Sebastian⚡ @sebastian@chaos.social

@sebastian I already tried a few standard approaches and I ended up need a ridiculous high order for the filter to get the selectivity resulting in incredibly small values for the components involved.

Hey #hamradio fediverse I've got a little challenge for you all.
The nice folks at DL0HSK set up a websdr for 2m an 70cm so that people listen to the spectrum from nice exposed position.
Problem: American military is using 141 142 an 143MHz for radio coms around here. Since they use a rather liberal amount of power the keep saturating the poor SDRs frontend causing weird effects all over the 2m band.
Is there a filter selective enough to block 141-143MHz while keeping 144-146MHz?

So I've just learned that this ham radio stuff comes with achievements... #ariss #ISS #SSTV

@sebastian Okay since I don't like pointless rants:
1:4 Current balun facts in 500 chars:
- Impedance of the balun should be ~10 times what you are trying to match e.g. 2k for 200Ohm
- The winding cables characteristic impedance should be around the geometric mean of the input and output impedance e.g. sqrt(50 * 200) = 100ohm
- The core will only conduct the common mode currents magnetically and should not get saturated by them
- Wires and core should beefy enough for your TX power

@sebastian This literally is a basic engineering problem and yet nobody can be bother to treat it like one.
Instead everybody keeps handing the specs of the balun that uncle of the guy at least fieldday build, which works great but he couldn't tell you the vswr because his uncle doesn't know how to measure it.

@sebastian Why is it so hart to write a proper guide on how to dimension your own balun?
Like starting with how would an ideal balun look like? What are the important parameters to look out for? Which of them should be maximized? which of them should be minimized?
What range are they typically in?
Next step: What is the physics behind them? How are they parameters connected? What are the design constraints?
And then finally 1 or 2 examples showing all the calculations and numbers.

@sebastian Third result is a forum thread were somebody points out exactly the same thing. Two fronts are formed and anecdotal evidence is exchanged. Not a single argument was made using actual theory or even worse fundamental laws of physics.
Anyhow by now I know I'll need either a 1:6 or a 1:4 balun. Depending on which antenna fan fiction you read.
So I google how to dimension baluns, because again I don't blindly follow howtos ... and oh well... Worse then the antenna discussions.

Why is reasearching HAM radio stuff always such a pain in the butt?
I though about building a Windom antenna. (roof is to short for a full dipole)
So I'd like to know the math behind just a little, to be reasonably confident about what I'm doing. I hate following howtos that do not include the reasoning behind their numbers.
The first two search results already disagree about the impedances, eventhough the dimensions given match.

@Bobo_PK @sebastian Symbolbild: Viresechzichhebuff

4:00 the sun is rising. I'm working on incorporating the last last-minute changes.
The birds just started singing oustide.

3:01 ... 6h into the final tweaks on my master thesis. I see little moving withe spots in my peripheral field of vision. Also I'm running out of clubmate.
6h remain until I'll have to print the final version.

https://network-dev.satnogs.org/observations/24756/
https://network-dev.satnogs.org/observations/24562/
https://network-dev.satnogs.org/observations/24561/

Found my observation for the fourth image.

https://network-dev.satnogs.org/observations/24559/

More SSTV from the ISS recveived by my Satnogs groudstation

https://network-dev.satnogs.org/observations/24557/
https://network-dev.satnogs.org/observations/24558/
https://network-dev.satnogs.org/observations/24560/

This weird feeling when you know something will be a really close call, but you also clearly see that you can make it and narrowly avoid the catastrophe. And at the same time you also know that there will be no rest until it's over and you'll have to fight through every second of it. That's when I feel alive. That's when I come up with the real awesome stuff.

Not to bad considering I only pointed the antenna in roughly the right direction and used my smartphone to decode the noises coming out the speaker ...

Waiting for the ISS to come back around...

Also if you managed to receive something stop by at https://ariss.pzk.org.pl/sstv/
where you can get an award that entitles you to some #hamradio bragging rights.