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



Hi Ed,

My first two guesses would be:
1.  Pointing Angle

Most of the DSS offset dishes have a "tilt angle" of around 68 degrees.
When the face of the dish is vertical the main beam is actually at 68
degrees elevation.  As a result the dish must almost always be tilted DOWN
to bring the elevation down to point at the satellite.  On mine I found the
mounting bracket on the back had elevation markings based on the support
post being vertical.  The lowest setting was 25 degrees so I turned the
elevation rotor up to 25 degrees and mounted the support post vertical.
When I was trying to point the dish by hand, I almost always underestimated
how far over I had to tilt the dish.

Check out http://www.qsl.net/n1bwt/app-5a.pdf and the preceding chapter for
lots of good information.


2.  Feed polarization

The feed should be wound for LHCP so that you get RHCP after reflecting off
the dish.  I used the same design from K5OE's website for both my 16 turn
helix and the 5 turn dish feed.  The only difference was the number of turns
and the direction of the turns.

I seem to be getting about 20dbi from my 18 inch offset dish and it is about
6db better than the 16 turn helix.

73s,
Lee-KU4OS
----- Original Message -----
From: "Edward R. Cole" <al7eb@ptialaska.net>
To: <amsat-bb@AMSAT.Org>
Sent: Sunday, May 20, 2001 5:24 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!

>From: "hasan schiers" <schiers@netins.net>
To: "Al Lawler" <alawler@us.oracle.com>,
>I put in a 5 dB NF and 20 dB gain for your Drake, 1 dB of coax loss for
your
>25' run and a 10 dB NF for your FT-736R and a receive bandwidth of 2500 hz.
>I put in 21 dBi for your antenna, subtracting 3 dB for it being linear to
>circular. If these values are anywhere near correct, the results follow:
>
>Your MDS (minimum discernable signal) is -136.4 dBm. The strongest signal
in
>the passband that is graphed is -133 dBm. So you will hear this VERY STRONG
>signal on 3.4 dB above your noise floor. The strongest signal permitted is
8
>dB below the beacon. The beacon is at -123 dBm. The loudest station should
>not exceed -131 dBm. Since your noise floor is -136.4 dBm, the loudest
>signal you should be able to hear will have a signal to noise ratio of 5.4
>dB.
>
>The Beacon should be 13 dB above the noise. You will hear only the very
>strongtest of signals.
>
>This was calculated at 65,000 km, or apogee, with 0 offpointing or squint
>(antenna dead on). If there is any squint, you will have even poorer
>signals.
>
>Hope this helps ya.
>
>hasan schiers, NØAN
>schiers@netins.net
>
>----- Original Message -----
>From: "Al Lawler" <alawler@us.oracle.com>
>To: <amsat-bb@AMSAT.Org>
>Sent: Saturday, May 19, 2001 3:52 PM
>Subject: [amsat-bb] Expected noise results with Drake converter (no
>satellite.)
>
>
>>
>>   Configuration:
>>
>>   Ft736r -> 25' rg6 -> Drake(With "easy mods" -> 24dbi bbq antenna
>> (and no satellite in view)
>>
>>
>>    What should I expect to hear with the above configuration
>> when I power up the converter?
>>
>> I'm not attempting to recieve any  satellites yet,
>> and understand the importance of a preamp when I do,
>> but for the purposes of debugging what I have built so
>> far,  should I expect to hear a change in the
>> "white noise" on the 2m reciever (tuned to 144.050 or so)
>> when I power up the converter?  Are there any "birdies"
>> generated by the converter which I might be able to
>> tune to to verify whether the converter is working or not?
>>
>>
>>   Thanks,
>>
>>   -al
>> ----
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>>
>
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