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Re: Orbit 250 apogee



Thanks, John! That was some good info on the ground to sky difference. I
just tried it with both my VR-5000 and the FT-847 receivers. The baseline
did a noticeable increase on the 5k and the Yaesu saw about 1.5 S units.

Thanks for a great no cost system test! We need more of this type of
info....

----- Original Message -----
From: John Stephensen, KD6OZH <kd6ozh@gte.net>
To: <amsat-bb@AMSAT.Org>
Sent: Thursday, May 17, 2001 7:24 AM
Subject: Re: [amsat-bb] Orbit 250 apogee


> An easy way to see if your system has a low enough noise figure is to
point
> the S-band antenna at the ground and then at the unobstructed sky. If the
> noise level doesn't go down by 6 dB (about one S unit) or more you
probably
> need a preamp.
>
> 73,
>
> John
> KD6OZH
>
> ----- Original Message -----
> From: "Edward R. Cole" <al7eb@ptialaska.net>
> To: <amsat-bb@AMSAT.Org>
> Sent: Thursday, 17 May 2001 06:07 UTC
> Subject: Re: [amsat-bb] Orbit 250 apogee
>
>
> > Hasan,
> >
> > You have addressed an issue that I must EMPHATICALLY agree on.  Now
Wayne,
> > I am not wanting to beat up on you.  In fact I think your comment is
> > serving a valuable service to all AO-40 operators, by voicing a valid
> > concern but a solution that will lead one wrong.  Over the last couple
> > weeks I have been reading the signal reports and getting a feeling that
> > this issue was coming to a head.
> >
> > First of all, all you "experienced" AO-10/13 mode-B operators out there
> > must remember the lesson learned: of the absolute necessity of having a
> low
> > noise preamp located right at the 2m antenna terminals for good
reception.
> > How many of us has worked the ham with no preamp, or with the preamp in
> the
> > shack who just couldn't hear us [while the rest of us with a proper set
up
> > were hearing fine]?  Yes, there are other parameters that affect
> reception,
> > but this is the first priority in building a working system; "you can't
> > work em if you can't hear em".
> >
> > To support Hasan's arguments more, I have run the numbers.  You seen me
do
> > this before.  This time we'll compare different NF and their effect on
> your
> > signal threshold [another way of saying SNR]:
> >
> > Drake-->Rx:
> > Tsky = 10K {estimate}*
> > Tant = 29K {estimate}*
> > NF = 6 dB {mounted directly to the antenna connector and assuming no
loss}
> > Gain = 15 dB {estimate}
> > Feedline = -3 DB {at 144 MHz: 50-foot RG-58 or RG-6, 100-foot RG-213,
> > 200-foot 9913 or LMR-400}*
> > Rx NF = 14 dB
> > Rx BW = 2.5 KHz*
> > System NF = 7.43 dB {note that this is higher than the NF for the Drake
> > because there is insuff. gain}
> > System Noise Temp = 1315K
> > Signal Threshold = -133.3 dBm
> >
> > Preamp-->Drake-->Rx:
> > *Note Tsky, Tant, feedline, and BW are assumed the same throughout so I
> > will not repeat them.
> > NF = 1.5 dB
> > Gain = 15 dB
> > NF-drake = 6 dB
> > Gain-drake = 15 dB
> > Rx NF = 14 dB
> > Sys NF = 1.93 dB
> > Sys Temp = 162.4K
> > Signal Threshold = - 141.6 dBm  {this represents a 8.3 dB improvement in
> > signal from a bare Drake}
> >
> > Preamp-->Drake-->Rx:
> > NF = 0.7 dB
> > Gain = 15 dB
> > NF-drake = 6 dB
> > Gain-drake = 15 dB
> > Rx NF = 14 dB
> > Sys NF = 1.05 dB
> > Sys Temp = 79.2K
> > Sys Threshold = -143.9 dBm {the better preamp gives you another 2.3 dB
> > improvement in signal}
> >
> > Preamp-->Drake-->Rx:
> > NF = 0.7 dB
> > Gain = 30 dB
> > NF-drake = 6 dB
> > Gain-drake = 15 dB
> > Rx NF = 14 dB
> > Sys NF = 0.71 dB
> > Sys Temp = 51.6K
> > Sys Threshold = - 145 dBm {the high gain preamp gives you another 1.1 dB
> > signal}
> >
> > So if you use a low NF [0.7 dB], high gain [>30 dB] preamp you will be
> able
> > to hear signals 11.7 dB weaker than using a bare Drake.  Or another way
of
> > saying it: the same signals will be 11.7 dB stronger using the preamp
> > [that's about 3 S-units].
> >
> > Now about the high cost.  What did your 2m low-noise preamp for AO-10/13
> > cost?  $80 to $140 probably.  A two-stage 2.4 GHz preamp [0.7 dBNF, 35
dB
> > gain] costs $145.  If you have a Drake the total is under $200, right?
> >
> > Another topic is how big a dish do you need?  Save that for another time
> :-)
> >
> > Ed
> >
> > >From: "hasan schiers" <schiers@netins.net>
> > >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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> > >
> >
> >
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