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*Subject*: [amsat-bb] Yet More on BBQ dish gain...*From*: Scott Townley <nx7u@xxxxxxxx>*Date*: Sun, 16 Mar 2003 13:30:34 -0700

...I am not trying to re-ignite the "BBQ Debate" (oh that's a bad pun). Only providing information that some may find interesting. In a previous note, I mentioned that I have modelled the K5GNA 38"x26" BBQ dish in NEC-2. It's an "exact" model in the sense that the grid spacing and wire diameter are as-measured from an actual reflector. Also, the feed is the stock AIDC3733 dipole-plus-reflector plate. The interesting bit from the previous modelling was that the maximum gain was calculated to be 25.52dBi. One difficulty in modelling this antenna outside of NEC is that it not a circular aperture. Often assumptions are made in equations etc. that the antenna can be described by a single angle. In the case of a circular-section parabola, you can define the parabolic surface intersection with a sphere with just the polar angle...in other words, the angle subtended by the rim of the dish. However, a rectangular-section parabola requires two angles (both polar and azimuth), so all the math that is normally reduced to a single variable of integration is out the window. In short, using tools like those provided by W1GHZ don't necessarily apply, or require special interpretation. Not a fault of the tools, only their application. How can the BBQ calculate out to 25.5dBi? That result requires a TOTAL (illumination+spillover+blocking) efficiency of -1.6dB (69%). As Jerry, K5OE correctly points out on his website, the BBQ grid is overilluminated; the edge-illumination is only 3dB down in the "long" plane (H-plane) and 5.3dB down in the "short" (E-plane) direction, based on modelling the AIDC3733 feed by itself (my feed model matches Jerry's very closely). Even with space attenuation, nowhere near the "optimum" -10 to -12dB edge illumination. So this means (relatively) high spillover loss, but also means (relatively) high illumination efficiency--remember that 0dB edge taper (uniform reflector illumination) is 100% illumination efficiency. With that in mind, I hand-integrated the modelled feed pattern of the AIDC3733 against hand-calculated pairs of angles that describe the outline of the BBQ dish. The calculation of spillover is pretty straightforward--count up all the energy that intercepts the reflector against the total energy radiated (in all directions) by the feed. For the BBQ feed I come up with a spillover efficiency of 72.8% (-1.37dB). That doesn't leave much for illumination efficiency, but remember that "poor" spillover means "good" aperture. Again, I hand-integrated the radiation striking the reflector using an integral formulation for illumination efficiency found in Balanis. The calculated illumination efficiency was 94.4% (-0.25dB), for a total (less blockage) efficiency of 68.7% (-1.63dB). This is extremely close to the result implied by the maximum gain calculation of 25.5dBi. It would be expected that there is very small feed blocking loss, because the reflector is only 0.45% of the total antenna aperture. So it would seem that the modelled result of 25.5dB is well founded. NOTE HOWEVER that modelling intrinsically assumes a mechanically perfect implementation; perfect focus, perfect adherence to the double-curved parabolic surface, etc. In the field, focus errors or mechanical distortion of the reflector will reduce the achieved maximum gain (aka YMMV). Scott Townley NX7U Gilbert, AZ DM43di http://members.cox.net/nx7u ---- Sent via amsat-bb@amsat.org. Opinions expressed are those of the author. Not an AMSAT member? Join now to support the amateur satellite program! To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org

**Follow-Ups**:**Re: Yet More on BBQ dish gain...***From:*nick pugh

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