Scalar Rings

[rebecca22]

Hello. I needed a feed horn from a supplier identical to one I already have that I know works. I gave them the picture of the feed horn (see below pink feed horn - four scalar rings) and they said they had the same one. The one they supplied (see below white feedhorn) has TEN scalar rings and the throat opening is about 30% larger. Now I don't know if it will work properly. Is it meant for a much larger dish do you think?
Rebecca

[gordon_s1942]

I think you can get 50 opinions regarding which will do what but the proof of the pudding as they say will be installing it and trying it out.
Obviously you already have the pink one removed so now is the time to set up the new one the same as the old one was.
Dont be surprised if the new one may need adjusting to suit the dish for the best reception.

[rebecca22]

Thanks. Really trying to understand the theory behind it if anyone can help
ta
'bec

[tristen]

A feed is usually designed to match the dish on which it is to be installed, hence dishes used in commercial microwave installations and satellite downlink and uplink sites, will come complete with their matching feed.

Dishes and feeds used in the typical home satellite TV installation are somewhat of a compromise and their comparatively low price reflects that.

The feed horns shown in your pictures appear to have conical scalar rings indicating their intended use on dishes having an high f/D ratio, typically in the range .5 to .7, whereas the common C-band centre-fed dishes in use for satellite TV reception are in the range .28 to .4 (f/D ratio).

Offset dishes commonly have a high f/D value.

I am inclined to agree with you that the recently-purchased feed is intended for use on a larger dish than the original feed horn. As it will still need to be mounted at the focal point of your existing dish, I expect that noise will increase, due to over-illumination of the dish.

Whether this will be acceptable to you can only be found by installing it, but exercise care when doing so to ensure that scratches, bolt marks etc., don't damage it and thus prevent you returning it for a full refund.

However, that said, as we have no other information except two photographs, can you post the specifications for each of the feeds? Specs for the new feed MUST be readily available.

Where did you purchase the new one?

You also ask whether we think the new feed is "meant for a much larger dish". What size dish are currently using (with the old feed)?

Finally, there is some useful information concerning feeds and scalar rings, which you might find enlightening, available at the following websites:

[tristen]

Ahah! The penny has dropped....at last.

When I first saw the feed shown in picture 2 (post#1), I thought that it looked familiar.

I believe that I was right. It looks very much like the feed from a Pacific Telecoms (VSAT) terminal, used for two-way communication via satellite.

I've reproduced the relevant section (below) as shown in one of their brochures.



It's designed for both transmit and receive purposes.