[Date Prev][Date Next][Thread Prev][Thread Next] - [Date Index][Thread Index][Author Index]

Re: 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.



----- 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
> 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
> 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
> 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
> 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
> 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
> >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
> >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,
> >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
> >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
> >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
> >the time, and they expose "compromise" systems. A downconverter with a 5
> >or greater noise figure,  without a preamp is just asking for lousy
> >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
> >a 1 dB NF system, I think it will become quite apparent what the problem
> >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
> >bucks for a top of the line downconverter, like the UEK-3000, or fairly
> >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
> >Technology). If that doesn't produce a signal to noise ratio greater than
> >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
> >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
> >that matter), on Mode B with AO-40 Mode S. I'm willing to bet however,
> >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,
> >loss, etc. on receive threshold improvements. It is downright SCARY how
> >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
> >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
> >if your receive threshold improvement was not  at least 7 dB, buy putting
> >low noise preamp in front of those surplus downconverters.
> >
> > (Note: improving the system noise figure will improve your ability to
> >weak signals by MORE than the difference in the two noise figures. That
> >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
> >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
> >or 7 dB, especially if the gain of the converter is insufficient to
> >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
> >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
> >
> ----
> Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
> To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org

Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org