RST Engineering ha escrito:
I tried simulating a Slim Jim in EZNEC and did NOT get the expected "aim at
the horizon" low-angle radiation simulated pattern. I got instead the
standard "hole in the donut" that you would expect from a vertical dipole
and a rather fat true donut shape rather than the expected pancake pattern.

I'm not at all sure how to connect a "voltage source" driver at the tap
point without being able to simulate the ground of that voltage source
tapped up from the bottom end as well.

Thoughts?

Jim

Hello Jim,

I think your simulations are right.

As the height of the Slim Jim is about 0.75lambda (common mode choke in
coaxial line, to avoid radiation from cable), the beam width in
vertical direction is limited by physics (will be similar to a dipole).
If you want an omni-directional pattern with small beam width in the
vertical plane, your antenna's vertical dimension must be large with
respect to wavelength (for example co-phases dipoles, full wave dipole,
1.25lambda dipole).

About the Slim Jim
The upper half wave section (with the fold back HW section) just acts
as a single end-fed dipole with current maximum in the middle. The
current in the two HW sections is in phase. The lower quarter wave
section acts as a matching circuit to transform 50 Ohms to a high
impedance to feed the dipole section. The current in both quarter wave
legs are opposite to each other and therefore has little influence on
the overall radiation pattern. There is a small common mode current in
the quarter wave section (just as in a normal end-fed HW dipole without
radials, like is done in many Citizens Band antenna's).

The two wires of the HW section are too close to each other to have
significant impact on the radiation pattern in the horizontal plane (so
it will be omni-directional).

If you want to simulate the effect of cable radiation, just put the
voltage source at the points where you want to connect the coaxial
cable. Draw a wire (that is the screen from your coax) and connect it
at the voltage source (where the screen of the coax will be connected).
Now you can see the common mode current in you coax and the change in
overall radiation pattern.

Best Regards,

Wim
PA3DJS