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Re: BBQ Dish Sanity???



Anthony
Point the antennas at the sun and measure the sun noise the go to the web
and get the SFU and calculate the g/t and compare the g/t between the
antennas. Ed Cole has a nice spred sheet to calcularte the g/t and theink
its on the seti web page
nick
----- Original Message -----
From: "Anthony Monteiro" <aa2tx@comcast.net>
To: <amsat-bb@AMSAT.Org>
Sent: Wednesday, August 06, 2003 10:54 PM
Subject: [amsat-bb] BBQ Dish Sanity???


> Dear friends,
>
> What is the real story on the BBQ dishes?
>
> I am specifically referring to the Andrew
> Model 26, the 2' x 3' grid dish.
>
> This dish is spec'd by the manufacturer at
> 24 dB (+/- 1 dB) gain at 2400 MHz and is
> linearly polarized. It has an included feed
> that has a short "pigtail" of LMR-240
> coax with a male N-connector attached.
>
> If you look at the numbers, such as in Lee McLamb's
> article in the recent AMSAT Journal, this dish,
> set up with a reasonable preamp, ought to be
> a PERFECTLY FINE ANTENNA for receiving AO-40.
>
> With 24 dB gain and -3 dB for the polarization loss,
> along with a 0.5 dB noise figure preamp, you should
> be at the transponder noise floor at 50,000 Km and
> only a little degraded all the way out at 65,000 Km.
>
> Yet, these are often criticized (especially
> on the amsat-bb) as only for "alligators" - what is going on
> here? I was curious and so, to find out, I purchased one and ran
> some tests. My dish was new and the parts were still sealed in
> plastic. The feed was marked 1P, 2.4 GHz
>
> Here are the results.
>
> ---
>
> TESTS
>
> I ran these tests this morning (Aug-6, EDST)  with a reference
> antenna and the Andrew Model 26.
>
> My reference antenna was a pyramidal horn, made of aluminum,
> that I carefully designed and constructed to provide
> 22 dB of gain. The horn apperture is 20.25" by 26.0". It uses
> a TEM01 probe feed and is linearly polarized.
> I should note that it has not been calibrated against a
> lab standard however.
>
>
> The receive system, basically a 2.4 GHz power meter, was as
> follows:
>
> 1. DEM preamp - measured 0.28 dB NF
> 2. 3' of 9913FX coax
> 2. AIDC preamp - measured 1.37 dB NF
> 3. 18 dB attenuator
> 4. Yaesu FT-817 receiver at 123 MHz - AGC off
> 5. HP, AC voltmeter calibrated in dBm0
>
> The FT-817 is very handy because you can shut the AGC off thus
> allowing you to set up a linear system to make power measurements.
>
> Note that I was not trying to measure S/N ratio or anything
> "exotic," just the received level of the AO-40 beacon.
>
> Both antennas are configured with an N-male connector so the
> receive system is exactly the same for both antennas. No
> switch or coax was used in front of the preamp. Both antennas
> were operated horizontaly polarized.
>
> Both antennas were located in the same position - on
> my picnic table on a second floor deck with a clear view
> to the satellite.
>
> The procedure was as follows:
>
> 1. Connect reference antenna and roughly point at AO-40
> 2. Tune in AO-40 beacon on FT-817 for max level on meter
> 3. Adjust antenna pointing to maximize level on meter.
> 4. Adjust output level of FT-817 to -4 dBm0 on meter (well
>     within linear range)
> 4. Note meter reading minimum and maximum as signal
>     varies about +/- 1 dB on the meter.
>
> Then:
>
> 5. Connect test antenna
> 6. adjust antenna for max level on meter
> 7. Note meter readings
>
> Finally;
>
> 8. reconnect reference antenna
> 9. verify same readings on reference antenna as before.
>
> ----
>
> I did the above a number of times to make sure there
> were no inconsistencies. When these tests were run,
> the satellite was at around 60,000 Km and less than 5 degrees
> squint. The results were surprising.
>
> The dish is spec'd at 24 DB gain and the reference antenna
> was designed for 22 dB gain so the dish should provide 2 dB
> more signal power than the reference antenna.
>
> However, the reference antenna consistently provided about
> 1.5 dB more signal strength than the dish. Hmmmmm????
>
> ---
> Since the above result was not what was expected, I did some
> qualitative, "on the air" testing as well.
>
> The FT-817 AGC was switched to "slow."
>
> An SSB QSO was tuned in and monitored with both
> antennas. Switching from one antenna to the other,
> the reference horn consistently provided
> a noticably better signal than the Model 26 dish.
>
> Hmmmmm again....
> ---
>
> To get a handle on the reference horn, I ran these
> tests:
>
> The FT-817 AGC was set to "SLOW."
>
> With receive system operating but the antenna disconnected
> from the preamp, the S-meter on the FT-817 showed no
> S-meter reading (S-0.)
>
> With the receiver tuned to 123 MHz (in the AO-40 passband)
> but not tuned to any signal, the meter reads S2 to S3.
>
> Tuned to the AO-40 beacon, the meter reads S-8.
> The beacon was strong with some noticable QSB in
> the audio level but no change was observed in
> the s-meter reading.
>
> A CW signal near the beacon showed S-8 on the meter
> and was Q5.
>
> JN1GKZ was heard calling CQ on SSB and his signal
> read S-6 to S-7 and was easily Q5 copyable.
>
> This was pretty much as expected at this range.
>
> ---
>
> So, the only conclusion I can come to is that while the
> dish is usable, it does not perform as specified and does
> not really have 24 dB gain. If it did, it would be a very good
> antenna for AO-40. Hmmmm????
>
> What is going on here?
>
> Has anyone tested a Model 26 on a range?
>
> What did I do wrong?
>
> Any other comments?
>
>
> Thanks and 73,
>
> Tony AA2TX
>
>
> ----
> Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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----
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