Case History Study (dish page)
Using Az/El Charts, Eirp Maps and SMW Link Budget
For Satellite Spacenet 4 as Seen From Saltillo, Coahuila, Mexico
(to Western Hemisphere,Eastern Hemisphere,Footprints by Dish Size)

When I installed my satellite antenna here in Saltillo, I had questions about the quality of reception I was receiving. So I went to the Internet in search of satellite footprints calibrated in dish diameter to compare to my system; I thought that would be a lot easier than doing a link budget calculation and, besides, surfing the 'net' is fun. Below is one of the footprints I found, along with the actual statement, by the company at that website, about the appropriate dish size to receive their satellite delivered services:

North America - Satellite Footprint - Spacenet 4: "The CQG satellite footprint covers almost all of North America and half of Central America. A 24"- 30" antenna should be used in the lightest shaded areas, while a 48" or larger antenna should be used in the medium shaded areas. CQG's satellite will not cover the darkest shaded areas on the map."

I had curiosity about the dish size claim, even though they were transmitting data instead of video, which requires much more bandwidth, their recommended antenna size seemed way too small. So I contacted them for further information. The following is the ensueing correspondence:

From: Becky McGraw <becky@cqg.com> 1/07/98 14:00 Subject: Re: Request for technical information (fwd) To: intracom@interclan.net > > Can you tell me what satellite you are using on the North American photo > on your web page and do you have a better footprint with EIRP values on > it? Thank you. Regards, Ricardo Roberts, Saltillo, Mexico> Dear Ricardo, The satellite that we use is Spacenet IV. With your location, you would be able to use our 30" satellite dish. You are in an area that is within the 32.0 dbW EIRP. The approximate coordinates for your area are: Magnetic Azimuth 173.9 and polarity is 61.3. If you have more questions, please let me know. We are working on being able to do business in Mexico. Please advise me of your needs for CQG and I will be happy to assist you. Best, Becky McGraw ------------------------------------------------------------------ > Becky McGraw > National Marketing Representative > CQG Inc. 201 Centennial St., Glenwood Springs, CO 81601 > Direct Line: 888-645-8088 Phone2: 800-525-7082 Fax: 970-945-4451 > email: becky@cqg.com

With this information, the first thing I did was to determine if they were using a 'C' or 'Ku' transponder. I already knew, using a spectrum analyzer, there was negligible, if any, Ku activity on this satellite and I was receiving excellent Ku on both sides of this satellite, but I thought I had better check. This was easy to determine by going to the Spacenet 4 EIRP footprint charts and checking which chart had contours for Saltillo, Coahuila, Mexico, ('x' marks the spot) at 32dB EIRP.

Next, I went straight to an azimuth/elevation calculation table to check my 'numbers', once again, to see if I had the correct angles for Spacenet 4 for my latitude and longitude - here are the results from the antenna alignment screen using the SMW link budget program. The elevation is 60.47° and the azimuth, from true north, is 180° (due south). Next I applied the magnetic deviation correction for my location, which is approximately 7.5° subtracted from the true value, giving me an azimuth of 173.5°. Just what I had thought from earlier analyses and these were values close enough to that quoted to me in the e-mail; however in the e-mail (see above) the elevation was given to me as a polarity number!! Seeing the polarity error in the e-mail, combined with my natural curiosity about the size dish required for data in comparison to video, it encouraged me to continue my efforts. The next step was to determine sizing information, so I input the az/el data into the link budget equation along with information about a dish system that I would consider to be excellent quality, using the size dish they recommend, and here are the results from the link budget program, note it is for video reception and not for data - my goodness, there are a lot of sparkles in it - now that is for video and not for data. Then I input the actual values of my satellite system into the link budget and here are the results. So what are my conclusions? 1)Based on the assumption the company offering the service must know what they are doing, it takes a smaller diameter satellite dish to receive data than video. 2)My installation was 'OK' for the top of the arc (remember, the look angle for Spacenet 4, for my location, is due south). 3)I could have saved a lot of time had I not been lazy and used the link budget software from the beginning!!! 4)Download the SMW link budget program and use it!!! (top of page)