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Gene's Spreadsheet (was: AO-40 Lids)


You are absolutely correct.  I played with the spreadsheet last night and
saw similar values to you.  There is a 20 dB difference between beacon level
and noise floor with my setup as well.

The statement and assertion by some that the problem operators who call CQ
on the beacon are using BBQ dishes just cannot be supported by the math!  If
you play on the spreadsheet it's clear that even with a 15 dBi linear corner
reflector you will still be able to EASILY hear the beacon.

If people are not hearing the beacon, then there is something else
drastically wrong. 

One of the reasons I took the AIDC survey last week was I wanted to know how
many were running an unmodified downconverter w/o the Murata filter.  I see
now that Bob is having the factory tune the internal comb filters for our
band - that's great.  But if you have an original unit designed for 2500 MHz
and unmodified, then your noise figure is 3 dB worse!  Add that to a small
BBQ dish of only 18 dBi and I can see how you might have problems hearing
the beacon.

Again, I'll be putting up some more hard numbers soon.  I want to calibrate
my S-meter tonite.  I couldn't last night as I didn't have a pass.



on 2/5/03 7:28 AM, hasan schiers at schiers@netins.net wrote:

> Not all systems that use a BBQ dish are alike and not all BBQ's appear to
> work the same. I've been using a HyperLink 39" with a UEK-3000 from day 1 on
> AO-40 and comparing my results using Gene's spreadsheet shows exactly what I
> experience. I have made many measurements with a step attenuator and I get
> precisely (or at times a bit better) SNR on the Beacon and on my signal that
> I should. It is readily apparent what difference I would get by going to the
> patch feed with 1 meter dish.
> For all this crazy discussion, how about simply plugging in the values of
> interest into Gene's spreadsheet and reducing all the speculation?
> The BBQ dish I use has 24 dBi linear (horizontal polarization). The 1 meter
> dish and patch feed produces 26 dBc, this is a net difference of 5 dB
> (correcting for linear to circular loss). Gene's prediction software shows
> how well I hear with the BBQ and I have verified it. Can I hear the passband
> noise floor at apogee? No. I do see the beacon at a 20 dB SNR. I produce a
> signal of 12 dB SNR with 12 w output power into my 14.2 dBc uplink antenna
> (assuming 2 dB of coax loss, 80' of LMR-400).
> So...what's the problem?
> Whatever system one is using as long as you can copy yourself at 8 dB BELOW
> the Beacon, your set for a LOT of contacts.
> Can you hear everyone on the bird? No. Can you hear the vast majority with
> this kind of setup, absolutely!
> With the BBQ Dish:
> Rx Noise Power -145 dBm
> Passband Noise Floor -145.487 dBm
> The passband noise floor cannot be heard by about 0.5 dB...these is pretty
> darn respectable. 0 Squint, 50,000 km
> With the 1 meter dish and circular patch feed:
> Rx Noise Power -145 dBM
> Passband Noise Floor -140.487 dBm
> The passband noise can easily be heard by about 4.5 dB....this is really
> good. 0 Squint, 50,000 km
> ===================================================================
> From the analysis above, I don't  think I could agree that we should be
> throwing away the BBQ dishes. That's just too harsh a judgement IMHO. If you
> have a really good D/C and a good BBQ dish, you are going to hear very well,
> with squints < 30 degrees. That is my direct experience. You won't hear
> nearly as well as with a 1 meter dish/patch, but to carry the point further,
> the 1 meter dish isn't going to hear "nearly as well" as a 10' TVRO either.
> Where do we stop?
> The assertion that I should throw my BBQ dish away, based on FACTS (both
> theoretical and empirical), not a bunch of speculation, is just not
> supportable.
> This discussion could take on a lot more variables such as manipulating
> squint, etc. The point being, put up the very best receiving setup you can
> afford and physically manage, as it will always benefit you. However, don't
> throw away your BBQ dish, IF Gene's program shows that you will get
> performance similar to mine.
> The problem may be two-fold (the apparent good performance of some systems
> using BBQ dishes)
> 1. Lousy downconverters. If you have a top of the line D/C, you may find
> SOME BBQ's quite sastisfactory.
> 2. Poorly performing BBQ Dishes. Not all of them are equal, that is quite
> apparent.
> I can't tell you the number of people I work using surplus D/C's that can't
> hear worth a darn when connected to moderate receiving antennas. Connect
> them to a 3 meter TVRO dish and they "seem to work really well", but it is
> an illusion. They still don't work worth a darn, but the massive antenna in
> front of them lets the "system" work well.  They are cheap, but good ones
> are not necessarily "automatic performers" upon purchase. Some are fairly
> decent, many are very marginal performers. Put these on even the best BBQ
> dishes and you get a lousy overall system.
> I really can't emphasize strongly enough sitting down with Gene's
> spreadsheet and "playing" with all the variables, especially the "front end"
> ones like antenna gain, circularity, coax losses, D/C NF and Gain, etc.
> You can then "see" exactly what effect various antennas and d/c's are going
> to have on your ability to hear the passband noise at apogee and under
> various squint conditions. The same analysis can be done on the uplink, with
> very little effort.
> ===========================================================================
> Bottom Line: Don't throw anything away until you use an objective tool to
> analyze what is going on, and then verify the predicted performance with
> some real "measurements". Then we can tell what should and shouldn't be
> "thrown away".
> ...hasan, N0AN
> (Much snipped and attribution removed to protect the guilty.)
>> You don't know what you are missing because you've never heard it. And you
> won't realize this fact until you throw that BBQ in the trash. I have used
> that setup, and lots of other guys have and either moved to something
> bigger, or gave up. I prefer they move to something bigger.
> ----
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Jon Ogden
NA9D (ex: KE9NA)

Citizen of the People's Democratic Republik of Illinois

Life Member: ARRL, NRA
Member:  AMSAT, DXCC

http://www.qsl.net/na9d   <- Updated on 1/22/03!!!

"A life lived in fear is a life half lived."

Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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