On Wed, 2 Mar 2016 14:26:28 -0800 (PST), Reg Williams
wrote:

I have been trying for some time to decode L band signals from the Inmarsat satellite 3-F2 with no success. I am located 20 miles west of Gloucester UK.Lat 51 degrees N.
There are no obstacles as far as I am aware to block the signal at 30 degrees elevation. The antenna is pointing in the direction of 54 degrees west. So I guess it may be down to the equipment. This is a 60cm dish with a Log periodic pcb antenna 850-6500Mhz manufactured by WA5VJB pointing at the dish. A short coax length is connected to the antenna via a pcb mount sma connector to a LNA4All Low Noise Amplifier and then to a rtl 2832u r820t dongle plugged into my laptop. The software is sdrsharp. I have tried receiving signals generally around 1.545 Ghz. Not sure about the log periodic. I guess the fact there is a short circuit when using an ohm meter between the outer and inner soldering points before fitting the sma connector is part of the characteristic of the antenna. From info I have read, the signal is quite strong and can be picked up easily, so it seems strange I cannot pick it up. Any thoughts or guidance i would be grateful.
Thanks
Reg

Your problem is the antenna and possibly the amplifier.

Inmarsat uses right hand circular polarization. Your antenna is
linearly polarized. That's good for a -3dB cross polarization loss.

LPDA antennas have great bandwidth, but very low gain. My guess(tm)
is that antenna has only about 4dBi gain. You can do better with a
simple patch antenna cut for Inmarsat frequencies.

The LNA4ALL is a broad band amplifier that goes from 28 to 2500 MHz.
Broadband amps generally have lower gain and worse noise figure than
narrow band amps. Gain and low noise are vital in this amplifier
because the rtl2832u have very low receive sensitivity.

You can probably get away with the 1dB NF of the LNA4ALL as long as
you don't care about terrestrial interference and mixing. When you
invite your entire RF neighborhood of strong broadcast signals into
your receiver, all you're going to see are mixes and intermodulation.
If you do care about this problem, you might consider replacing the
amp with something more narrowband and possibly inserting a 1545 MHz
bandpass filter before your amplifier.

You will have problems finding a dedicated Inmarsat receive only
amplifier. Most amps are made to go on satellite terminals, which are
both TX and RX. They also tend to be built into the antenna radome
assembly. I found a few under "L-band amps".

RX amps:
http://www.rtl-sdr.com/tag/low-noise-amplifier/
http://www.uhf-satcom.com/lband/

Inmarsat antennas:
https://www.google.com/search?q=inmarsat+antenna+circular+build&tbm=isch

Inmarsat bandpass filters:
https://www.google.com/search?q=inmarsat+bandpass&tbm=isch

Good luck.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558