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*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 ***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 ---- Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA. To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org

**Follow-Ups**:**Re: Helix feed for dish***From:*Douglas Braun & Nadia Papakonstantinou

**Re: Helix feed for dish***From:*Lee

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