[amsat-bb] BBQ Dish Sanity - Part 2

[Anthony Monteiro <aa2tx@xxxxxxxxxxx>]

Hi Guys,

I did some more tests this mornig here is a recap:

Background:
-----------

I tested the Andrew Model 26, 2' x 3' BBQ dish
and found it did not meet it's spec of 24 dBi (+/- 1dB)
of gain. I measured 20.5 dBi (linear) gain.
This would be OK but certainly not a great antenna
for AO-40.

Cliff suggested the feed might be positioned
incorrectly.

I checked the dish geometry and Jerry, K5OE, graciously
checked my work (thank you Jerry!)
Sure enough, the feed was in the wrong spot!

I made a new feed arm that positioned the feed at the
spot I calculated would be the dish focus.

Test 2
-------

This morning I re-ran the test with the new feed arm.

The gain didn't seem to be all that sensitive to
the feed position, so I set it for minimum noise
(i.e. closer in than I calculated as the focus.)

Amazing - the Model 26 now measured 22.5 dBi gain.
This is within spec given the range of error of
the measurement using my simple set-up.

The background noise level measured less than
.5 dB more than the horn so the G/T looked about
the same as the reference horn - which is pretty good.

The difference, while only ~2 dB gain, is actually night
and day in terms on-the-air performance. With
the old feed position, signals are OK but not great.

This morning, the signals were perfectly
Q5 out at 60K Km. I heard NM2A, NZ4BM, and AC4EA
among others, perfectly clearly (Q5) using the stock
SSB filter on the FT-817 (not a "good" filter)
and the built-in speaker with no DSP or other filters.

This is an amazing difference from the other day
with the stock feed.

Also, the "sweet-spot" was very clear and easy to
identify, unlike the "mushy" peak using the stock feed.

I am trying to track down if this was a specific
problem with my dish or a general Andrew Model 26 problem.

Using AO-40 as a test signal is limited in accuracy so
when I get a chance, I will lug the test equipment out
into the back yard and actually measure the gain in
a controlled environment.

The next step would be to measure the G/T accurately as well but
I don't think it is all that bad given how good the signals
were this morning.

As Lee suggested, l think  it would also make sense to
cut the pigtail and put the preamp right at the feed
which should get ~ 1 - 2 dB further improvement in S/N.

I won't have a chance to run these tests for another
couple of weeks given my current schedule. I will try
to get another report out in September.

The rest is my opinion:
-----------------------

With 23 dB gain (linear) this could be a VERY REASONABLE
antenna for AO-40, with only minor mods if one is willing to
use a decent (.5dB NF) preamp.

Even At 24 dB, this dish would still only be running 50% efficiency.
I haven't even looked at a "good" feed for this dish. It
seems as though these could be very good antennas for AO-40
with the right feed mechanism.


Comments welcome (after all it's just a hobby!)

73,
Tony AA2TX




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