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Re: Orbit 250 apogee



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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