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Re: Orbit 250 apogee


It is a an Excel spreadsheet.  I presented it at SETI-Con01 last month and
is to be placed on:

I checked last night and it has not shown up yet.  Exec. Director Dr. Paul
Schuch makes weekly updates on the weekend [Saturday usually], so it may be
there this next week.  My companion sw g-t.xls is there, but I have revised
an error in the algorithm so you should get the new version.

BTW I tried http://home.hiwaay.net/~nmarcus/ and the page is not there?
have you a better link to it?

A complete downlink analysis program is what is needed to answer all these
questions from the bb.  Your description implied that the nmarcus sw does
this.  It was something I have been thinking about but haven't gotten
around to.  Hey, the S2 signal is up there so real measurements can be made
vs. theoretical analysis.  I think everyone forgot about the power sharing
penalty on the link budget [10-13 dB].  But everyone should know by now [if
they are reading the bb].

The Tsky and Tant are estimates; no calculations done.  Tsky may be
available from radioastronomy sky-noise maps, but above 15 degrees
elevation 10K is pretty close.  If you are in a dense urban RF environment
then 30K may be more realistic, but with antenna elevation that should

Finding Tant is much harder.  You can arrive at it by using g-t.xls and
measuring your system G/T ratio, plug in what you think your antenna gain,
sky temp, and receiver temp [NF] is and solve for Tant:  Tsys = Tr + Tsky +

I just took 10% of To = 290K [ambient air temp] as a good target for Tant.
If you limit dish edge spill-over to -15 dB or better you'll approach this.
 For helical antennnas I don't have any idea what their Tant is.  Low Tant
is tied to low sidelobes in the antenna pattern, since they can look at
thermally hot ground.  A clean antenna pattern gives a low Tant.

Regarding the NF of the FT-847, this is not theoretical.  It was measured
as 14 dBNF [with the internal preamp OFF] and 8 dBNF with the preamp ON by
a well-known eme'r two years ago...sorry!  If you look at sensitivity vs.
NF, it is very predictable for a given receiver bandwidth.  

With your:
>Rx ant: 26 dBi
>UEK-3000-144: .7 dB NF and 30 dB gain.
>2 dB of coax loss @ 144 mhz
the NF of the FT-847 is negligible [turn OFF the internal preamp as it will
be overdriven by the conv].


At 02:41 PM 5/17/01 -0500, you wrote:
>That is a WONDERFUL piece of work. I'm just thrilled to see it. Now I want
>to be able to do it myself.
>I would LOVE to have a spreadsheet, or if not available, the base equations
>for doing the more complete analysis you outline below. I especially need to
>include the values for:
>Antenna gain/noise in Tant
>I also noticed you used Tsky as 10K while Domenico used 30K.
>If you could send me your notes, or if you did it in a spreadsheet, the
>spreadsheet itself, and any instructions that might be necessary to use it
>I can pretty well put things into perspective:
>Tsky 10K or 30K ? What is Tant for 26 dBi gain parabolic section dish (and
>how to derive it?)
>Rx ant: 26 dBi
>UEK-3000-144: .7 dB NF and 30 dB gain.
>2 dB of coax loss @ 144 mhz
>FT-847, assume NF of 5 or 6 dB and Rx BW of 2.5 Khz (I don't think the 847
>NF is 14 dB <grin>)
>tnx so much, 73

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