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BBQ Dish Sanity - more info

Hi Guys,

I found out a little bit more about my dish.

The reflector is an Andrew model 26 magnesium
alloy grid dish. The dish measurements are:

39.25" (h-plane)
23.5"  (e-plane)

Focal length is around 12.75"

The required feed beamwidths are
150 degrees (h-plane)
100 degrees (e-plane)

Note that for maximum gain, the feed should be down
around -10dB at the above angles.

This would not be the optimal feed for the
best signal to noise ratio (for satellites)
but the antenna was designed for terestrial use.

However, the feed supplied with my dish is NOT
an Andrew model 26 feed. The feed is for a
Pac Wireless PAWDCA-24 which is also a grid dish
but is slightly larger than the Andrew dish and
the focal length is 16"

The required feed beamwidths for the PAWDCA-24 are
134 degrees (h-plane)
82 degrees (e-plane)

Notice that the required feed beamwidths are
noticeably narrower than for the Andrew dish.

The dish and feed were surplus from an
MMDS service provider and were used together.
Apparently you can buy just the grid reflector
from Andrew and supply your own feed.

This is really interesting because the
Pac Wireless feed would be out-of-focus when
mounted with the supplied feed arm onto the
Andrew dish. Apparently the MMDS service provider
didn't notice or didn't care!

***** This explains a couple of things *****

First of all, I can't make any claims at all about
the gain of an Andrew model 26 since I did not test
the dish with the Andrew-supplied feed. I apologize
for any misleading statements in this regard.

It also explains why my dish seemed to have a "mushy"
sweet spot and why the gain was low. The feed arm
is too long for the dish putting the feed out-of-focus.
The feed also over-illuminates the dish when mounted
this way.

Now, assuming that the Pac Wireless feed was designed for
max gain, or something close to it, this feed would have
a narrower beamwidth than the Andrew feed. I don't actually
know what is inside the little feed box and I don't know what
the actual pattern might be. I would guess it is a
dipole + reflector probably stamped out of a single piece of aluminum.

When the Pac Wireless feed was mounted on the
arm that I made, the dish performed reasoably well. Not
only would it then be in-focus but if it had been
designed for max gain on the Pac Wireless dish, it would
under-illuminate the Andrew dish (perhaps only in the
h-plane though.) This would tend to improve the noise
performance of the dish over using the stock feed. I didn't
measure the noise performance so I can't say if this
is what was happening for certain.

So, while the dish actually worked pretty well, it is not
really comparable to a stock Andrew model 26.

I still think that the Andrew grid reflector, if fitted with a
proper feed that matched the required beamwidths, would work
even better in both gain and noise performance. I'll take a look
at what this would require when I get a chance.

Thanks and 73,

Tony AA2TX

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