R: [amsat-bb] Heard the Bird

["i8cvs" <domenico.i8cvs@xxxxxx>]

Hi   David, KY0DR

At 03:30 UTC may 23, orbit 258, MA=62.9  the AO40  squint was 16.4° from
Kansas City ,KS and AO40 was at a range of 51282 Km with  AZ=101.7° and
EL=15.1°
The actual ALON/ALAT transmitted by the TLM beacon at orbit 258 this morning
was 0/-5
At MA=62.9 the transponder was OFF and it went ON from MA=70 to MA=209

I was operating SSB at 05:30 UTC and made good QSO with VE3NPC,i1TEX and
WA6FWF. Also W0EOZ was heard very strong

If you hear the beacon very week in to the noise with 60 cm dish i suggest
you to check if your 2,25 turn elix feed has been wound LHCP or not to get
RHCP from reflection from the dish.

Also you should investigate on the image rejection in dB of your Drake
converter and see if the Drake has inside a bandpass filter to attenuate the
noise contained in to the image band before it arrive to the mixer.

I dont know the Drake at all but as a general example,if the LO of your
downconverter is 2256 MHz  than
2256+144=2400 MHz and this is the desired frequency
2256-144=2112 MHz  and this is the image frequency

If your donconverter has no bandpass filter to attenuate the image band
before it arrive to the mixer it happens that a signal at 2112 MHz and a
signal at 2400 MHz will be received both overlapped in the same
IF frequency of 144 MHz as for the example.

The same is true for the noise but the power noise contained in to the image
band and the power noise contained in to the desired band are constantly
present as random noise or "withe noise"

If no filter or poor filter to attenuate the image is included between
the downconverter input and its mixer,both noise will add deteriorating  by
3 dB the noise figure NF  for  the wanted band of 2400 MHz

The problem is that if we arrange a NF measurement using a PANFI we will get
a noise figure wich value cannot  take in consideration the above situation
and so,without filtering out the image frequency band  a measured NF of 1 dB
will result in real practice a  NF= 4 dB or 3 dB worse at 2400 MHz

This happens because the noise head of the PANFI  instrument injects in to
the  wanted and in to the image bands the same amount of noise power when it
is OFF and  when it is ON  rendering unchanged their Y ratio

In other words in this situation the noise figure is 1 dB for the PANFI
instrument but 4 dB for your ears.

This is why along considering for example a good downconverter noise figure
of 0,6 dB as a final task,it is necessary to measure the image rejection as
well in order to establish if the real NF at 2400 MHz  is 1 dB or possibly
higher.

In order to keep the noise figure not corrupted by the image noise,the image
rejection of downconverter should be at least 40 dB and it requires a good
filter to attenuate as little as possible 2400MHz and the maximum is
possible 2112 MHz in the above example.

As a result of this discussion my homemade DB6NT downconverter has a
measured image rejection of 40 dB and a noise figure NF=1,5 dB and
a conversion gain G=21 dB

In order to improve the receiving performance of my system i have added a
hombuilt  WB5LUA preamplifier just in the focal point of my 1,2 meter dish
and it is directly connected to the 3 turns elix  SMA connector.

The WB5LUA preamp has 0,5 dB NF and G=13 dB with the ATF-10135 and PCB is
available from DEM

Following  this preamp i have a second postamplifier,the  same WB5LUA
and so the gain before the downcoverter input is 25 to 26 dB

In order to improve the image rejection of the system i have included an
interdigital bandpass  filter between the postamplifier output and the
downconverter input.

The total  image rejection at 2112 MHz is now 60 dB from the filter plus 40
dB from the downconverter for a total of 100 dB of image rejection.

In this conditions using 1,2 meter dish the AO40 beacon is 25 to 30 dB over
the noise depending on MA

The measured sun noise is ranging from 5 to 6 dB with the actual solar flux
in sfu and the received S2 transponder noise is fluctuating from 5 to 8 dB

All of this to point out that noise figure alon is not sufficient to define
a receiving system performance without considering the value of the image
rejection as well because a very low image rejection  of downconverter will
deteriorate the best averall noise figure of  the system.

73 de i8CVS Domenico


----- Original Message -----
From: Scott Townley <nx7u@arrl.net>
To: <ky0dr@arrl.net>; <amsat-bb@AMSAT.Org>
Sent: Wednesday, May 23, 2001 7:21 AM
Subject: Re: [amsat-bb] Heard the Bird


> My NOVA sez 20.4 degrees, hearing anything at that squint is an
> accomplishment.
>
>
> At 11:13 PM 5/22/01 -0500, David Rush wrote:
> >I stayed up a little later than usual tonight, and finally heard the
> >AO-40 beacon at about 2401.320 MHz.  Very weak, though - S0.  It took me
> >a while to find it.  This is with a 60cm dish, 2.25-turn helix, Drake
> >(simple mods only), 50 ft of cheap Rat Shack TV coax, to my FT736 (no
> >preamp).
> >
> >Can any of you tracking software jockeys tell me what the squint angle
> >was, to a station in Kansas City at around 03:30, May 23 (UTC)?  I'm at
> >39d 00.74m north, 94d 40.87m west.
> >
> >I'm trying out Predict 2.1.2 under Linux, and it doesn't seem to provide
> >squint angle.
> >
> >David, ky0dr
> >----
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> >
> ------------
> Scott Townley
> nx7u@arrl.net
> Gilbert, AZ  DM43
>
> HF/VHF/UHF/Satellite MOBILE station
> http://www.primenet.com/~nx7u
>
> ------------
> "When the Going gets Weird, the Weird turn Pro"
> Raoul Duke, Sportswriter
>
> "Don't worry Boys, we'll weather this Storm of Approval and come out as
> Hated as Ever"
>
>
> ----
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>






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