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Re: Masthead preamp question



At 05:02 PM 5/4/2005 +1000, Jack wrote:
>Wow what a lot of good data.
>
>More clearly here is my setup.
>
>
>Wimo crossed quad (11db) with V H RH and LH remote polarisation switching on 
>2m and the same for 70cm (13dB).
>
>60 cm dish with patch feed and SSB downconverter conected to the patch
>
>2m coax loss to receiver input 0.5dB using LMR600
>70cm coax loss to receiver input 0.8dB using LMR600
>The SSB downconverter runs to the shack on 2m also on LMR600.
>
>Q:
>
>With these figures and a Kenwood TS2000 do I need a preamp at all.  Echo 51 
>belts in here most passes but since there are no current HEO birds I have no 
>experience with them.  
>
>From my readings however, the downlink on proposed HEO birds is S band or 
>higher.  One has 2m but also 2.4Ghz
>
>Is the S band signal worse than AO40.  I have read at length the systems 
>required and I feel my disk, patch and downconverter should cut it.
>
>Any comment for a amateur and AMSAT newbie (Dec 2004)
>
>
>-- 
>73
>
>Jack Swart
>VK2TRF
>35758
>QF56oe

Jack,

Sounds like you are building a first class station for satellite.  But you
question whether to use preamps.  Every time a new satellite is launched we
have many new operators ask that same question ....or not even know to ask
that question.  And every time we have "deaf" stations cranking up their
uplink power or jamming the uplink trying to hear something...meanwhile
many stations running reasonable or low power are blocked from use of the
satellite.

The old rhyme is: "You cannot work them if you cannot hear them"!

ALL commercial radios, including the TS-2000 are built with receivers
designed able to handle high RF environments and thus do not have good
noise figures.  On average they run 9 to 14 dB NF on VHF.  Good preamps
bring that NF to the 0.5 dB NF range which makes a basically deaf radio
hear.  As you go up in frequency into the microwaves low NF becomes
increasingly harder but increasingly more important for good receiver
sensitivity.  Do not judge your weak-signal capability an the ability to
hear repeaters.  I would ask you how far can you hear stations on 146.52
simplex?  If not 50 miles+ you have a deaf radio!

You mention you have an SSB 2.4/144 convertor with a 2-foot dish.  But you
say you had no experience on AO-40.  That set up was the minimum needed for
good hearing.  Often hams with that had marginal copy of AO-40 signals when
the satellite squint angle was high...yet those with 3-foot and larger
dishes and running 0.6 to 1.0 dB NF preamps had no troubles.

We heard on the bb during AO-40 from lots who maintained they heard just
fine yet they had to run uplink power ten times that needed.  If you have
never had the experience of hearing with a good system then you may think
you are fine.

The SSB convertor's NF is about 1.0 dB NF (to my memory) so that may be
sufficient with P3E and Eagle (both HEO's).  My Drake converter with 5-7 dB
NF was deaf on 2.4 GHz.  Adding a MKU-232 preamp with 0.6 dB NF made it
first rate!  My dish is 33-inch offset feed.  I could copy AO-40 telemetry
to squint = 45 degrees at apogee (65,000 km).

You spend all this money building a satellite station yet leave off the
most important part...the preamp.  Get preamps, you will never be sorry you
did.  This is true at VHF, UHF and mw.  It has been true since AO-7 was
launched.  It was true on AO-10, 13 and 40.  It will always be true (even
with AO-51).  I understand that in VK land the monetary exchange rate is
murder buying US or EU made equipment.  But you have some good mw mfrs down
there.  

Amsat member since 1976

73's
Ed - KL7UW  <new call>
===================================
BP40iq,  Nikiski, AK      http://www.qsl.net/al7eb
50-144-432-1296-2400-3456*-10368-24192* MHz
*Under construction
===================================
----
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