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Re: Helix feed for dish



Actually I'll be using a pitch angle of 12.75 degrees or .1925 wavelengths.
So using the Kraus formula, I get 5.5 turns for a 90 degree, -10 dB beam
width.  Since I had already built the 16 turn helix on K5OE's web site, I
used the left over materials to build the feed for the dish.  The feed is
done but the mounting isn't working out quite like I had planned.  I guess
it will have to wait until I get back from LA Wednesday to continue.

BTW with the 16 turn helix I'm hearing AO40's beacon quite well.

Lee-KU4OS

----- Original Message -----
From: "Edward R. Cole" <al7eb@ptialaska.net>
To: <amsat-bb@AMSAT.Org>
Sent: Sunday, April 08, 2001 5:43 AM
Subject: Re: [amsat-bb] Helix feed for dish


> Lee and Joe,
>
> I'll just make a couple replies to you both here on the -bb.  My guess is
> there are some "arm-chair" readers out there following the topic, also.
My
> replies inserted with ***before them ;-)
>
> >From: Joseph Murray <k0vty@juno.com>
> >
> >Hi Ed
> >
> >>From Kraus:
> >The number of turns required for the helix feed antenna depends on beam
> >width desired.
>
> ***Yes, this is obvious; I suppose this comment was for general
distribution.
>
> >For the pattern to be 10 dB down at the edge of the parabolic dish
> >reflector ,  the
> >required number of turns is approximetly given by:
> >
> >     n ~ 8400 / ( BW ( deg ) required, squared ) times turns spacing in
> >wavelength
>
> ***That is new to me...guess I need to buy a copy of "Antennas".
>
> >Thus    8400 / ( 0.21 ( 14 degree pitch ))   times ( 115 (deg  required
> >BW)  ^ 2)
> >
> >8400/ (  0.21 * 13225 )  = 3 turns
>
> ***The solution for an angle of 115 degrees.
> ***So if I want 71 degrees:  8400/{0.21*(71)^2} = 7.9 turns, or
> ***For Lee: 8400/{0.21*(90)^2} = 4.9 turns
>
> ***Now lets compare these results with those from the
> ***W1GHZ (ex N1BWT) formula that Lee found.
> ***For a -10 dB illumination angle of 115 degrees:
> ***HPBW(feed) = A*SQRT(3/taper) =115*SQRT(3/10) = 62.9 degrees
> ***For a -10 dB illumination angle of 90 degrees:
> ***HPBW(feed) = A*SQRT(3/taper) =90*SQRT(3/10) = 49.3 degrees
> ***For a -10 dB illumination angle of 71 degrees:
> ***HPBW(feed) = A*SQRT(3/taper) =71*SQRT(3/10) = 38.8 degrees
> ***Now to arrive at the number of turns, you would need to
> ***convert HPBW into gain and solve the reverse formula for finding
> ***gain from number of turns.  Hopefully you would arrive with Kraus'
> ***formula provided by Joe.
>
> ***In fact the 5-turn helix I modeled has a HPBW = 59 deg. and
> ***-10 dB BW of 99 deg.  Falling somewhere between the two formula.
> ***This is the dilemma of all antenna development, coming up
> ***with formula that accurately model reality.
>
> >Remember I told you about the pitch spacing adjustment to fit -10 dB down
> >at the edge of the dish.
>
> ***This presupposes either you can measure the antenna pattern or
> ***have software to analyze effects of changing the pitch angle.
> ***In a practical sense though, the helix pitch could be adjusted for max
> ***signal into the dish while observing a stable far-field signal source
or
> using sun noise.
>
> >Most formula for helix seem always are for max gain.   In the dish feed
> >gain is not the driver .   BW is .
>
> ***Yes, I am aware of this.  I only stated dish gain,
> ***since this is of interest for path-link analysis.
> ***Feed gain is not of particular interest except as
> ***it pertains to illumination angle [i.e. feed directivity].
>
> >This is the Kraus method.
>
> ***Good info, thanks!
>
> >I have not dealt with offset feeds as you,  so I don't know if offset and
> >non offset respond the same.
> >Another thing  ,  The above  is good for transmit in non offset dish
> >feeds but have no idea about transmit on offset dishes.     The need to
> >have zero phase difference across the aperture in transmit makes things
> >different.
> >
> >Enjoy
> >
> >Joe  k0VTY
>
> ***PS: I'm not the expert on offset antennas.  Actually in ham radio
> ***circles Paul Wade, W1GHZ, is.  Lee gave the link to
> ***Paul's on-line antenna primer where he discusses offset dishes
> ***as well as many other microwave antennas.
> ***Paul is widely published on antenna subjects:
> ***QST, QEX, Microwave Update Proceedings for example.
>
> Ed
>
>
>
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>

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