Hello everyone, this is my first post. I am reposting a few questions that I've asked on other forums, as I've gotten no response. To summarize, I was initially interested in building a dual or quad-biquad feed for a parabolic dish on 2.48 Ghz, to help with my wireless N and G reception. After doing some research, I decided that constructing a dual-mode feedhorn based on the information at http://www.qsl.net/n1wbt/contents.html would be nearly as easy, and provide me with superior performance and some more useful practical experience in antenna/feed/horn design and construction. The Trevor Marshall biquad is essentially a tuned coathanger in front of a reflector, while this feedhorn is a bit more sophisticated, and allowed me the opportunity to learn some of the maths involved in antenna design.
Using an estimation of the f/D for the parabolic reflector I'm using (an offset satellite dish) that I got from a very nifty web publication on antenna design (http://www.qsl.net/n1bwt/contents.html), I've come up with some preliminary calculations for the feed. (the nifty little calculator doesn't run on Mac, so I got some extra practice in algebra.)
Assuming an f/D ratio of 0.8, the diameter of the output aperture of the waveguide would be 1.88 wavelengths, using the approximation Dw=2.35(f/D), which translates into 231.05 mm for this project. The length of this section, calculated by some ninja trickery having to do with the TE11 and TM11 modes, turns out to be about 257.882612 mm. I just did the math without really understanding anything about the modes, other than that when they are aligned 90 degrees out of phase at the end of the aperture, they cancel out interference patterns along the edge and add to each other in the phase center, thus (theoretically) eliminating performance-robbing sidelobes from the finished feedhorn. If anyone would care to explain some basic theory on these 'modes', I'd appreciate it.
The critical flare section was calculated using the formula Angle=(44.6(w))/D, where w is the wavelength and D is the diameter in mm of the output aperture. I came up with 23.72 degrees of flare.
So, I've eliminated, on my own, the original seven questions, and have a couple of new ones.
1. I know that this is probably pathetically simple for you RF wizards, but I need to know how to calculate the optimum diameter and length of the input waveguide for a given frequency wavelength, such that it will not accidentally propogate additional frequency modes that will interfere with the operation of the feedhorn.
2. Another simple-simon question... How in the blazes do I connect the feedhorn to the coaxial cable once I've constructed it? Should I follow the instructions for a cantenna, and how should I adapt the measurements to fit this application? Throw me a bone here. Direct me to an accessible resource where I can find the info myself. Something. Please.
3. Since the goal when illuminating a reflector is to have ideal edge taper, would it be possible to create a system with near-ideal characteristics by adding a lens in front of any high-efficiency feed to shape the RF into a precise shape? I could consult an optometrist about lens design, they're pretty much experts at lenses, and from what I understand, lenses can shape RF as well as light. A lens oriented so that the convex side is facing the feed could be made, I think, to refine the signal received by both the parabolic reflector and the feed.
Thank you, RF Ninjas, for your help. I'm a little bit geeked out by the prospect of homebrewing a high-gain antenna, and with your help filling in some rather crucial gaps in my knowledge, I hope to start a community-wide broadband sharing program. Line of sight within my community is very good, the area is compact, and sharing a T1 line is about the only way for us to get decent internet speed. I certainly can't afford 1500 bucks a month, and paying 60 bucks amonth for a mobile broadband card is (nearly) intolerable.
Using an estimation of the f/D for the parabolic reflector I'm using (an offset satellite dish) that I got from a very nifty web publication on antenna design (http://www.qsl.net/n1bwt/contents.html), I've come up with some preliminary calculations for the feed. (the nifty little calculator doesn't run on Mac, so I got some extra practice in algebra.)
Assuming an f/D ratio of 0.8, the diameter of the output aperture of the waveguide would be 1.88 wavelengths, using the approximation Dw=2.35(f/D), which translates into 231.05 mm for this project. The length of this section, calculated by some ninja trickery having to do with the TE11 and TM11 modes, turns out to be about 257.882612 mm. I just did the math without really understanding anything about the modes, other than that when they are aligned 90 degrees out of phase at the end of the aperture, they cancel out interference patterns along the edge and add to each other in the phase center, thus (theoretically) eliminating performance-robbing sidelobes from the finished feedhorn. If anyone would care to explain some basic theory on these 'modes', I'd appreciate it.
The critical flare section was calculated using the formula Angle=(44.6(w))/D, where w is the wavelength and D is the diameter in mm of the output aperture. I came up with 23.72 degrees of flare.
So, I've eliminated, on my own, the original seven questions, and have a couple of new ones.
1. I know that this is probably pathetically simple for you RF wizards, but I need to know how to calculate the optimum diameter and length of the input waveguide for a given frequency wavelength, such that it will not accidentally propogate additional frequency modes that will interfere with the operation of the feedhorn.
2. Another simple-simon question... How in the blazes do I connect the feedhorn to the coaxial cable once I've constructed it? Should I follow the instructions for a cantenna, and how should I adapt the measurements to fit this application? Throw me a bone here. Direct me to an accessible resource where I can find the info myself. Something. Please.
3. Since the goal when illuminating a reflector is to have ideal edge taper, would it be possible to create a system with near-ideal characteristics by adding a lens in front of any high-efficiency feed to shape the RF into a precise shape? I could consult an optometrist about lens design, they're pretty much experts at lenses, and from what I understand, lenses can shape RF as well as light. A lens oriented so that the convex side is facing the feed could be made, I think, to refine the signal received by both the parabolic reflector and the feed.
Thank you, RF Ninjas, for your help. I'm a little bit geeked out by the prospect of homebrewing a high-gain antenna, and with your help filling in some rather crucial gaps in my knowledge, I hope to start a community-wide broadband sharing program. Line of sight within my community is very good, the area is compact, and sharing a T1 line is about the only way for us to get decent internet speed. I certainly can't afford 1500 bucks a month, and paying 60 bucks amonth for a mobile broadband card is (nearly) intolerable.
