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Inmarsat L band Antenna problem
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Reg Williams
Mar 2, 2016, 5:26:29 PM3/2/16
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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
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
Jeff Liebermann
Mar 3, 2016, 11:15:33 AM3/3/16
to
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 je...@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
Jeff Liebermann
Mar 3, 2016, 1:43:17 PM3/3/16
to
On Thu, 3 Mar 2016 16:28:07 +0000, Jeff <je...@ukra.com> wrote:
>To me what you have seems like a bit of an odd antenna setup. For it to
>work well the LPD active section at the Inmarsat frequency will have to
>be at focus of the dish, which is mechanically difficult. You may be
>better of looking for either a different antenna completely, or using a
>circularly polarized feed for the dish (which may be a bit too small for
>the frequencies involved anyway).
>
>Jeff
Oh, I didn't see the part about the dish. Since the size of the dish
isn't specified, it's difficult to determine if it will work. For
example, if the dish were a common Ku band DBS dish, it is too small
and will not have much gain at 1.5GHz[1]. If it's a converted kitchen
wok, as is common in wi-fi circles, it's also probably too small and
insufficiently close to a parabolic surface. If the dish is a
converted C-band dish, it will probably be very difficult to aim as
the beamwidth can be extremely narrow.
[1] I ran the numbers for a DBS dish at cellular frequencies a few
years ago:
<https://groups.google.com/forum/?hl=en#!original/alt.internet.wireless/7KUW6sGRXls/hhS4ZsRouwUJ>
Plug in 1500MHz instead of 850/1900MHz to see what theoretically can
be done with a DBS dish assuming everything else is perfect. My guess
is you'll do about the same as a single CP patch antenna (about
10dBi).
>To me what you have seems like a bit of an odd antenna setup. For it to
>work well the LPD active section at the Inmarsat frequency will have to
>be at focus of the dish, which is mechanically difficult. You may be
>better of looking for either a different antenna completely, or using a
>circularly polarized feed for the dish (which may be a bit too small for
>the frequencies involved anyway).
>
>Jeff
Oh, I didn't see the part about the dish. Since the size of the dish
isn't specified, it's difficult to determine if it will work. For
example, if the dish were a common Ku band DBS dish, it is too small
and will not have much gain at 1.5GHz[1]. If it's a converted kitchen
wok, as is common in wi-fi circles, it's also probably too small and
insufficiently close to a parabolic surface. If the dish is a
converted C-band dish, it will probably be very difficult to aim as
the beamwidth can be extremely narrow.
[1] I ran the numbers for a DBS dish at cellular frequencies a few
years ago:
<https://groups.google.com/forum/?hl=en#!original/alt.internet.wireless/7KUW6sGRXls/hhS4ZsRouwUJ>
Plug in 1500MHz instead of 850/1900MHz to see what theoretically can
be done with a DBS dish assuming everything else is perfect. My guess
is you'll do about the same as a single CP patch antenna (about
10dBi).
Jeff Liebermann
Mar 4, 2016, 12:36:09 PM3/4/16
to
On Fri, 4 Mar 2016 07:22:39 +0000, Jeff <je...@ukra.com> wrote:
>On 03/03/2016 18:43, Jeff Liebermann wrote:
>> On Thu, 3 Mar 2016 16:28:07 +0000, Jeff <je...@ukra.com> wrote:
>>
>>> To me what you have seems like a bit of an odd antenna setup. For it to
>>> work well the LPD active section at the Inmarsat frequency will have to
>>> be at focus of the dish, which is mechanically difficult. You may be
>>> better of looking for either a different antenna completely, or using a
>>> circularly polarized feed for the dish (which may be a bit too small for
>>> the frequencies involved anyway).
>>>
>>> Jeff
>>
>> Oh, I didn't see the part about the dish. Since the size of the dish
>> isn't specified, it's difficult to determine if it will work. For
>> example, if the dish were a common Ku band DBS dish, it is too small
>> and will not have much gain at 1.5GHz[1]. If it's a converted kitchen
>> wok, as is common in wi-fi circles, it's also probably too small and
>> insufficiently close to a parabolic surface. If the dish is a
>> converted C-band dish, it will probably be very difficult to aim as
>> the beamwidth can be extremely narrow.
>>
>>
>> [1] I ran the numbers for a DBS dish at cellular frequencies a few
>> years ago:
>> <https://groups.google.com/forum/?hl=en#!original/alt.internet.wireless/7KUW6sGRXls/hhS4ZsRouwUJ>
>> Plug in 1500MHz instead of 850/1900MHz to see what theoretically can
>> be done with a DBS dish assuming everything else is perfect. My guess
>> is you'll do about the same as a single CP patch antenna (about
>> 10dBi).
>a 60cm dish was stated in the OP, which is a bit small for teh
>frequencies involved!!
>Jeff
Argh, I also missed the 60cm. This article:
<http://www.rtl-sdr.com/rtl-sdr-tutorial-decoding-inmarsat-std-c-egc-messages/>
suggests various antenna and amplifier combinations. For example, a
RHCP patch antenna and 15dB of amplifier gain assuming a very short
coax cable run. The article is not very specific about using a dish,
but does mention that it should use a LHCP patch with about the same
total gain. My guess(tm) is about 30dB total gain between the dish
and the amplifier.
My obviously impaired brain didn't think of using a modified GPS
antenna, but that should also work. Note that the physically larger
patch antennas do not have more gain than the tiny antennas used in
smartphones. The difference is that smaller antennas have a higher Q
and therefore a narrower bandwidth. In this case, bigger is better.
<https://www.digikey.com/Web%20Export/Supplier%20Content/tallysman-1526/pdf/tallysman-selecting-antennas-gps-gnss.pdf?redirected=1>
>On 03/03/2016 18:43, Jeff Liebermann wrote:
>> On Thu, 3 Mar 2016 16:28:07 +0000, Jeff <je...@ukra.com> wrote:
>>
>>> To me what you have seems like a bit of an odd antenna setup. For it to
>>> work well the LPD active section at the Inmarsat frequency will have to
>>> be at focus of the dish, which is mechanically difficult. You may be
>>> better of looking for either a different antenna completely, or using a
>>> circularly polarized feed for the dish (which may be a bit too small for
>>> the frequencies involved anyway).
>>>
>>> Jeff
>>
>> Oh, I didn't see the part about the dish. Since the size of the dish
>> isn't specified, it's difficult to determine if it will work. For
>> example, if the dish were a common Ku band DBS dish, it is too small
>> and will not have much gain at 1.5GHz[1]. If it's a converted kitchen
>> wok, as is common in wi-fi circles, it's also probably too small and
>> insufficiently close to a parabolic surface. If the dish is a
>> converted C-band dish, it will probably be very difficult to aim as
>> the beamwidth can be extremely narrow.
>>
>>
>> [1] I ran the numbers for a DBS dish at cellular frequencies a few
>> years ago:
>> <https://groups.google.com/forum/?hl=en#!original/alt.internet.wireless/7KUW6sGRXls/hhS4ZsRouwUJ>
>> Plug in 1500MHz instead of 850/1900MHz to see what theoretically can
>> be done with a DBS dish assuming everything else is perfect. My guess
>> is you'll do about the same as a single CP patch antenna (about
>> 10dBi).
>frequencies involved!!
>Jeff
Argh, I also missed the 60cm. This article:
<http://www.rtl-sdr.com/rtl-sdr-tutorial-decoding-inmarsat-std-c-egc-messages/>
suggests various antenna and amplifier combinations. For example, a
RHCP patch antenna and 15dB of amplifier gain assuming a very short
coax cable run. The article is not very specific about using a dish,
but does mention that it should use a LHCP patch with about the same
total gain. My guess(tm) is about 30dB total gain between the dish
and the amplifier.
My obviously impaired brain didn't think of using a modified GPS
antenna, but that should also work. Note that the physically larger
patch antennas do not have more gain than the tiny antennas used in
smartphones. The difference is that smaller antennas have a higher Q
and therefore a narrower bandwidth. In this case, bigger is better.
<https://www.digikey.com/Web%20Export/Supplier%20Content/tallysman-1526/pdf/tallysman-selecting-antennas-gps-gnss.pdf?redirected=1>
Sal M. O'Nella
Mar 5, 2016, 4:46:16 PM3/5/16
to
"Reg Williams" wrote in message
news:31dc8c07-04b1-4634...@googlegroups.com...
that transmit and receive are reciprocal, so the term applies to receive,
also.)
The feed should illuminate the entire dish but the illumination should fall
off outside the area of the dish. First, the feed point has to be placed at
the focal point of the dish, which is easily found by the formula here:
http://www.analyzemath.com/parabola/parabola_focus.html
Second, the feed's geometry determines the illumination of the dish. Having
too many elements will illuminate only a small part of the dish; having just
a single dipole will waste the available signal.
60 cm is a standard Inmarsat dish size, so don’t let that bother you. At
L-band, such an antenna doesn't have a particularly narrow beam; US Navy
ships use unstabilized Inmarsat dishes but pitch and roll do NOT cause a
loss of signal, if that helps.
"Sal"
Reg Williams
Mar 7, 2016, 3:26:05 AM3/7/16
to
Reg
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