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Re: Orbit 250 apogee



If the converter noise figure is 5 dB that is a noise temperature of 917 K.
Down East Microwave sells a LNA that they guarantee has a noise figure less
than 0.7 dB and a gain of greater than 16 dB for $85 as a kit or $120
assembled. This should vastly improve reception. I'm sure that there are
equivalent products from other suppliers.

A 0.7 dB noise figure is a noise temperature of 51 K. If you put that preamp
ahead of a 5 dB NF converter the noise figure degrades to 51 + 917/40 = 74
K. The typical parabolic dish adds 29 K because the feed picks up ground
noise and the sky has an average temperature of about 3 K at 2400 MHz so you
probably get a system temperature of 106 K. This would improve SNR by a
factor of 9 or 9.5 dB when the antenna is not pointing at the horizon.

73,

John
KD6OZH

----- Original Message -----
From: "hasan schiers" <schiers@netins.net>
To: "Estes Wayne-W10191" <W10191@motorola.com>; <amsat-bb@AMSAT.Org>
Sent: Wednesday, 16 May 2001 23:48 UTC
Subject: Re: [amsat-bb] Orbit 250 apogee


> N.B. The comments below are an attempt to address Wayne's perception that
> AO40's downlink signal is "disappointing". I'm not sure if it is or isn't.
> What I wrote below is an attempt to explain why it may seem that the
> downlink signal is not what one would like. Please take it in the light it
> is offered. I'm not trashing the surplus downconverters...I think they
> were/are a great find....but they leave some important work to be done,
and
> if it isn't, it may lead to conclusions about AO40 that are not
> "reasonable".
>
>
>
> Wayne said,
>
> "I'm still disappointed in the downlink signal
> strength of AO40.  For me, the AO40 downlink is less readable than what I
> got from AO13 with much less RX antenna gain."
>
>
> (I bet you had a MUCH better system NF on 2m for AO13, than you have for
> AO40 on Mode S!)
>
> Are we comparing apples and oranges?  From what I've been reading on the
> list, the NF of your downconverter is around 5 dB (or worse, depending on
> mods done or not done) . My "guess" is, if that is accurate, your
threshold
> signal is poorer by at least 7 dB than it should be. A 5 dB NF is just
> totally inadequate for weak signal work. I'm betting you had a much better
> NF on AO-13 mode B.
>
> Again, what I glean from reading the reports here, and the discussions on
> the #amsat chat channel is the following:
>
> 1. System NF of 1 to 1.5 dB, certainly less than 2 dB.
> 2. Rx antenna of at least 20 dBi
> 3. Squint < 10 degrees.
> 4. No buildings or trees to impede line of site
>
> produces VERY good signals, nothing to be disappointed about. The problem
> appears to be largely squint induced. Pointing angles are very poor much
of
> the time, and they expose "compromise" systems. A downconverter with a 5
dB
> or greater noise figure,  without a preamp is just asking for lousy
signals,
> unless everything else is super-optimal, including the squint. There are
> also significant obstruction losses on 2.4 gig that are minimal by
> comparision at 2m (but still there).
>
> If one compares the MDS (minimum discernable signal), of a 5 dB NF system
to
> a 1 dB NF system, I think it will become quite apparent what the problem
is.
> A 5 dB or greater NF is like having a 7 dB attenuator on the antenna.
> Unfortunately, getting a 1 dB NF ain't cheap. There are two choices....big
> bucks for a top of the line downconverter, like the UEK-3000, or fairly
big
> bucks for a low NF preamp, ahead of the downconverter. (Or ...a humongous
> antenna that you have trouble pointing)
>
> I may end up sharing your assessment, but I hope not. I'll be using a low
> system NF setup along with a 26 dBi gain parabolic section dish (HyperLink
> Technology). If that doesn't produce a signal to noise ratio greater than
10
> dB at squint angles less than 5 degrees, then I will be disappointed. The
> antenna I'm describing is 39 inches wide and about 15 inches high. I don't
> consider it "big", but it has substantial gain.
>
> I just don't see any way to fairly compare the ease of AO-13 (and AO-10
for
> that matter), on Mode B with AO-40 Mode S. I'm willing to bet however,
that
> a BBQ linear antenna with an antenna mounted preamp into these surplus
> downconverters will produce impressive signals. Expecting a 5 dB NF
> converter to produce decent signals without a gigantic antenna, is, to my
> mind, unrealistic.
>
> I'm basing this "assessment" on comments on the #amsat chat group, and an
> article I wrote for Amsat Jounal when AO-10 was launched. The article
> examined the effects of antenna gain, receive system noise figure,
feedline
> loss, etc. on receive threshold improvements. It is downright SCARY how
much
> you can improve your ability to hear by paying STRICT attention to the
> overall SYSTEM NOISE FIGURE. This is precisely what the surplus
> downconverters (with no preamp) do NOT accomplish and may explain why some
> people are disappointed.
>
> Even if you are hearing AO-40 at an acceptable level with the 5 dB NF
> downconverters, you have no idea what you're missing. I don't have the
> figures or equations in front of me at the moment, but I would be
astounded
> if your receive threshold improvement was not  at least 7 dB, buy putting
a
> low noise preamp in front of those surplus downconverters.
>
>  (Note: improving the system noise figure will improve your ability to
hear
> weak signals by MORE than the difference in the two noise figures. That is
> why I "guessed" that a 4 dB improvement in NF, will result in a 7 dB
> improvement in receive threshold.)
>
> (Note 2: The 5 dB NF for the downconverters being discussed is NOT the
> system noise figure. The loss in the coax to the radio at 2m must be added
> in, as well as the noise contribution of the front end of the radio. As a
> result, the system noise figure of the downconverter setups could well be
6
> or 7 dB, especially if the gain of the converter is insufficient to
overcome
> the noise contribution of the 2m feedline to the radio).
>
> Compare the predicted S/N ratio of a 1.5 dB system NF to a 7 dB system NF
> and you are talking nearly 10 dB improvement in weak signal reception., as
I
> recall. I wonder how AO-13 would have sounded with a 10 dB attenutator at
> the antenna? That's precisely what a mediocre NF system behaves like for
> Mode S on AO40.
>
> I apologize if my remarks offended anyone, that was not my intention. I
also
> apologize for the length of the post.
>
> 73
> hasan schiers, NØAN
> schiers@netins.net
>
>
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