Building an L-band helical antenna for Inmarsat

Previously in August of this year we wrote an article showing how to decode Inmarsat satellite STD-C NCS EGC messages with an RTL-SDR. Inspired by this article, RTL-SDR.com reader Mario Filippi, N2HUN has written in to show us how he built an L-band helical antenna to receive these signals. A helical antenna is one of the better choices for receiving Inmarsat signals as it will provide higher gain when compared to a patch antenna, however the disadvantage is that it is much larger. Of related interest, Adam 9A4QV also recently showed us a video detailing the correct dimensions for building an air gap patch antenna.

Mario’s Inmarsat antenna consists of a 90cm Ku band dish, a homebrew L-band LHCP helical antenna and an inline amplifier. He used the assembly instructions found on UHF Satcom’s page at http://www.uhf-satcom.com/lband and scavenged most of the parts from his junk box. To help others with the construction of a similar antenna Mario has also created a document detailing the construction of the antenna with several useful build images (.docx file).

Helical Inmarsat antenna feed for a 90cm Ku band dish
Helical Inmarsat antenna feed for a 90cm Ku band dish

Mario has also recently given a presentation about the RTL-SDR to the Mid Atlantic States VHF Conference entitled “SDR Dongle for VHF/UHF Reception”. The presentation is an overview of the RTL-SDR dongle and many of its interesting applications, including several screenshots of software in action (dropbox) (mega mirror).

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