Hello Rich and the SETI list:
I'll take a stab at this. Remembering that antenna gain is a function of
aperature, Rich has proposed an antenna with an aperature of 10x24 feet.
The "Little Ear" should be visualized as a 24 foot dish that is half sunk
into the ground so that the feed point is at the surface of the ground in
front of the dish. To further simplify construction, suppose we then cut
the top of the dish off along a line parallel to the ground at a height of
10 feet. What we have now is a parabolic sector antenna.
For purposes of discussion lets assume that the f/d ratio is 0.6. The feed
point of this antenna will be 14.4 feet from the center of the hypothetical
24 foot dish. To simplify calculations lets assume that the physical
aperature (area) is 10x24 ft. In reality it isn't rectangular but the ends
curve inward toward the top. Gain=4*E*pi*A/lamba^2. A=10*24=240 ft^2, or
22.3 m^2. converting to wavelenths (lamba): A=22.3/(.21)^2=505.7 lamba^2.
So G=12.56*505.7*E. Lets assume that the efficiency factor E=0.5.
G=3177.4 (or converting to decibels: Gdb= 10*Log(G)=35 db).
Remember we assumed a rectangular aperature, so the actual gain will be
less. Coincidentally, if we calculate the gain of a 16 foot (5m) dish, it
is 34.4 db. so it looks like this parabolic sector antenna would be
equivalent in gain to a 16 foot dish.
To design a feed horn, I used a program called hdlant* written by Paul
Wade, W1GHZ (ex N1BWT). The horizontal illumination angle is approx. 90
degrees, and the vertical angle is 40 degrees. Two horn design options are
offered by the program:
W1GHZ Design: Rect. waveguide size: 210x105 mm. Horn: Length=16.89 mm,
H-plane=312.1 mm, E-plane=231.2 mm, gain=10.8 db.
G3RPE Design: Rect. waveguide size: 210x105 mm. Horn: Length=35.9mm,
H-plane=241 mm, E-plane=192 mm, gain=9.68 db. (optimized for f/d=0.6)
Oh, BTW the horn has to be half sunk in the ground, as the horn program
assumes a dish of 24 foot diameter. (Remember that you also need a tilting
plane reflector, and a conductive ground plane between the two reflectors).
Well, this is a pretty crude design approach and others may wish to correct
my errors.
*If you are interested in Paul Wade's program you can download it from:
http://www.tiac.net/users/wade
BTW Paul is featured in this month's QST Magazine (ham radio mag).
73, Ed
Alaska SETI Coord.