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Re: Expected noise results with Drake converter (no satellite.)



Hi Ed,

Two observations:

1. You can be pretty sure that your system is not performing as well as the
assumptions built into the model, and that's important!

2. Your BBQ antenna is working quite a bit better than the dish. (Should it?
What's the predicted gain of the dish with the helical feed? (You do have
the sense of the helix right...don't you reverse it for the reflection...in
other words, it has to be wound Left Hand Circular pointed at the reflector
and becomes RHC with the "reflection" ?)

I played with the numbers to try to see what might be happening ..of course,
everything really points at the NF and Gain of the Drake not being what we
had assumed, plus there is coax loss between the antenna and the
Drake...wonder how much?

New numbers:

Ant > Drake Loss: .2 dB
Drake NF: 7 dB
Drake Gain: 15 dB
Drake > Radio Coax Loss: 2.5 dB
Rx NF: 10 dB
Ant Gain: 18 dBi (subtracting circular to linear loss, IF that's
appropriate)
Range: 63,000 km
Squint: 6 deg

Results:

System NF: 7.6 dB
MDS -133.2 dBm
Beacon S/N: 6.7 dB

Looking at the graph, you wouldn't hear ANY signals and just barely hear the
Beacon, which is what you seem to report.

The good news is that if you put a  .6 dB NF preamp  with 25 dB gain in
front of what we show for your system now... the Beacon S/N improves to 19.5
dB...an increase of nearly 12 dB.

Who knows for sure what is "fungo" in your system at this point, but you can
certainly see a light at the end of the tunnel if you put a good preamp on.

Good luck...I'll be joining you in the struggle this week, if the guys get
back from Dayton with my "package". 73

p.s., I ran numbers approximating your friend's setup and they show he
should have a Beacon S/N of 23 dB...which is what you are reporting. What
fun!

...hasan


hasan schiers, NØAN
schiers@netins.net

----- Original Message -----
From: "Edward R. Cole" <al7eb@ptialaska.net>
To: <amsat-bb@AMSAT.Org>
Sent: Sunday, May 20, 2001 4:24 AM
Subject: Re: [amsat-bb] Expected noise results with Drake converter (no
satellite.)


> Al and Hasan,
>
> Eary this morning, after trying my 18 inch DSS RCA dish/5-turn Helix/Drake
> [435 MHz IF...xtal changes only], 50-foot Rg-213 [2.5 dB loss], to a
> FT-736R [a freind's], I have to report total failure to hear the beacon!
I
> then subtituted a BBQ style dish with 21 dB gain and dipole feed, and
could
> just detect the beacon about 1-2 dB above noise.  This at about 63,000 km
> and squint ~6 degrees.  This is not quite the expected and estimated 7 to
9
> dB S/N!!!
>
> Not sure what the problem with the offset dish/helix is: pointing
probably,
> feed adjustment, bad feed [swr?], not enough antenna gain, too high NF,
not
> the expected IF gain from the Drake, ...?  All the above?
>
> I was visiting the qth of my freind Mike, WL7BQM, and his Meyers dish [~24
> dB] dipole combo, Conifer preamp [NF?], Rg-6 coax, FT-736R is doing
> fantastically well.  We heard the beacon from 25 to 35 dB above the noise.
> He has 100% copy on the TLM with ao40Rcv.  We had fun working a few
stations.
>
> So folks, there is theory and there is reality.
>
> I'm back to the "drawing board" and the bench.  Waiting for my Kuhne
> preamp.  Looking at setting up a 4-foot dish.  Wondering why the little
> dish performed over 35 dB down from Mike's system?  As they say...stay
> tuned for future developments.
>
> Ed
> Al, with no satellite you should see a significant rise in Rx noise when
> you turn on the preamp.  I see about two s-units; some see up to S-9!
>


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