On Sat, 08 Oct 2005 06:58:43 -0700, Wes Stewart
wrote:

On Sat, 08 Oct 2005 13:36:37 GMT, "Frank"
wrote:

"David" wrote in message
...
Does anyone know how I can model a coaxial sleeve antenna
on EZNEC ?

I know how to enter a standard Vertical 1/2 wave but not sure how
to represent the coax going up through the tubing(sleeve) in the lower
portion.

Thanks in advance.

Just model a vertical dipole. The presence of a coaxial cable, within the
lower conductor, will not effect its performance.

But the presence of the coax below the antenna sure will.

Let me expand on this. If---big if---the sleeve is the correct length
and lossless, so that it acts as a quarter-wave choke, then in theory
it does decouple the coax from the radiator.

A few points however:

1. The Vp of the choke will depend on the effective dielectric
constant of the air/outer jacket combination. The choke so formed
will likely be fairly lossy as the outer jacket is probably not the
lowest loss dielectric around.

2. Since the choke Vp will be lower than an air-dielectric one, it
must be physically shortened to optimize the choking function. This
combined with the usual diameter differences between the sleeve and
the upper radiator (rod) makes the vertical dipole asymmetric, i.e.
unbalanced, when the rod length is adjusted for resonance. In effect
the feedpoint is moving up and down as the rod length is adjusted. In
practice, this is no big deal but offered for completeness.

3. You can model this antenna to some extent. Below are some
coordinates for a model I used. The wire size was #12, all lengths in
inches and the frequency was 100 MHz. I used Multinec invoking EZNEC 4
for the calculations. In Multinec I made the Z a variable and could
vary the height above ground programmatically. The sleeve is
represented by four stubs; more would perhaps more accurately
represent a cylinder, but I don't believe it's necessary. Since the
"coax" connects to ground, MiniNEC ground (avg) was used. To observe
NEC limitations, all segments are (very nearly) the same length and
adjacent segments are aligned. The sleeve length was adjusted to
minimize the current on the "coax" below the antenna (wire #4) and the
rod length adjusted for zero reactance.


End 1 End 2
X Y Z X Y Z Dia Segs.

Source wire
0.00 0.00 354.09 0.00 0.00 355.09 #12 1

Upper radiator (rod)
0.00 0.00 355.09 0.00 0.00 382.47 #12 29

"Coax" inside sleeve
0.00 0.00 326.84 0.00 0.00 354.09 #12 29

"Coax" below sleeve
0.00 0.00 0.00 0.00 0.00 326.84 #12 337

Top of sleeve
0.00 0.00 354.09 0.00 2.00 354.09 #12 2
0.00 0.00 354.09 0.00 -2.00 354.09 #12 2
0.00 0.00 354.09 2.00 0.00 354.09 #12 2
0.00 0.00 354.09 -2.00 0.00 354.09 #12 2

Sides of sleeve
0.00 2.00 326.84 0.00 2.00 354.09 #12 29
0.00 -2.00 326.84 0.00 -2.00 354.09 #12 29
2.00 0.00 326.84 2.00 0.00 354.09 #12 29
-2.00 0.00 326.84 -2.00 0.00 354.09 #12 29



4. If SWR is your criteria for "goodness", you will be in for a big
surprise. The above example places the midpoint of the antenna at 3
wavelengths above ground. The feedpoint Z is a nice 56.6 +j0 but the
elevation for maximum radiation is 71 degrees above the horizon. If
satellite work is your goal, this is your antenna.

5. Despite the "choke", the transmission line is part of the antenna.
Skeptics of the model above can remove the sleeve and put a trap at
end 2 of the coax and see similar results.