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RE:Help with 2880 (was Orbit 250 apogee)



With my modified Drake converter, I don't think I could tell a 1-1.5 S unit
change.  The baseline noise is NOT steady and jumps from around S5-S9
rapidly.  I've seen it as high as 20dB over 9 at times.  I've heard of
stations on this reflector using attenuators in the IF coax to bump this
baseline noise down.  What does this do to the important factors like NF and
overall gain?  I've installed the new RF amp modification and Toko filter
for a hopeful NF of <2.0.  This is not counting the feed line from the dish
(3 feet of RG8 from Myers BBQ dish), insertion loss of the 2880, or the coax
feed from the IF output to the rig.

I know I've read allot on this reflector about baseline noise in the Drakes,
but I'm still a little confused as to whether I need a pre-amp, an
attenuator in the IF or inductors across the input or what.  I'm about to
rip my 10 foot TVRO dish feed off and stick a feed on it for AO40.  Although
I don't think this will help me with my primary problems of noise, NF and
gain of the converter.  I've seen the calculations and can understand them
for the most part, but I need more info in "laymen's" terms.  As Danzel
Washington said in the movie "Philadelphia", "explain this to me like I'm a
6-year old so I can understand." d:^)  I know I'll get this stuff sooner or
later.

Any help would be appreciated to this newbie to the world above 435MHz!!

Thanks in advance,
K3TZ

PS - It looks like it's going to be a good pass on Saturday night, and I
will try in earnest to see what I can do with what I have.  My luck the
transponder will be turned off to run the arcjet motor!!  Listen for me!!!!
I may be weak (10W into a 30ele CP yagi on 435).


-----Original Message-----
From: owner-AMSAT-BB@AMSAT.Org [mailto:owner-AMSAT-BB@AMSAT.Org]On
Behalf Of John / NS1Z
Sent: Thursday, May 17, 2001 6:53 AM
To: John Stephensen, KD6OZH; amsat-bb@AMSAT.Org
Subject: Re: [amsat-bb] 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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