Bill, W6WRT wrote:
> ORIGINAL MESSAGE:
>
> On Fri, 19 Mar 2010 22:22:44 -0600, Larry Benko <xxw0qe@comcast.net>
> wrote:
>
>
>> 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!
>>
I see one problem with the expected results and I'm not sure how to
phrase it. Instead of the conductor being a constant diameter you are
going to add a very short section that is a larger diameter which also
adds capacity loading. The formula for resonance then becomes a
summation for each diameter along the length where i = a to n with a
being the regular diameter and n being the disk diameter so it's not
going to just add length, diameter, or capacitance. and I'm at a loss as
to how to write that out.
IOW the formula is a series of very short antennas added together until
you get the desired length. All of those antennas but one will have the
wire diameter while the one man out will have the disk diameter.
However this still ignores the capacitive loading.
73
Roger (K8RI)
>
> REPLY:
>
> Sounds good, but don't forget to also measure with the disk connected
> at its edge as well as its center. The disk adds some capacitance to
> the antenna and lowers it's resonant frequency a bit because of that.
>
> 73, Bill W6WRT
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