Sunday, December 17, 2017

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: https://community.libre.space/t/uhf-helical-v4-instructions-and-source/617

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 wavelength ratio: 1
CM Reflector perimeter to wavelength ratio: 0.8 = 553 mm
CM Winding diameter: 220mm
CM Winding circumference: 691mm
CM Winding spacing: 172 mm
CM WInding length: 5696 mm
CM Reflector perimeter: 553mm
CM Total length = 172 mm x 8 t = 1376 mm
CM Reflector plane 1 mm below zero to avoid a short with start of helix
CM
CM Feedpoint(1) - Z: (111.340 - i 1152.400) I: (0.0001 + i 0.0009) VSWR(Zo=50 Ω): 99.0:1
CM Antenna is in free space.
CM Directivity: 11.08 dB
CM Max gain: 12.51 dBi (azimuth 0 deg., elevation 90 deg.)
CE
GH 1 500 0.172 1.376 0.110 0.110 0.110 0.110 0.003
SM 10 10 0.277 -0.277 -0.001 0.277 0.277 -0.001
SC 0 0 -0.277 0.277 -0.001
GE
EX 0 1 1 0 1 0 0 0 0 0 0
FR 0 1 0 0 434 0
RP 0 90 90 0 0 0 1 4 0 0 0
EN


This design will work over the whole 2 m band and is good for Hamradio, Weather and Cubesats.

What I like about satellite work, is that it is low power, sensitive work, so you need to know what you are doing, but you will not zap anyone touching your antennas when you happen to be transmitting at 3 or 5 Watt.

The modern earth observation satellites produce amazing pictures and they change all the time, due to the sun angle, seasons and weather http://www.geo-web.org.uk/index.php

Notes

Helical antennas superimpose nicely.  That is, if you would wind four helixes together, each 2 turns, every 90 degrees, it would be equivalent to a single 8 turn helix, just much shorter.  Here is the seminal work on the quadrifilar helix http://www.uhf-satcom.com/sband/sbandQFH.pdf

La voila!

Herman

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