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Re: Orbit 250 apogee


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]:

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

*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}

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}

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

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 :-)


>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
>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.
>hasan schiers, NØAN
>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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