choke the coax, feed it at right angles to
the dipole and make measurements in my basement with a VNA. Comments
before I start!
73,
Larry, W0QE
Bill, W6WRT wrote:
> 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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