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Re: Helix feed for dish



Doug,

Ideally the feed for a dish would be constant gain over the total angle
that the dish needs illumination an then drop to zero gain [an analog would
be a band width filter with constant gain and vertical skirts...nice but
hard to achieve].  In practise this is not achievable.  The eggbeater
example is essentially a "loop dipole" over a reflector.  By varying
spacing from the ground plane you can achieve wider beamwidth, but it does
not satisfy the steep gain roll-off criteria needed at the edge of the
dish.   

One approach is using a microwave lense on a microwave horn to modify
illumination taper.  On large dishes a cassegrain feed using a shaped
subreflector can be used to correct illumination, but this comes with
blockage of the dish by the subreflector which needs to be many wavelength
in diameter [thus is used only on large dishes typically >60 foot at 2.4
GHz].  Most succesful feed designs for microwave are using some kind of
interference choke ring at the edge of the feed, or playing with complex
phase relationships.  BTW this is where offset feed dishes have some
advantages.  The feed does not block the dish since it is not in the beam
of the dish, so feed size and design geometery is more flexible.  Paul Wade
has shown dish efficiencies approaching 80%, theoretically [and has measure
70% at 10 GHZ with the RCA DSS 18 inch dish, I believe].  Center feed
dishes can only approach 60%.  This directly affects the effective dish
gain [i.e. 80 vs 60, how many DB improvement is that? ...hint 10*Log (80/60)].

At amateur dish sizes our choices are somewhat limited, but that does not
mean that the challenge is diminished in the design of dish feeds...in fact
it is exactly the opposite.  This is where the main focus {no pun intended}
in ham radio dish design lies.

Ed
So Doug it is a good question...keep asking.  

At 10:36 AM 4/8/2001 -0400, Douglas Braun & Nadia Papakonstantinou wrote:
>This talk started me wondering:
>
>Is it possible to make a directional antenna that has
>a better "shape" factor than usual?  Ideally a
>dish feed antenna would have a relatively constant gain
>over a relatively large angle, and then the gain would
>quickly drop once this angle was exceeded.
>
>I have seen plots for some eggbeater style antennas that
>had two gain peaks at about +- 45 degrees, and then a
>partial null (maybe -6 dB) straight up.  This
>is the sort of thing that I had in mind.
>
>Doug
>NA1DB
> 

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