[TowerTalk] EZNEC Question

[David Gilbert]

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 at 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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