[TowerTalk] Lunar Dipole

[jimlux]

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.