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



  Just a suggestion - I wonder if folks with very high ambient
noise-levels aren't having some sort of a parasitic oscillation
problem?

  -al (still modifying my drake -- hope to be qrv by the weekend.)

John / NS1Z wrote:

> 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, NXAN
> > > > >schiers@netins.net
> > > > >
> > > > >
> > > > >----
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> > > > >
> > > >
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