Modeling an Ant questions

[Jlerch 0]

So, I'm attempting to model our simple turtle necked dipole in a program called EZNEC ARRL

No point really, other than to attempt at getting a grasp as to what's going on.

So, my question is this, does this look right?

I created two vertical wires, each 1/4 wave long, with a tiny space between the two.

I've connected two sources, one to each of the two wires, on each side of the tiny space, and put them 180 degrees out of phase.

I've played with a lot of optical design, so this feels kinda familiar, but I think I've done something odd. Here's what it looks like:

[PeterB 0]

Jlerch, I have 4 ideas for you to try.

1. You need to ensure the current (or voltage) sources are applied where the wires are nearly touching. It appears that you have applied one source 5% from the bottom end (E1) of wire 1, and source 2 at 5% from the far opposite end of wire 2 (i.e. 95% away from the end of wire2 closest to wire 1). I would try and apply one source at End 2 of wire 1 and the other 180deg out of phase source at End 1 of wire 2

2. There seems to be a discrepency in the View Antenna window, the XY plane is shown between the two wires, but in the Wires window, you have specified both wires to be above the XY plane (e.g. one end of wire#1 begins at 10 units above the XY plane etc)

3. Have you tried using voltage sources rather than current sources ? (set type to V rather than I)

4. Is there a ground plane ? If so , is its position causing the asymmetrical pattern ? Try removing the ground plane all-together if one exists.

regards

Peter

So, I'm attempting to model our simple turtle necked dipole in a program called EZNEC ARRL

[Mr.RC-Cam 129]

An example of a vertical coaxial dipole (bazooka) modeled in EZNEC: http://www.hamhelpdesk.com/antennas/jpole-...c-shootout.html

[Jlerch 0]

Peter, I screwed up and posted the wrong image, Try this one. Its "more right"

With regards to your 4 points

#1, I can only model 20 "segments", so I only get 10 segments per wire, and the resulting 5% discrepency.

#2 The X/Y image of the ant is centered on the ant/

#3 tried both volt and current sources, the patterns were a touch different but not much.

#4 Yes, there is a ground plane, its ~10 wavelength (3 meters) below the antenna. If I turn it off and use free space, I get this perfect looking double stacked donut.

Oh NEAT!! Just played around with putting the ant "On the ground". Very interesting result. It appears the ground reflects the signal, and the two reflection constructivly interfere with each other.. I Seem to recall a thread about how much better the reception was improved by placing the receiver on the ground, I wonder if this explains it? Here's the "on the ground" results:

Need to do some more experimenting here...

[PeterB 0]

Glad to see some experimentation Jlerch. If you remove the ground plane to simplify things, you should aim to get this toroidal shaped radiation pattern (and not a 'double doughnut').

Once you get that put the ground plane back in and you can be more confident of a realistic simulation.

Let us know how you go.

regards

Peter

[Jlerch 0]

Glad to see some experimentation Jlerch. If you remove the ground plane to simplify things, you should aim to get this toroidal shaped radiation pattern (and not a 'double doughnut').

Once you get that put the ground plane back in and you can be more confident of a realistic simulation.

Let us know how you go.

regards

Peter

Ok,

Here is what I get for a center fed dipole in free space

Now here is what I get for our Coax "dipole" in free space

[PeterB 0]

Jlerch,

the centre fed radiation pattern looks spot on, well done.

The coaxial dipole pattern looks way too symmetrical in the Z-Axis. To solve that you need to place at least 3 additional lower wires to represent the coax shield (check out the link Mr RC-Cam posted above) which act like a ground plane thus approaching the pattern of a 1/4 wave monopole/unipole antenna sitting perpendicular to a ground plane.

regards

Peter

[Jlerch 0]

Jlerch,

the centre fed radiation pattern looks spot on, well done.

The coaxial dipole pattern looks way too symmetrical in the Z-Axis. To solve that you need to place at least 3 additional lower wires to represent the coax shield (check out the link Mr RC-Cam posted above) which act like a ground plane thus approaching the pattern of a 1/4 wave monopole/unipole antenna sitting perpendicular to a ground plane.

regards

Peter

After asking a few questions, it appears that the turtled necked dipole and the one modeled on Mr.RcCam's link are not the same. The one on the link has the lower legs connected to the top radiating element. In esesence its just an odly shaped dipole

The lower half of our turtle necks are not connected to the top radiator, right?