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Re: 2.4GHz S band helix feeds, et al.


Yes, you need roughly a reflector of diameter = 0.6 wavelength behind the
helix to direct radiation toward the dish and not pickup ground noise with
the back lobe that results with out a reflector.

If you are using a f/d = 0.3 to f/d = 0.4 center-fed dish, the helix needs
to be quite short (about 3-1/4 turns) to properly illuminate the dish
(about 6-dB gain).  If you are using an offset feed dish, then the helix
needs to be about 6-1/4 turns as the effective f/d = 0.7 (about 10-dB gain).  

The WiMo helix may be too long for either use.  It probably was intended
for long-range WiMo use and runs too much gain to be used as a dish feed
(gain is probably 14-18 dB).  Too much feed gain results in the feed only
illuminating the center of the dish, effectively reducing the overall gain
of the dish.  Dish gain (dB) = 10Log [12.57*(diam/wavelength)^2] . The gain
of the feed does not add to the gain of the dish; it only functions to fill
the surface of the dish with RF (light it or illuminate it).

The proper position for a helix feed is to put the first turn ahead of the
reflector at the focal point.  I recall that others found that the AIDC
feed position did not work when they subsituted a helix feed.  I have never
worked with a BBQ dish so cannot comment more.  For an offset-feed dish
such as the Direct-TV dish place your helix first turn right ahead of the
clamp for the satellite feed.

I hope that helps.

73's Ed - AL7EB

At 04:25 PM 3/16/2005 -0500, ka8syx2003@aol.com wrote:
>Hi, Everyone,
>Does a helix feed for S band require a reflector?  The feed I ordered a
while back from WiMo doesn't seem to be available any longer, so I cannot
tell you what the model number is.  It is a plastic-sealed unit, about 5-6
inches long and has a diameter of about 3".  Is that a sufficient diameter
to suggest that it does have a reflector at the base, just forward of the
N-type female jack?
>I have integrated this feed with my AIDC 3731 S band downconverter,
purchased around the same time as the feed from Bob, K5GNA, and am thinking
about how best to mount it to my DirecTV dish.  The IF rig and signal
source I'm using to determine the focus and best orientation for the feed
WRT the dish seem to work best together when the rig is listening in FM
mode -- that seems to provide the best indication of the sweet spot when I
wave the connected feed and DC in front of the dish, in the vicinity of
where the DirecTV LNA/LNB would mount.  But the location of maximum signal
pickup isn't quite in the same location as where I estimated the helix of
the TV LNB/LNA would be.  Are there reasons for this?
>I also have some large BBQ type dishes, one of which I was thinking about
covering with aluminum screen and using instead if the DirecTV dish doesn't
cut it.  Ultimately, I'm hoping this will all work on P3E S band.
>Any advice would be great.  Am I missing anything here?
>73, Ray KA8SYX
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