## Measuring a mains choke with Hermes-Lite VNA

I have made a mains choke for my HF station, following Ian GM3SEK’s design, which involves twisting the three mains wires together and passing as many turns as possible through a Fair-Rite 0431177081 snap-on ferrite core. I wanted to measure the choke’s impedance to get an idea of its performance, so I’ve used my Hermes-Lite 2.0 beta2 in VNA mode.

English summary: Slides and recordings for the two talks I gave yesterday in IberRadio. One of the is about gr-satellites and the other one is about Linrad. All the material are in Spanish.

Ayer estuve en la feria IberRadio, en Ávila, dando dos charlas: una sobre gr-satellites y la otra sobre Linrad. Las diapositivas en PDF de las charlas se pueden descargar aquí:

He grabado las charlas usando mi cámara. El enfoque y la exposición no son muy buenos, pero he editado el vídeo incluyendo encima las imágenes de las diapositivas, lo que facilita seguir el vídeo de la charla. Por contra, las demostraciones en directo en la charla de Linrad se ven un poco mal.

Actualización: David EA1FAQ también hizo grabaciones de las charlas. En sus grabaciones se ve mejor el proyector, por lo que las demostraciones en directo durante la charla de Linrad se siguen mejor. Incluyo links más abajo.

### Grabaciones por EA1FAQ

Having to deal with DSP texts written by engineers, I have sometimes to work a bit to get a good grasp the concepts, which many times are not explained clearly from their mathematical bases. Often, a formula is just used without much motivation. Lately, I’ve been trying to understand critically damped systems, in the context of PLL loop filters.

The issue is as follows. In a second order filter there is a damping parameter $$\zeta > 0$$. The impulse response of the filter is an exponentially decaying sinusoid if $$\zeta < 1$$ (underdamped system), a decaying exponential if $$\zeta > 1$$ (overdamped system) and something of the form $$C t e^{-\lambda t}$$ if $$\zeta = 1$$ (critically damped system). Critical damping is desirable in many cases because it maximizes the exponential decay rate of the impulse response. However, many engineering texts just go and choose $$\zeta = \sqrt{2}/2$$ without any justification and even call this critical damping. Here I give some motivation starting with the basics and explain what is special about $$\zeta = \sqrt{2}/2$$ and why one may want to choose this value in applications.

## Polarization in Voyager signal from Green Bank Telescope

A few days ago I read the paper about the Breakthrough Listen experiment. This experiment consists in doing many wideband recordings of different stars using the Green Bank Telescope, and (in the future) Parkes Observatory and then trying to find signals from extraterrestrial intelligent life in the recordings. The Breakthrough Listen project has a nice Github repository with some documentation and an analysis of a recording they did of Voyager 1 to test their setup.

I have also been thinking about how to study the polarization of signals in a dual polarization recording (two coherent channels with orthogonal polarizations). My main goal for this is to study the polarization of the signals of Amateur satellites in low Earth orbit. It seems that there are many myths regarding polarization and the rotation of cubesats, and these myths eventually pop up whenever anyone tries to discuss whether linearly polarized or circularly polarized Yagis are any good for receiving cubesats.

Through the Breakthrough Listen paper I’ve learned of the Stokes parameters. These are a set of parameters to describe polarization which are very popular in optics, since they are easy to measure physically. I have immediately noticed that they are also easy to compute from a dual polarization recording. In comparison with Jones vectors, Stokes parameters disregard all the information about phase, but instead they are computed from the averaged power in different polarizations. This makes their computation less affected by noise and other factors.

As I also wanted to get my hands on the Breakthrough Listen raw recordings, I have been computing the Stokes parameters of the Voyager 1 signal in their recording. Since the Voyager 1 signal is left hand circularly polarized, the results are not particularly interesting. It would be better to use a signal with changing polarization or some form of elliptical polarization.

I have started to use Jupyter notebook. This is something I had been wanting to try since a while ago, and I’ve realised that a Jupyter notebook serves better to document my experiments in Python than a Python script in a gist, which is what I was doing before. I have started a Github repo for my experiments using Jupyter notebooks. The experiment about polarization in the Voyager 1 signal is the first of them. Incidentally, this experiment has been done near Voyager 1’s 40th anniversary.