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*Subject*: Re: [amsat-bb] Helix feed for dish*From*: "Lee" <lee-fl@xxxxxxxxxx>*Date*: Sun, 8 Apr 2001 16:31:16 -0400

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

**References**:**Re: Helix feed for dish***From:*Edward R. Cole

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