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Showing posts from December, 2017

Parasitic Quadrifilar Helical Antenna

This article was reprinted in OSCAR News, March 2018: If you want to receive Satellite Weather Pictures , then you need a decent antenna, otherwise you will receive more noise than picture. For polar orbit satellites, one needs an antenna with a mushroom shaped radiation pattern .  It needs to have strong gain towards the horizon where the satellites are distant, less gain upwards where they are close and as little as possible downwards, which would be wasted and a source of noise.  Most satellites are spin stabilized and therefore the antenna also needs circular polarization, otherwise the received signal will flutter as the antennas rotate through nulls. The helical antenna, first proposed by Kraus in 1948, is the natural solution to circular polarized satellite communications.  It is a simple twisted wire - there seems to be nothing to it.  Various papers have been published on helix antennas, so the operation is pretty well understood. Therefore,

Satcom 2 m band Helical Antenna

To receive Satellite Weather Pictures , you need a special antenna that will handle the rotating signal. Dimitris Papadeas at SatNOGS built a variety of 2 meter band helical antennas: To see how well it works, I modeled his design with NEC2 using CocoaNEC on my Mac.  The radiation pattern looks cool.  With 12 dB gain, it would be good on a simple tracker or for permanent pointing at a Geo sat.  You cannot just point it up at the sky to catch a polar bird, since then a satellite would have to pass almost directly over head, which doesn't happen very often. 2 m Band Helical Antenna Pattern At this size, a crossed Yagi would be rather easier to build though. A helix is more suitable for the 70 cm band, where it will be much smaller and easier to support. CM Helix, SatNOGS, v4 CM Frequency: 434 MHz CM Wavelength: 691 mm CM c=299792458 m/s CM Radius = 110 mm CM Turns: 8 CM Circumference to wavele

Raspberry Pi 3 Headless Server With SSH

If you wish to make a standalone router, file server, or satellite weather station, then you need a little computer to make things happen.  At this time, the most popular embedded Linux machine is the Raspberry Pi .  It is a delightful little design - tiny and very useful. The default system is Raspbian , which is loosely based on Debian .  This is excellent.  Years ago, I tried a Beaglebone Black and it came with a cripple version of Angstrom Linux which I didn't like and the board is consequently lying somewhere in my junk box. The Raspbian system is aimed at clueless newbies and the ssh daemon is disabled by default.  To use it as an embedded server, without having to plug in a keyboard and screen, you need to add one line to a configuration file, before you plug the SD card into it.  Here is how to do all that. Get a Pi and a SD Card Image Get your RPi3 from here: Download a Raspbian image zip file from here:

Satellite Weather Maps, on a Macbook

There are about 3000 active communications and earth observation satellites flying over our heads at all times (plus about 2000 dead ones).  Many of the earth observation satellites broadcast useful data which anyone can receive, once you acquired the necessary equipment and know-how.  See this   Clouds in the UAE Desert Weather satellites are generally considered to be the most useful of the lot, since the data is open and not encrypted and the signals are quite strong.  The NOAA operates both geostationary and polar observers.  The geo satellites can only be received if you happen to live in its antenna footprint (North America), while the polar satellites pass overhead twice a day wherever you are. This article describes how to get an image from one of the NOAA polar satellites, using a cheap ($25) little RTL-SDR radio receiver.  These pictures are interesting, since the weather is always changing.  You also need to make a decent antenna , else you

Weather Satellite Turnstile Antennas for the 2 meter Band

NEC2, 2 m band, 146 MHz, Yagi Turnstile Simulation and Build This article describes a Turnstile Antenna for the 2 meter band, 146 MHz amateur satcom, 137 MHz NOAA and Russian Meteor weather satellites.  Weather satellite reception is described here .  A quadrifilar helical antenna is described here .   Engineering, is the art of making what you need,  from what you can get. Radiation Pattern of the Three Element Yagi-Uda Antenna Once one combine and cross two Yagis, the pattern becomes distinctly twisted. The right hand polarization actually becomes visible in the radiation pattern plot, which I found really cool. Radiation Pattern of Six Element Turnstile Antenna Only a true RF Geek can appreciate the twisted invisible inner beauty of a herring bone antenna... Six Element Turnstile Antenna Essentially, it is three crosses on a stick.  The driven elements are broken in the middle at the drive points.  The other elements can go straight throug