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RE: RE: Feeding 1.2 meter offset dish for AO-40,



Hello Ken

First, sorry for the length of the post, and thanks for the bandwidth.
Hopefully what's here will help others too.

> I'm working on a similar offset dish. The helix is only 3 turns because
> of the illumination requirements for the dish. Four turns is only
> about 55 deg beam width and this isn't enough to illuminate 90%
> of the dish I'm using, which has a lower f/d ratio than the one
> you show in the pictures (.6?)

It seems that when calculating beamwidths for feeds for dishes (offset fed
or otherwise), it's the 10dB beamwidth that's important, not the 3dB
beamwidth. Matching the 10dB beamwidth to the _illumination angle_ of the
dish (the angle that the dish covers when seen from the feed's phase centre)
is what's important. Check out http://www.qsl.net/n1bwt/chap5.pdf for Paul
Wade's W1GHZ excellent discussion on offset fed dishes in particular. The
term f/D becomes confusing with offset fed dishes: there's the real f/D
(which unlike standard feed dishes bears little relation to the illumination
angle) and the effective f/D which is simply derived from the illumination
angle.

There's even a program to help you calculate the geometries and dimensions
of offset fed dishes. Use HDL_ANT http://www.w1ghz.cx/software/hdl_3b3.exe
and select the O option... beware: the program's 3dB calculation is for a
horn and is not necessarily applicable to a helix. Use the 10dB beamwidth
instead.

A discussion on beamwidth feeds occurs at
http://www.qsl.net/n1bwt/app-6a.pdf where it seems to suggest that one
should vere on the side of under-illumination rather than over-illumination.

The 10dB point comes from a trade-off between what you lose in over-spill
and ground noise from the over-spill, compared to under-utilising the dish
area.

In my limited experience of three different offset fed dishes with
home-built helix feeds, it's wise not to get hung up too much on perfection
on these issues - I'd suggest giving it a go with good educated guesses and
optimise it later. It's tough to be more than a couple of dB out based on
some good guesses. You can then optimise it experimentally.

Here's some beamwidth calc's I made on the G3RUH feed via EZNEC (feeds other
than G3RUH feeds may give different results)...

Turns   3dB beamwidth  10dB beamwidth
2.25       58 deg          113 deg
3.25       48 deg          86 deg
4.25       45 deg          80 deg
4.75       43 deg          79 deg
5.25       40 deg          77 deg
5.75       39 deg          74 deg

Match your illumination angle to the 10dB beamwidth above and away you go.

I doubt that there's an easily measurable difference between, say, a 4.25
turn helix and a 5.25 turn helix when used to feed a dish - the beamwidths
are within 3 degrees. But there is quite a difference between a 2.25 turn
and a 3.25.

If the dish you have came with a feed and feed arm, use that to take an
educated guess at where the focal point is. If you don't have the feed arm,
HDL_ANT will pretty accurately get it for you.

> How did you determine where to put the helix in relation to the
> focal point? You mentioned "phase center at .7 inches from the reflector"
is where the
> focal point would be placed. I haven't
> been able to find a reference to this subject anywhere and had
> planned to post a question on this BB about this.

This is another example where one shouldn't get too stressed out - I've seen
many suggestions from 0.7" to 1.5" from the reflector (see Ed Krome's S
Band - The Book, 2001 edition pp 118-119). It depends on the exact feed.
This gives you an idea of the point of phase center - in practice you can
optimise it experimentally later if you wish.

The point I'm trying to make here is that at 2.4Ghz, it's one of those
things in life where it's worth going ahead and having a go, and then
working out why it works so well later.

73 Howard G6LVB

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