Ahh ... I just reread my post and realized I said relative permeability
... I meant relaitive permittivity.
Dave AB7E
On 1/28/2025 10:50 AM, David Gilbert wrote:
Hi, Jim.
I've attached a copy of the EZNEC file to this reply. It won't get
through the reflector but will hopefully get to you directly. I'd
love to hear your thoughts.
The loop is intended to represent half of a QFH antenna for 435 MHz
satellite reception (or transmission depending upon the satellite),
although I didn't try to center the unloaded loop exactly on 435MHz
because I was just trying to model the effect. My idea was simply to
be able to slide a sleeve along the wire to tweak the resonant frequency.
The vertical legs of the loop are broken up into multiple wires (seven
on each side) so that I can simulate a sliding dielectric sleeve (3D
printed PETG) by selectively adding or subtracting insulation
(relative permeability = 2.5, thickness = 5mm) to individual wires.
The feedpoint is at the top of the loop so at least as I understand
the theory, the middle wires on each side should show the greatest
effect, but in the model adding insulation in the upper or lower side
wires shows the greatest frequency shift.
The seven vertical wires on one side of the loop has the following
wire numbers in the model. I've shown the resulting frequency
resulting from adding the dielectric to each wire individually.
#3 424 MHz
#4 426
#5 428
#6 430 (center of vertical wire)
#7 428
#8 426
#9 424
No insulation at all gives 430 MHz, and insulation on wire #3 as well
as its directly opposite wire (top wire on opposite side of the loop)
gives a resonant frequency of 418 MHz.
I greatly appreciate your offer of help!
Dave AB7E
On 1/28/2025 4:53 AM, Jim Lux wrote:
Can you post an example?
(I can run an example on nec4 and see what it looks like. Yes, I’d expect
dielectric to have the most effect where the E-field is biggest)
On Mon, 27 Jan 2025 20:33:55 -0700, David Gilbert<ab7echo@gmail.com> wrote:
I've been using EZNEC Pro/2 to try to model the effects of dielectric
loading on a wire antenna ... specifically a full wave loop on 435 MHz.
I split the sides of the loop up into multiple wires (each with multiple
segments) so I could individually declare a thick dielectric ... i.e.,
insulation in the wires table ... separately for each portion of the
loop. To my surprise the loading effect seems to be greatest at or near
current maximums, not at voltage maximums where I would have presumed
the electric field would have the most effect. This has me greatly puzzled.
Any thoughts?
Dave AB7E
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