Page Index Toggle Pages: 1 Send Topic | Print
Normal Topic Feed Horn calculation (Read 643 times)
Semrc
New Member
*
Offline


Posts: 1
Joined: 20th Nov, 2017
Feed Horn calculation
20th Mar, 2018 at 3:32pm
Print Post  
hello everybody

i need to use a 110 antenna , can i use it with a feed horn designed for 2.4 m antenna ?

if it's not , how the new feed horn should be calculated ,
  
Back to top
 
IP Logged
 
Admin1
Forum Administrator
*****
Offline

Welcome to the new forum

Posts: 103
Location: UK
Joined: 2nd Jun, 2017
Re: Feed Horn calculation
Reply #1 - 20th Mar, 2018 at 6:47pm
Print Post  
The feed diameter affects the beamwidth of the beam from the feed towards the dish.

The beam-width of the beam from the feed must correspond to the angular diameter of the dish, as seen from the feed.

F = focal length of the dish, either the distance from the feed to the centre of the dish in the case of a true circular axi-symmetric dish or, typically, the distance from the feed to the lower edge of the dish in the case of an offset dish (slightly non-circular).

D = diameter of the dish.

A = Angular diameter of the dish, as seen from the feed.

If the F/D ratio of the dish is small, e.g. 0.25 the feed will be small in diameter since from its position the dish looks very wide and the feed needs to project a wide low gain beam towards the dish.

If the F/D ratio of the dish is large, e.g. 0.8 the feed needs to be larger diameter since from its position the dish looks narrower and the feed needs to project a narrow higher gain beam towards the dish.

Offset VSAT dishes are about F/D=0.6 to F/D=0.8   0.8 gives better cross-pol performance but the longer feed support arms means higher stress due to weight of the ORU/LNB/Feed assembly.

Edge taper is the difference in power level at the dish centre compared with the edge.  The general idea is that the edge taper should be betweewn -10dB and -16dB at the edge of the dish.  -10dB gives the best gain but suffers higher adjacent satellite interference. -16dB gives fractionally lower main beam gain but lower sidelobes so lower adjacent satellite interference and is a good compromise for VSAT dishes. For a transmit dish you must use an approved feed+dish combination, otherwise you may cause interference to other satellite services.



This picture illustrates 2 offset dishes, each with the same F/D ratio (approx 0.8). Angle A is the same in both cases and the same size feed is appropriate.  It does not matter what is the dish diameter. It is angle A that counts for feed diameter.
  

Admin1
Back to top
WWW  
IP Logged
 
Page Index Toggle Pages: 1
Send Topic | Print
Bookmarks:    Facebook    Google    Google+    Linked in    reddit    StumbleUpon    Twitter    Yahoo