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Re: BBQ Dish Gain Data



Hi Cathryn,

I recently measured the gain of an Andrew model 26
grid reflector. This dish is 39.5" by 23.5".
It measured around 23 db (+/- about 1 dB)  which is
within the manufacturers specs of 24 dB +/- 1 dB.
The Andrew catalog shows that the "typical" gain
should be about 23.9 dB at 2.4 GHz.

I have been working on improvements to the BBQ
dish feed and this morning I measured a S+N/N
ratio of 18.5 dB on the middle beacon with the
satellite at 62,500 km and 4.4 degrees squint using
the same dish but with an improved feed. My feed
improvement is just an extra reflector to minimize
the e-plane over-illumination inherent in the dipole feed.
I do not believe it increases the gain at all, just
reduces the noise temperature from the spill-over.

As a quick "back of the envelope," I would estimate the raw
gain of the dish at around 24.2 dB at the feed minus .3 dB
for the LMR-240 pigtail yielding a net gain of 23.9 dB
which is exactly what the manufacturer says it should be.

The problem with the BBQ dishes tested at the central
states conference seems to be the feed, not the
raw capability of the dish.

I believe I can squeeze another .5 to maybe 1 dB or so more
out of this dish with a little more work. So, if you can
receive the middle beacon at 18-19 dB S/N ratio at 62,500 km,
there doesn't seem any need to make a really big one.

Best 73,
Tony AA2TX

---

At 11:56 AM 9/15/2003 -0700, Cathryn Mataga wrote:

>Edward R. Cole wrote:
>>Thanks, Gene:
>>Central States and other VHF/UHF groups do a good job of making the
>>"reality" measurements to compare with our theoretical assumptions.  To
>>summarize, it looks like the larger BBQ dish (labeled 38x26-inch and
>>submitted by K5GNA) measured at 20.8 dBi, while the smaller BBQ 17x22-inch
>>dish came in at best gain of 18.3 dBi.
>>These two data points kind of establish the upper limit of performance that
>>one should expect...I would use these gains in the AO40v2.0 spreadsheet for
>>estimating purposes.
>>Interesting to note that a 0.9m dish submitted by K5VH measured at 22.2
>>dBi...I am using an 85cm-offset feed dish which is approx the same size (he
>>uses a loop feed vs my helix).  This is approx. -3dB of my est. gain.
>>Also interesting to see the effects that choice of feeds make.
>
>Okay, so 38x26 = 988 sq inch.
>.9m = 35.4 inches area = pi r^2 = 3.14 * (35.4/2) * (35.4/2) = 983 sq inch
>
>Would that be about 22.2db versus 20.8db or about 1.4db difference for the
>BBQ factor?   Is that the whole difference?  Or is there something else
>going on here?
>
>Has anyone ever tried to make a monster BBQ dish, like 10ft orsomething?
>It'd seem to me, that'd be easier to rotate than a solid/mesh dish of the
>same size.
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