# Re: Helix feed for dish

• Subject: Re: [amsat-bb] Helix feed for dish
• From: "Edward R. Cole" <al7eb@xxxxxxxxxxxxx>
• Date: Sun, 08 Apr 2001 01:43:57 -0800

```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
***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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