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RE: AO-40 beginner having some probs!




>From: "Richard Mason" <info@m0exe.org.uk>
>Hi Rick,
>
>I assume your S3 noise level is produced by the downconverter.
>
>>> Yep, theres nout on the meter with it switched off.
>
>Try aiming your dish up into the cold sky and then at your house or the
>ground.
>
>>> Ok, did try that yesterday during the day too.  The signal does vary
>slightly but not significantly different when pointed at the Sun, but if I
>am to expect a 4Db noise difference with a 1m dish I guess that would be
>less with the 60cm dish I'm using.  Therefore hardly noticeable on the
>signal meter (6Db to an 'S' point normally)??
>
>Regards,
>
>Richard
>M0EXE

Richard,

It sounds like your 3731 is working OK.  I'm not an expert on them so I
differ to those experienced with them.

1.  If you have a noise increase in the speaker and an s-meter increase
when you power on the converter...that is a good indication that it is
working...most is internal noise generated by the conv.
2.  If when your antenna is pointing at the ground or a stand of trees you
see a small rise in the s-meter that also shows that is working and picking
up the thermal noise from those objects...it should drop when you point the
antenna up to the sky (probably only 1/2 s-unit).
3.  The sun will not show much rise in s-meter with a 60cm dish, but if you
can see a little, then there is another indication things are working.
4. Realise that most commercial radios do not exhibit s-meters with 6 dB
per s-unit.  That is the theoretical goal, but most will run 3 to 4 dB per
unit!  The only way to know is to calibrate the FT-726 receiver s-meter by
using a calibrated signal generator.  I did this for my FT-847 and found
not only did my s-meter vary form 6 dB/unit, but it was wildly non-linear
(S1-S2 was 4 dB; S5-S6 was 7 dB...probably a function of the agc which
messes up true readings).  {hint if you can turn off agc, do so when making
s-meter measurements}
5.  So next you need to know if you are pointing correctly...don't assume
it is!  Use your computer tracking program to predict the azimuth and
elevation for the sun and then point the antenna and see if you see it at
that position, or if it is a little off!  If it is off note how much in az
and el.  You will need to offset your pointing when looking for AO-40,
correspondly.
6.  Next, you need to know what freq. your conv. is on....assumptions will
really mess you up on this!  Best way to know is to make a test signal
source.  It is also nice to reassure yourself the thing is working when
problems hearing AO-40 come up.  There are several sources of info on
making a signal source.  Most conv. LO's are off freq. a bit so each one
needs to be determined, then you can compensate in tuning the FT-726!  My
Drake conv. is only 5 KHz low so makes it fairly easy, but they can be many
KHz off!
7.  After all that is accomplished you can confidently look for
AO-40....but there is doppler on the signal so you have to tune around to
find the MB (hint it will be +/- 25 KHz of 2401.323 MHz).

You should realize that I am using a 33-inch (85cm) dish and a super
low-noise preamp.  Yet, I only see one s-unit of ground noise over the cold
sky and slightly less on the sun!  The 60cm dish is a bit small for best
performance on AO-40.  It will work but you may still have some difficulty
hearing weaker stations on AO-40.  GL

Ed, AL7EB
PS:  see my paper in the 2001 Symposium Proceedings (avail from Amsat) for
requirements and steps in getting on AO-40.  There are several other good
papers in there on antennas, etc.  {unabashed promotion for Amsat}


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