Both C & Ku / Tuning LNB - F/D F/L

[Hermitman]

I'll throw my two cents in this technical discussion. While a parabola has a true single focal point, a satellite receiving dish has more of a focal cloud. The size of the focal cloud depends on the received frequency (lower frequency-larger cloud, higher frequency-smaller cloud). On c-band, the focal cloud is about 2 1/4 inches, the size of a c-band feedhorn throat. On ku-band, the focal cloud is about 3/4 inch, the size of a ku-band feedhorn throat. The opening of the feedhorn is positioned so the focal cloud is slightly inside the feedhorn's throat, 1/4 inch for c-band and 1/8 inch for ku-band - the difference because of the focal cloud size. Once the focal cloud is in the feedhorn's throat, the feedhorn becomes a waveguide, bringing the signal to the antenna down inside the throat. From there its amplified, downconverted and sent to the receiver. To get the best signal, you want to try and get as much of the focal cloud at the correct point in the feedhorn's throat. And any imperfection in the dish's parabolic shape (dents, damage, etc.) don't allow the received signals from the satellite to become part of the focal cloud and never make it into the feedhorn. Have fun and good luck.

 

[caddata]

"....Once the focal cloud is in the feedhorn's throat, the feedhorn becomes a waveguide, bringing the signal to the antenna down inside the throat...."

The waveguide must be designed to be a resonate cavity at the C Band frequency. It's my contention now, and has been since the dual band BSC621-2 style feeds were introduced, that the C Band cavity is not the correct size and operates with reduced efficiency, as compared to the Corotor II. If you'll measure them, you'll note the differences. Remember that .001" makes a difference.


"....any imperfection in the dish's parabolic shape (dents, damage, etc.) don't allow the received signals from the satellite to become part of the focal cloud...."

The imperfections you spoke of, cause the microwave energy reflected by those imperfect areas to arrive at the focal point out of phase with the other "perfectly timed" energy. Those "out of phase" signals seek out and cancel a similar "perfectly timed" wave. Reducing the efficiency of the reflecting surface.

 

[B.J.]

"....Once the focal cloud is in the feedhorn's throat, the feedhorn becomes a waveguide, bringing the signal to the antenna down inside the throat...."

The waveguide must be designed to be a resonate cavity at the C Band frequency. It's my contention now, and has been since the dual band BSC621-2 style feeds were introduced, that the C Band cavity is not the correct size and operates with reduced efficiency, as compared to the Corotor II. If you'll measure them, you'll note the differences. Remember that .001" makes a difference.

Are you sure about this? I really don't think that you want a feed's waveguide to function as a cavity. If it did, it would have a rather narrow frequency range. Ie there is no "C band frequency", it is a band at least 500 MHz wide. I think that the dimensions of the feed would have more effect on the bandwidth and the beamwidth. The feed needs to be big enough for the desired freq band, but other than that, I don't think .1" would make much difference, let alone .001". I think the position of the probe relative to the back of the waveguide is more important than the length/diameter of the waveguide. I've seen numbers suggesting ~ 1/4 wavelength spacing is appropriate, but the length/diameter dimensions of the waveguides are not nearly so critical from what I've read.

"....any imperfection in the dish's parabolic shape (dents, damage, etc.) don't allow the received signals from the satellite to become part of the focal cloud...."

The imperfections you spoke of, cause the microwave energy reflected by those imperfect areas to arrive at the focal point out of phase with the other "perfectly timed" energy. Those "out of phase" signals seek out and cancel a similar "perfectly timed" wave. Reducing the efficiency of the reflecting surface.

While the out of phase thing is certainly one factor, I tend to think the more important factor is that the reflected signal just doesn't hit the feedhorn at all. I am thinking that, because the out of phase thing would seem to have to be due to the imperfect surface being closer or further from the feedhorn, but still at the proper angle to reflect to it, but your typical dent, would have a surface that isn't parallel to the theoretical surface, and wouldn't reflect the energy into the waveguide.

But both of my comments above are just intuition, and not based on any direct knowledge or experience. Just my opinion.

 

[caddata]

....don't think that you want a feed's waveguide to function as a cavity....

If there's an antenna probe in it, it's a cavity and it should be designed to be resonate at mid-band. The resonate cavity functions like a bandpass filter at RF frequencies. The closest thing we use to a waveguide is the 90 degree ell on some of the Corotor II feeds.

....there is no "C band frequency"....

The key word is "band". Used to include 3.7 to 4.2 Ghz now it includes 3.4 to 4.7 Ghz.

....I tend to think (because of dents and dings)....the reflected signal just doesn't hit the feedhorn at all....

Yes, they cause some energy to be reflected off into space. They (dents and dings) make the focal point larger and irregular in shape and portions outside the throat of the feedhorn can't be processed by the receiver. Any deviation from a perfect parabolic surface causes some energy to travel a longer distance to the focal point----arriving out of phase. Dents, dings, large holes, and less than a true parabolic surface make a dish function like one of a much smaller size.

Harold