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Re: BBQ Dish Sanity???



Tony, Cliff,Regarding the observation of less than "calculated 24 dBi gain" of these MMDS dishes, I did a geometric analysis of these dishes a couple of years ago for a presentation at amsat-na (Atlanta).  An excerpt from that report follows:-----------------------------------------------Wade, in his authoritative on-line text, states the general feed design goal succinctly: "in order to have a very efficient dish illumination we need to increase energy near the edge of the dish and have the energy drop off very quickly beyond the edge" (4). The oft-quoted feed efficiency rule of thumb is illumination power equal to -10 dB at the dish edge, with perhaps -13 or -15 dB for slightly less efficiency but better G/T ratio (5). A little trigonometry shows the E-plane edge-of-the-dish-cutoff (E) is at 46 degrees and the H-plane cutoff (H) is at 63 degrees for the author's surplus dish: 

E = arctan [13 / (15.5-2.75)]E = 46 degreesH = arctan [19 / (15.5-5.75)]H = 63 degrees
Note the dipole patterns above, at the calculated cutoff angles, are closer to -5 dB and -4 dB, leading to significant spillover loss--thus efficiency is low for this dish, especially in the H-plane dimension. The standard dipole array feed simply does not have enough gain to meet the -10 dB design goal. Given this, the de facto 50 percent efficiency  used above appears optimistic and was shown to be just that at a recent microwave conference in Texas. A few iterations of these formulae will indicate a f/D greater than 0.5 helps achieve the desirable characteristic by compensating for the antenna's beamwidth pattern; i.e., the prime focus is closer to the edge than the center. For comparison, an f/D of 0.4 requires about 1.4 dB stronger signal at the edge (inverse square law) than at the center of the dish for constant illumination, while an f/D of 0.6 requires only 0.7 dB at the edge (6). These values are derived from applying the classic power equation to the dish geometry: 
P = 10 * Log10 [opposite / hypotenuse], where P is in dB --------------------------------------------------------------
The whole article ("MMDS Dishes") can be viewed at:  http://members.aol.com/k5oe(note some of the other pages are not available as my swbell server is down temporarily).
You don't have to guess at the efficiency.  If you wanted to get a better value for the actual efficiency of these dishes, you could take the modelled patterns in Figure 2 (http://members.aol.com/homebrewpics/mmds-feed/figure2.gif) and enter the data in W1GHZ's FEEDPAT program.  I am sure I ran this 2 years ago but honestly forgot where the files are :-)  I suspect an actual efficiency more like 40-45 %.
With all that said, nothing is implied here that 21 or 22 dBi is not enough gain to work AO-40 under good conditions.  The 18" offset dish I took to amsat-uk last week only tested out to have 17 dBi gain (I predicted 18), yet I have had both CW and SSB QSO's with it sitting in the bed of my truck.73,Jerry, K5OE
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