Scientific Computing

A few quick start tips for using EZNEC antenna design software for yagi antennas. This is assuming a yagi antenna for 440 MHz band.

• Ground: start with “free space”. If the antenna model is too close to the ground, the simulation can be unrealistic. Once satisfied in free space, consider real ground (non-free space)
• Number of segments: suggest 5 per wire, with free EZNEC license
• Number of yagi elements: 3..5 elements are possible with free EZNEC at 440 MHz.
• Source: 1 source, middle (50%) of “driven” wire

Example antennas

• commercial yagi datasheets
• coat-hanger yagi page 19 starting point 4 element yagi

Typical yagi performance

How good is good enough?

• Gain: > 6 dBi
• Backlobe (front/back ratio) (opposite direction of desired mainlobe): < -15 dB or so
• Sidelobes (off side of desired direction): < -15 - -20 dB or so
• SWR (439-441 MHz): Ideally less than 2.0. 3.0 is perhaps tolerable.

SWR is a measure of how efficient an antenna is at coupling energy to/from the air to a radio device.

Γ = (SWR - 1) / (SWR + 1)

• Mismatch loss [dB] (energy loss due to SWR): -10 * (1 - Γ)^2

Example

Suppose: SWR = 3.0

• Mismatch loss: -10 (1 - ((3-1) / (3+1))^2) = 1.25 dB

Power loss (gain) factors:

• 3dB ~ factor of 2
• 10dB ~ factor of 10
• 20dB ~ factor of 100

CMake by default searches for shared libs before static libs. Sometimes a project needs to specifically link external or internal static libs, even if shared libs are present. A simple cross-platform way to do this is to set CMAKE_FIND_LIBRARY_SUFFIXES before any find_library() or find_package():

set(CMAKE_FIND_LIBRARY_SUFFIXES .a .lib)

This takes advantage of de facto static library extensions for macOS, Windows and Linux, using CMake system variables.

The default values for CMAKE_FIND_LIBRARY_SUFFIXES are set by (depending on platform and compiler) CMake

Modules/CMakeGenericSystem.cmake
Modules/Compiler/
Modules/Platform/

Reading RINEX files in Python or other languages historically required compiling or buying complex software. The GeoRINEX Python 3 program and library to be used with RINEX OBS and NAV files, including Hatanaka compressed files or other compression wrappers like ZIP.

GeoRINEX can even read only every N seconds from a RINEX file with option georinex.load(..., interval=N). This greatly speeds up reading where coarser time intervals than the RINEX file provides is needed. GeoRINEX uses performance-oriented techniques to read RINEX files at speeds approaching compiled languages, using Pure Python + Numpy and Xarray for metadata rich results.

If you use GNSS and RINEX in your work, you may like GeoRINEX for Python 3.

For those dual-booting Windows and Linux, it’s useful to disable Windows Fast startup so that you can access the Windows partition from Linux. Disabling Fast Boot may add a few seconds to boot time. When you shutdown your PC, Fast Boot actually reboots and then hibernates. This locks the Windows partition so that Linux can’t access it.

Disable Fast Boot: ⊞+r powercfg.cpl → “Choose what the power button does”. We typically disable hibernate as well to save on SSD wear.

BitLocker and dual-booting: If BitLocker is enabled, Linux cannot access the Windows partition.

Fast Boot SSD wear: Fast Boot just like hibernate increases solid state drive (SSD) wear, since you are dumping an image ~ RAM size to disk on each Fast Boot shutdown.

ℹ️ Windows now has

OpenSSH built-in. Cygwin isn’t necessary for Windows SSH servers any more.

The robust OpenSSH server on Cygwin allows SSHing into a Windows PC for secure port forwarding Windows RDP. Use Public Key Authentication – it’s far stronger than any human usable keyboard password.

From Windows Command Prompt:

setup-x86_64.exe -Bq -P openssh

Cygwin SSH server setup: start Cygwin64 using “Run as Administrator” and type

ssh-host-config

• Privilege separation: yes
• strict modes: yes
• sshd as service: yes
• value of CYGWIN for daemon: (just press Enter key, don’t type anything)
• use a different name: no

The Cygwin SSH server starts itself on reboot of Windows, but if you need to start it manually (e.g. for the first time upon installing)

cygrunsrv -S sshd

Test the SSH server:

• login to the Windows Cygwin SSH server from another PC.
• Under services.msc observe the Status “started” for service “CYGWIN sshd”, and under the General tab the “path to executable” should be something like c:\cygwin64\bin\cygrunsrv.exe

Configure Cygwin SSH server in /etc/sshd_config. To make changes take effect:

cygrunsrv -E sshd
cygrunsrv -S sshd

Disable Cygwin Windows SSH server with Cygwin “Run As Administrator”:

cygrunsrv -E sshd

cygrunsrv -R sshd

Reboot and verify the former SSH server is not operating. CYGWIN sshd should not be in services.msc anymore.

system error 1069 has occurred. The service did not start due to a logon failure

check the password on the “privileged server” cyg_server.

Related:

• Linux to Windows SSH / RDP
• Windows to Windows SSH / RDP

Python pathlib is part of Python standard library. When using Python subprocess, in general, do str(mypath) where type(mypath) is pathlib.Path.

Example:

#!/usr/bin/env python
from pathlib import Path
import subprocess

P = Path('~').expanduser()

O = subprocess.check_output(['dir', str(P)])

print(O)

if you don’t cast pathlib.Path to str:

TypeError: argument of type ‘WindowsPath’ is not iterable.

Notes

You should rarely if ever need shell=True. On Windows in particular, give the full path to a non-system executable.

Related: Idiomatic pathlib Python

Find and replace text in files recursively on Linux, macOS and Windows Subsystem for Linux with recursive_sed.sh:

#!/usr/bin/env bash
#
# The quoted variables allow using backticks and other "nasty" characters in sed.
#
# usage:
# ./recursive_sed.sh  pathtofiles yuck yay
#
# References:
# https://stackoverflow.com/a/1585810
# https://unix.stackexchange.com/a/128758

set -o errexit
set -o nounset

path=$1
old=$2
new=$3

echo "${old} => ${new}"

find $path -not -path '*/.git*' -type f -execdir sed -i 's/'"${old}"'/'"${new}"'/g' {} +

Pdoc is vastly simpler to use than Sphinx for small projects, with good HTML formatting.

In distinction from Sphinx, pdoc does not parse RST docstrings, but Pdoc does allow the following docstring formats:

• Markdown
• numpydoc
• Google-style docstrings

At this time, pdoc3 doesn’t consider Python type hinting.

Install

pdoc is a light minimal install.

pip install pdoc3

Usage

We use the pymap3d project with auto-generated docs as an example.

From the top-level pymap3d/ directory, type

pdoc --html pymap3d

This creates several *.html files under html/pymap3d, including index.html.

Check the output by:

firefox html/pymap3d/index.html

If the output seems satisfactory, move the files to the gh-pages branch, or at least to a docs/ folder:

mv html/pymap3d/*.html docs/

Publish the docs to the web by:

1. In the Github repo Settings → Github Pages → Source: “gh-pages”
2. git commit and push. It takes a few seconds for Github Pages to deploy HTML.

Python users typically use a non-system Python distribution such as Miniconda. Python distributions typically allow easy switching of Python version. Where Linux system Python must be used, the default Python version can be switched persistently with update-alternatives.

Update-alternatives without sudo: a one-time setup.

Configure shell to use ~/.local/bin instead of system-wide /usr as follows:

mkdir ~/.local/bin

Add to ~/.profile:

export PATH="$HOME/.local/bin:$PATH"

Enable switching Python default between Python versions with these one-time commands:

update-alternatives --install $HOME/.local/bin/python python /usr/bin/python3 20

After the setup above, at any time select the default system Python version without sudo:

update-alternatives --config python

GNU Radio is trivially easy to install on Ubuntu, which includes the UHD driver.

apt install gnuradio

DO NOT use GNU Radio from a virtual machine in general with an SDR, as typically many packets will be lost and the data will not be useful. The typical best choice is to use a GNU Radio live USB image. Frequent GNU Radio users often dual boot into Ubuntu for working with SDR.

Build GNU Radio from source

Optionally compile GNU radio yourself