[NLRS] [BC'ers] Fwd: Re: Anybody have experience with multi-band feeds?

[Dr. Gerald N. Johnson]

Sometimes the dishes that are broader than they are high for use with 
multiple LNB for multiple satellites are more spherical than parabolic 
in the long dimension so to lose less gain when the feed is to the side 
of the proper focus and so don't give added gain to match the added wind 
load (dish area).

The Direct TV dish, 20" x 18" projects a circular aperture when tilted 
into proper position and has an F/D of about .7 which can be feed 
efficiently with a horn feed. I suspect the Vivaldi feed is broader and 
so the efficiency will be down. It will be interesting to see how the 
gains compare to theoretical which the horn feeds generally accomplish. 
Typically 32 dBi at 10 GHz with a horn designed by the W1GHZ on line 
antenna handbook or by converting the LNB into a feed.

There have been dual band horn feeds made for the Direct TV dish that 
work well, most often 5760 and 10 or 10 and 24 because at 3456 and 2304, 
the 18" dish doesn't outperform a loop yagi by enough to make it worth 
the effort to try.

Focus is another problem. Its quite likely the phase center of the 
Vivaldi moves towards the coax connection as you go higher in frequency 
and that makes it hard to optimize on all bands. There are more complex 
Vivaldi described as double ridged waveguide horns where there is a horn 
around the bit that Kent puts on a PC board and the discussion on Kraus' 
4edition Antenna text book (Indian version with added chapters on 
propagation) mentions that varying phase center. As a compromise since 
the effect on gain is more of a wavelength effect, its probably best to 
optimize the highest band (shortest wavelength and hence most critical 
position) and take what you get on the rest. As a guess I'd say the 
opening of the Vivaldi should be about an inch closer to the dish than 
the opening of the original feed horn on the LNB. It would be a benefit 
to make that position movable along the axis of the original feed horn 
to be optimized on the antenna range. It would be beneficial to know the 
optimum position for each band and how much gain is lost by various 
compromise positions. Then you can trade off what position is optimum 
compared to having separate feeds or antennas for each of the bands. 
There is something to be said for having fewer antennas even if the 
multi band antenna isn't absolutely optimum. There is interaction on the 
rover between closely spaced yagis too. My presentation at Aurora 
promises to discuss that from experiment and computer modeling for VHF 
bands. Loop yagis are hard to model correctly so the extent of their 
interaction is a lot harder to know.

73, Jerry, K0CQ

On 3/14/2011 7:44 PM, David Palm wrote:
>
>
> I have Aurora 2011 marked out on my calendar and am going to do my best to
> be there with KC9JTL and KF9OP.
>
> Now, I want to get this dish with a multi-band feed ready to try on the
> range.  I will probably have a dedicated dish for 10 GHz (I'll probably be
> asking more about modifying a dish for that later.)  This would be for
> 3456/5760 and, possibly but it seems a stretch, 2304.  I need some advice.
>
> I have the PCB Vivaldi antenna from Kent Britain for the multi-band feed.  I
> currently have two dishes.  Both are offset feed.  One is a normal DirecTV,
> 18" wide and about 20" tall.  The other is a Channel Master, 24" wide and
> 18" tall.
>
> Which, if either of these, would be better and why?  The CM obviously has a
> little more surface area.  If neither of these would be a good match, what
> would you recommend and where would I get it?  And any good ideas on
> mounting the PCB Vivaldi, so that it can be focused for optimum
> illumination?
>
> Thanks and 73,
>
> David  W9HQ
>
>