BATC Forum

Hi

About 20 years ago, I bolted 3 lengths of aluminium tube (flattened and drilled through at each end) as a sort of tripod to reach the focus of this 60cm dish. I use a flat plate at the focus to mount the 10 GHz horn as seen here, but I have another plate with a 2.4 GHz helical feed for the ISS HamTV. You could use a similar arrangement for your log periodic.

It was a rush job that I do not think would last, but it is still in one piece!

The semi-rigid is connected to a 5.6 GHz dipole at the feed point. Unbelievably it received G8GTZ at 80km.

Dave

There is also the "Penny Feed" which on 6cms would need a larger disc, and the Shepherds Crook. Both of these can be used with PCB antennas. Both suffer from feed blockage, which matters more with small dishes. A better way is to use an offset dish. Mount it 90 degrees to normal to allow it to point at the horizon. In all cases, pay attention to the F/D of the dish and the illumination efficiency. Best resource is W1GHZ online antenna book http://www.w1ghz.org/antbook/contents.htm

Mike

Hi, all

I'm posting this here, at Noel's suggestion as it's vaguely related (I did start another thread, but I'll keep an eye on both places for replies).

***
Here's a general question about dishes for the consideration of the collective wisdom …

Apart from the obvious 'advantage' of being able to feed it from directly behind the dish, what are the advantages of cassegrain dishes? Any disadvantages?

Also, I'm getting conflicting info from Internet searches about what shape the main dish is - is it the same shape as a normal dish (ie parabolic)?
What I'm really asking, I suppose, is that if I have a cassegrain dish, can I remove the sub reflector and use it as a PF dish?
***

John

Shepherds crook sounds like something I had considered where Semi Rigid would come through the dish at some point and have a U bend just behind the Antenna.

Presumably it can enter from any angle but possibly not directly from the centre since it would then be in the line of radiation from the antenna

And I had forgotten all about "Penny feeds", I'll look at the website you linked at lunchtime.. (I should be working)

Thanks Guys?

G8PEF wrote: ↑

Wed Jan 31, 2018 9:39 am

Hi, all

I'm posting this here, at Noel's suggestion as it's vaguely related (I did start another thread, but I'll keep an eye on both places for replies).

***
Here's a general question about dishes for the consideration of the collective wisdom …

Apart from the obvious 'advantage' of being able to feed it from directly behind the dish, what are the advantages of cassegrain dishes? Any disadvantages?

Also, I'm getting conflicting info from Internet searches about what shape the main dish is - is it the same shape as a normal dish (ie parabolic)?
What I'm really asking, I suppose, is that if I have a cassegrain dish, can I remove the sub reflector and use it as a PF dish?
***

John

Quite a few advantages to the cassegrain, mostly not important for amateur radio. Large antennas, radio astronomy, extended F/D. Generally, though not always the reflector will be parabolic. It doesn't have to be but usually is. This means you can remove the plate and put an appropriate feed at the focus, which will not be where the sub-reflector was but some distance beyond. Are you sure it is not a Gregorian, not that that matters overly much.

https://en.wikipedia.org/wiki/Cassegrain_antenna

You can also use a lens horn, or a horn antenna. Horn antennas are very easy to make, very good for short ranges but get a bit impractical at higher apertures. It's a good choice as a feed for a larger dish. Much better than a PCB antenna.

Mike

Thanks, Mike.
I had made a guess that the biggest 'advantage' of dishes with sub reflectors was to give a bugger f/D ratio - but I'm not sure /why/ that's an advantage …

Re: cassegrain vs gregorian - how would I tell?
(EDIT: I've now read the link you posted and see that the difference is in whether the sub reflector is convex or concave. Mine is convex, so definitely cassegrain.)

I should add that the dish I have is about 900mm-1000mm diameter and originally had a 11GHz feed/LNB With a ferrite polarisation rotator fitted. To complicate matters, the subreflector is nutated.

Hi Wolfie

You'll certainly do better with a log periodic at the feed than with a dipole. They are very broad, and you would lose over half the signal from a dipole as direct radiation from the front. My experience here is that it is better to get on and try something that might appear sub-optimal than to prevaricate in pursuit of perfection. (Sorry that's a bit philosophical, but it seems to work!!)

Dave