On Fri, 4 Mar 2016 07:22:39 +0000, Jeff wrote:

On 03/03/2016 18:43, Jeff Liebermann wrote:
On Thu, 3 Mar 2016 16:28:07 +0000, Jeff 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



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