ComPhys @comphys@qoto.org

@rvlobato while I'm a Firefox advocate, I also don't really see an issue with this. Chromium is open source, but that doesnt mean any associated services like Sync have to be made freely available. Google are looking out for google, and having complete control of the browser market would be a huge advantage for them. If you don't think they would be suitable custodians of a browser monoculture, then don't use their browser (chrome or chromium) while that is still an option; also give Mozilla some money.

Online examination systems seem to be causing issues for students with Linux computers at some universities. Surely these universities can do better than make windows laptops mandatory.

@freemo no, it's completely out of the blue!

@freemo I was confused earlier on by a minicircuits spec sheet for a 180 degree hybrid [1]. They only seem to sell three port varieties, I assume they fourth port is internally terminated or it isn't included somehow. Now the thing which confused me was that they referred to one of the ports as the SUM port. Hybrid couplers are often shown as 4 port devices with Sigma, Delta 1 and 2 ports. I would have thought the "SUM" port would be the sigma port, but then the output of the splitter would be two in-phase signals and it wouldn't really be much of a "180" degree" hybrid. Their SUM port must correspond to the Delta port, that is, the signal out of the "sum" port would be large if the inputs at ports 1 and 2 were entirely out of phase, I.e. the difference. It does behave this way, I tried it today, but my question is: why would they call it a "sum" port when if anything it's subtracting?! Am I alone in finding that confusing?

[1] - https://www.minicircuits.com/WebStore/dashboard.html?model=ZFSCJ-2-1

@AmpBenzScientist I need the passband to be at 202 MHz, with a few MHz of bandwidth. I'm no expert on filter technology, but this frequency seems a bit high for quartz and their passbands apparently tend to be very narrow. It is unfortunate for me, because they'd be ideal otherwise.

I need to maintain the phase between two signals and to filter at least one of them. At the moment I've just made a simple LC ladder and put an identical filter on both signals, so that hopefully when they drift, they drift together. The temperature stability of this approach doesn't seem terrible actually.

Does anybody have any experience with designing thermally stable HF filters (100's of MHz)? Any references appreciated :)

@freemo ah fair enough, I was looking out for a single resistor. Yes that works.

@freemo I can see you've got one on the reflected_SIG, but I can't see one on forward_SIG.

@freemo the dev board of the ad8302 has a 50 ohm shunt resistor at the inputs (R1 & R2). Is there a reason you haven't included these?

@freemo I used the AD8302 recently as part of an analogue phase detection system for a linear accelerator. I hadn't thought about how they could be used in a VNA.

Finally made my 2D magnet post. In this example I do a simple demonstration of finding a magnetic field from a coax, and verify the result with an analytical solution. I then use the same method to find the field inside a dipole magnet of the type used at the LHC; a cos(phi) magnet. https://comphysblog.wordpress.com/2020/08/19/2d-magnetostatics-cos%cf%86-dipole-magnet/

#fenics #physics #finite_element_analysis

Half way through the magnetostatics post I promised a year ago. I've finished an introductory coax example and now I'm writing the intro theory for a cos(phi) magnet. I think it'll be ready in a week :) After that I see two options to progress: eigenmodes or time dependence.

If I go with the eigenmode, I can write an example for how the finite difference method works and talk about linear algebra methods. I haven't tried any time dependent problems in FEniCS yet, so that'll be a whole new area to learn.

Surprisingly, the blog is apparently getting a citation of some kind.

Is there a good way to identify low Q resonances over long cables with a VNA reflection measurement? The long cables can mean the phase doesn't cross zero, the low phase gradient of the resonant termination means there isn't a jump in phase and the BW of the amplitude makes it hard to identify a peak/ trough...

"Everybody believes in the law of errors [the final result of many small, independent, random errors is normally distributed], the experimenters because they think it is a mathematical theorem, the mathematicians because they think it is an experimental fact" - Poincare, Calcul des Probabilites

I've just compared my formula for the characteristic impedance of an eccentric coax with results from a series of finite element simulations with FEniCS. They look excellent. I've added them to my post https://comphysblog.wordpress.com/2020/06/28/conformal-mapping-1/

@revistazunai As somebody that has never touched a philosophy book, where would you suggest that I start?

New post - Conformal Mapping Example, the Eccentric Coax

https://comphysblog.wordpress.com/2020/06/28/conformal-mapping-1/

This post is different from all my others. Rather than stepping through solving a problem with FEniCS, I step through solving a problem with the mathematical method of conformal mapping.

I introduce the method by solving the far easier problem of a pair of slanted parallel plates with a potential difference.

After that I get to the main point of the post: finding the characteristic impedance of a coaxial cable where the centre conductor isn't in the middle; where the cylinders are eccentric. Although the process has quite a lot of algebra, the final solution is simple and very usable.

I've really enjoyed doing this example, because I think it's a great demonstration of using analytical methods to solve a problem with strange boundaries. These days we'd probably just solve this kind of thing numerically, but the analytical solution gives insight and a formula that can be used over-and-over.

I'll add a numerical comparison in the coming days.

#RF #HAM #physics #maths #mathematics

@coldwave It's a bit old now so, some things might have changed, but this tutorial is excellent https://python-packaging.readthedocs.io/en/latest/

@Lwasserman I used to do voluntary tech support for Firefox on the forums. 95% of the time this happened it was because of extensions or weird setting mods rather than Firefox itself. I recommend you run the FF profile manager, make a clean profile, don't install any extensions and see whether the problem persists.

I also discovered that WordPress.com supports latex. Excellent.