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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


----
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