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Re: RE:Help with 2880 (was Orbit 250 apogee)



I have two Drakes that I modified and I don't see the ambient level
fluctuations that you speak of (NOT steady and jumps from around S5-S9).
This may indicate a poor solder joint or other malady. Both of mine give
steady signals from stable sources (either my Weak Signal Source from DEM or
the satellite). I could easily hear the satellite with either of them using
my 24dB gain BBQ dish..

I added the DEM ULNA pre-amp between the dish and the Drake. This brought
signals up quite a bit. I don't run the radio's pre-amp fo r2 meters now and
the ambient level seems to be aroundd S1-2 on the IC-746.

I would have to agree with the others: the addition of a pre-amp is WELL
worth the expense. I am not sure which is better the DEM or the SSB but I
intend to buy a SSB pre-amp soon to find out. I know it has more gain....

Please, keep us posted on your progress. There are a lot of lurkers out
there who don't say a thing while they are in the development stage - they
just sit back and learn, learn, learn.........

----- Original Message -----
From: Timothy S. Zibrat <tzibrat@adelphia.net>
To: John / NS1Z <ns1z@arrl.net>; John Stephensen, KD6OZH <kd6ozh@AMSAT.Org>;
<amsat-bb@AMSAT.Org>
Sent: Thursday, May 17, 2001 12:35 PM
Subject: 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
> > > >
> > > >
> > > >----
> > > >Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
> > > >To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org
> > > >
> > >
> > >
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> >
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