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*Subject*: Re: [amsat-bb] 2.4 Ghz Helix--stacking distance*From*: "Edward R. Cole" <al7eb@xxxxxxxxxxxxx>*Date*: Wed, 23 May 2001 23:30:30 -0800

Pieter, and the rest of the helix stackers: I was waiting to see who would get it correct. The best comparison is stacking of yagis. In both cases the physical size is smaller than the effective aperture. For best gain optimum stacking occurs approx. when the individual apertures just touch. Wide spacing will cause sidelobes to enlargen. Smaller helps the sidelobes but lowers gain. For a transmitting antenna the optimum is when the apertures just touch. For receiving, noise pickup from the ground is more critical, so sacrificing 0.5-1.0 dB of gain is acceptable to minimize the sidelobes. As Pieter says, If you know the gain of an individual antenna, then you can solve its aperture from this formula: G = 4*pi*Ae/WL^2... or Ae = G*(WL^2)/4*pi Of course Ae is the effective aperture and should be circular in shape for a helix [a yagi has an elliptical aperture] Thus Ae = pi*R^2, and the Tx stacking distance is equal to 2*R. My guess is for 2.4G downlink antennas you should try 1.6*R to 1.8*R. Now the big question is what is the gain of your helix? Good luck on that one ;-) Ed BTW did everyone see my reply on stacking gain? You will not get 3 dB for a 1x2 or 6 dB for a 2x2 array. Subtract .25 dB for every doubling [rule of thumb used by experienced eme'rs who stack yagis]: 1x2 add 2.75 dB, 2x2 add 5.5 dB. Another rule of thumb for yagis is that stacking distance is half the boom length. >From: "Pieter Ibelings" <elpieter@hotmail.com> >Jerry, > >If you look at the article "A Mode-L Helical Antenna Array for OSCAR-13" in >the 1993 page 23-35 and older handbooks, they have 4 helical antennas >stacked. They use long helix antennas with 28.5 turns. They calculate the >gain of each antenna to be 19.2 dBic and from that number arrive at a 2.59 >lambda spacing. It is understandable how lowering the number of turns will >force the antennas to move in closer. > >If you know the gain of an individual helix. Can this gain be used to >calculate the effective apperture and this in turn be used to calculate the >propper spacing? > >Pieter > >----- Original Message ----- >From: <K5OE@aol.com> >To: <w4mvb@juno.com> >Cc: <elpieter@hotmail.com>; <amsat-bb@amsat.org> >Sent: Wednesday, May 23, 2001 23:45 >Subject: Re: [amsat-bb] 2.4 Ghz Helix--stacking distance > > >> Jess, Pieter, >> >> > I've run the numbers I found in the the UHF/Microwave Experimemter's >> > Manual and they seem to incicate that the optimum stacking distance for >> > two 20 turn helicies is about a foot. Can someone tell me if that is >> > close to the correct value? It seems a little close to me considering >> > the boom length is about 2 feet. >> >> Exactly what do they say? That would be 2.5 wavelengths, double the >> "typical" stacking distance for a Yagi--typically 1.25 lambda. If capture >> area and beamwidth are the contributing elements, then a 16 dB antenna of >any >> type has a "similar" pattern (of course the sidelobes are worse in a >helix). >> The ARRL Handbook and a QST article from 1963 both specify 1.5 WL for >> stacking 2x2--or about 7.5". The famous AF9Y 2x2 array has spacing that >> scales to 10.5" at 2401 MHz. My limited Xerox copies of Kraus' work do >not >> address this issue. I hope somebody that has more authoritative >references >> chimes in and offers some input, as I already have one based on 1.5 WL in >the >> works (garage). >> >> > Yea, I know, I should buy a dish but I'm hardheaded! >> DItto! And I have both a 2' and a 3' dish... but I want this to work :-) >> 73, >> Jerry, K5OE >> >---- >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

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