On 1/4/20 10:56 PM, Dan Maguire wrote:
On 1/3/2020 10:21 AM, Jim Lux wrote:
FWIW, this is calculating the input impedance of a 900 m dipole 1mm in diameter
near the surface of the moon, 3+0.00503j [edit: minus, not plus?] is the
complex permittivity of Lunar regolith
GN 3 0 0 0 3.000000 -0.005030
https://www.colorado.edu/project/lunar-farside/
Well golly, doing the calculations for a low dipole on the surface of
the moon sounds like it's just too much fun to pass up. Here's an
animation of the elevation patterns from 0.011 to 0.015 MHz with the
complex dielectric constant (permittivity) held constant at
3.00-j5.03E-3. In the lower-right corner of each frame, variable "S"
is the ground conductivity in nS/m required to get the desired
permittivity and variable "G" is the gain in dBi at 90° elevation (0°
theta). The outer ring of the polar plot is fixed at -14.24 dBi, the
gain at 0.015 MHz.
https://i.postimg.cc/tC1p4Dbk/Lunar-Dipole-Pattern.gif
And here are the feedpoint impedances at each frequency.
https://i.postimg.cc/13Gx8jjG/Lunar-Dipole-Rand-X.gif
So if you ever want to set up a remote station on the moon for some
nice DX in the 20000m band this should get you started. :)
Dan, AC6LA
__________________
The real trick is that unlike ham bands where you can use traps and
such, we want continuous coverage from about 100kHz to 20-30 MHz. And
we want a single lobe for all frequencies. We don't care about the
match - like most low frequency receive only systems, you run the
antenna into a High Z LNA, so it's more a "voltage field probe" than a
"resonant antenna".
So I run several hundred frequency points at 2% spacing from 10kHz to 50
MHz (with minimum delta of 1kHz, since NEC only has 3 digit precision
for frequency)
This is somewhat relevant for folks building antennas on Earth: NEC4.2
has a very much improved handling of the ground models - one of the
features is that you can set the permittivity as a complex number, as
opposed to as a epsilon and sigma. However, there's some traps.
Setting the "soil conductivity" negative sets the permittivity's
imaginary part, so entering 3, -0.00503. sets the epsilon to 3+j0.00503.
You have to reset it for each new frequency, and you have to do the
frequencies as separate runs. If you do a FR with steps, it "adjusts"
the imaginary part of the permittivity for each frequency, essentially
keeping the sigma (conductivity) constant.
This isn't obvious from the documentation - it *does* show up in the NEC
output file with the different values for each frequency.
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