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, 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 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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