C-band vs Ku-band signal levels

[ayelvington]

I just swapped out the classic LNBF installation on our dish at work for the integrated BSC621C C/Ku band dohicky. The point was to get away from the moving parts and use voltage switching for polarity.

I used an Agilent spectrum analyzer to help out with the phase angle and focal point, but ran into an issue.

It wasn't difficult to set the rotation and depth of the LNBF using the Ku band side because the signal was HUGE and the sweet spots were obvious. Unfortunately, the C-band side was significantly smaller and I'm not sure why. I tried depth settings, but there is only so far you can move that doer without having to take the entire support mechanism apart.

So, here're the questions: Are C-band transponders typically at lower power levels? Is there a magic process of measurements that I can take on the dish to figure out where the C-band sweet spot should be?

The plan is to have a digital receiver as the primary receiver, and connect an analog receiver to the IF output of the digital receiver for those odd moments. This configuration has worked in the past.

Many thanks in advance,

A

[rainman]

Originally Posted by ayelvington

I just swapped out the classic LNBF installation on our dish at work for the integrated BSC621C C/Ku band dohicky. The point was to get away from the moving parts and use voltage switching for polarity.

I used an Agilent spectrum analyzer to help out with the phase angle and focal point, but ran into an issue.

It wasn't difficult to set the rotation and depth of the LNBF using the Ku band side because the signal was HUGE and the sweet spots were obvious. Unfortunately, the C-band side was significantly smaller and I'm not sure why. I tried depth settings, but there is only so far you can move that doer without having to take the entire support mechanism apart.

So, here're the questions: Are C-band transponders typically at lower power levels? Is there a magic process of measurements that I can take on the dish to figure out where the C-band sweet spot should be?

The plan is to have a digital receiver as the primary receiver, and connect an analog receiver to the IF output of the digital receiver for those odd moments. This configuration has worked in the past.

Many thanks in advance,

A

what size of dish are you using. it's hard to receive c band on anything smaller than 6 foot.

[ayelvington]

12' SAMI that has been hosting C-band for about 10 years.

[pmb1010]

all I can add here is since Ku seems to be more fussy, peak for that and let C band fall where it may signal wise in your setup.

Since the BSC621 cannot adjust C and Ku independently, you're kinda stuck with the above.

It's hard to say if Ku is "stronger" than C band, as the different sats & signals are all over the place strength wise.

[ayelvington]

Many thanks. That's all the validation that I need to leave the dish alone until spring. The weather here in DC just shifted and I have no desire to stand on a tall ladder in the rain futzing with the doohicky. Take care, Al

[wejones]

I agree, that for adjusting the focal length, that I would start out at least with C-band. With C-band, the center lobe is more gradual, but there always *IS* a center . With Ku, if your focal length is off a bit (and it doesn't take much), instead of getting a center peak with some side lobes, the center lobe can dissappear, and all you are left with is the side-lobes. Ie a donut shaped pattern. Because of this, you can often be fooled, and focus on the side lobe rather than the center lobe. When you do this, you can get big signals on Ku, and yet be significantly off in the aim on C-band. I have done this several times, and it gets confusing. Since C-band generally gives you only the center lobe, I'd peak on C-band first, then see if you can touch up the adjustment with Ku if necessary. C-band and Ku *SHOULD* focus at the same place. If they don't, I start to suspect focusing on Ku sidelobes.
Adjusting the focal length is the hardest part of aligning a dish, as far as I'm concerned, particularly with dishes with quad arm feed supports. I've tried many times to peak the FL, but usually end up just using an tape measure and using theoretical measurements.

[ayelvington]

Can you share a quick link to a formula for those theoretical measurements? I've forgotten this part longer than I knew it. The dish does have quad arms and we have to rent a lift or special ladder to reach the LNBF assembly. ay

[rainman]

this site works for me.
Footprints by Dish Size - Adjusting the Polar Mount for Prime Focus Antenna - C/Ku-Band Satellite Systems - Tuning, Tracking, Azimuth, Elevation, Declination Angles, F/D Ratio, Focal Distance, Inclinometer, LNB/Feedhorn Assembly, Actuator Assembly, C

[wejones]

Originally Posted by ayelvington

Can you share a quick link to a formula for those theoretical measurements? I've forgotten this part longer than I knew it. The dish does have quad arms and we have to rent a lift or special ladder to reach the LNBF assembly. ay

I always forget too,

but I think it's Focal length = (dish diameter)^2/(16 * (dish depth) )

or just use one of the computer programs or on line calculator, such as:

BJDISCALC2

[ayelvington]

Perfect! Great link too. I'll take one more shot at it if we get a weather window after Thanksgiving. al