Question about dish's selectivity regarding reception of close satellites (1 Viewer)

Register Today to see less ads! It's Free!

polgyver

Creative Tinkerer
Pub Member / Supporter
Sep 21, 2010
466
321
Toronto
A few months ago I found interesting article by John Legon, mostly known as a mathematician researching Egyptian Pyramids.
John also is involved in satellite reception and devised some equations for finding parameters of unknown dishes.
I took a screenshot of part of his article.
What is puzzling for me is his statement that the left and right sides of a dish whose top and bottom were "truncated" for
making its shape more "modern" - that these sides enhance the dish's selectivity for receiving adjacent satellites.
I think that the only factor which determines the angle of a cone what the dish "sees" - is its focal length.
If the dish's focal length is relatively long, it will "see" just one bird. If its focal length is short, it will be vulnerable to adjacent satellites.
I am curious what other Members think about it?
thumb_IMG_0350_1024.jpg
 
Register Today to see less ads! It's Free!

a33

SatelliteGuys Pro
Feb 4, 2015
580
377
netherlands europe
The phenomenon is called "beamwidth", and indeed seems to be dependent on width of the dish, not focal length.

An equation for beamwidth is e.g. here: Parabolic antenna - Wikipedia
I've once read an understandable explanation of it, but I don't recall where....

BTW John's article (from 2012 or thereabouts) is excellent. But he wasn't the first to discover the equation for the focal length of offset paraboloid dishes using the deepest depth as input; the french were earlier, I discovered.

Greetz,
A33
 
  • Like
Reactions: polgyver
Register Today to see less ads! It's Free!

mikekohl

Prehistoric Satellite Guru
Supporting Founder
Jun 4, 2004
805
191
Montfort, Wisconsin
The phenomenon is called "beamwidth", and indeed seems to be dependent on width of the dish, not focal length.

An equation for beamwidth is e.g. here: Parabolic antenna - Wikipedia
I've once read an understandable explanation of it, but I don't recall where....

BTW John's article (from 2012 or thereabouts) is excellent. But he wasn't the first to discover the equation for the focal length of offset paraboloid dishes using the deepest depth as input; the french were earlier, I discovered.

Greetz,
A33
Here's an odd twist from the height of c-band terrestrial interference: study the placement of feedhorn scalar rings purposely slightly off the highest signal level, but also working in tandem with a perimeter screen mounted roughly perpendicular along the outer edge of the dish. Without an edge on the perimeter of the dish, interference from the side can enter the feedhorn or LNBF and sometimes completely wipe out desired signal. If you install a perimeter edge in this situation, it not only can block interfering signals from getting to sides of the focal point, but despite physically blocking a slightly larger outer surface area of the the antenna, you have reduced enough interference to result in a larger desired signal. Further playing with adjustable scalar rings and Chaparral GOLD RING add ons, or Deep Dish feeds such as those from ADL like their former RP-2 line (designed for 0.275 to 0.335 f/d ratios), can drop side interference further and result in more signal.
This is a 30 year old lesson that can be useful when encountering cell phone interference in the new C-band environment. In addition to, or instead of filtering, various screening methods can keep problem interference out of your dish's feed area. The lesson learned back then was that digital interference is sometimes impossible to filter out, and the better solution is to physically avoid it by careful placement and construction of dish sites. You might encounter situations where you wish to receive C-band frequencies on international satellites that may still be using frequencies that have been sold to cell phone companies in the USA. Such physical filtering could allow one to use "old" C-band feedhorns and LNBFs that may still cover down to 3400 MHz. A new feed or LNB/LNBF system has at least 500 MHz of spectrum above 3400 MHz filtered out, and it may not be capable of those lower frequencies, so it could be an alternate fix. Your mileage may vary.
 

cyberham

SatelliteGuys Pro
Jun 16, 2010
3,825
1,911
Nova Scotia
Your knowledge could be useful when I'm using my mini-BUD 1.2m dish with its wide beamwidth on C-band. Minimizing adjacent satellite interference is one way to improve reception and increase the number of receivable satellites.

Sent from my SM-G950W using Tapatalk
 
  • Like
Reactions: polgyver
Register Today to see less ads! It's Free!

Users who are viewing this thread

Users Who Are Viewing This Thread (Total: 0, Members: 0, Guests: 0)

Latest posts

Top