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W4BSG
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Joined: 15 Jun 2009
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Location: Alabama

 Posted: Mon Jun 15, 2009 1:26 am    Post subject: Definition help Hello- I have a question, based on several articles I have reviewed today. I have seen "Effective Aperture" related to the expression: Ae=(lambda)^2/(4 Pi) But I thought it should be: Ae=Pi (lambda)^2 /4 (Both equations should have efficiency terms applied) I am questioning this because Kraus has the second, but Wikipedia and another web reference haves at least two places where the first is given. Have I lost my marbles? Thanks-Bill

Joined: 03 Jan 2007
Posts: 205

 Posted: Mon Jun 15, 2009 9:45 pm    Post subject: Effective Aperture I have only seen the effective aperture defined as: Ae = (lambda^2)*Gain/(4*pi) Note that the effective aperture is defined as the amount of power delivered to the load for a plane wave incident upon the antenna: Ae = [Power Delivered - watts]/[Incident Power Density from a plane Wave - watts/meter^2] See: http://www.antenna-theory.com/basics/aperture.php I'm not sure where Kraus's definition comes from. If he defines things differently it could very well show up as he has it....however, I believe the definition above is now standard.
W4BSG
Antenna-Theory.com Newbie

Joined: 15 Jun 2009
Posts: 3
Location: Alabama

 Posted: Tue Jun 16, 2009 3:13 pm    Post subject: Definition help I am still confused. I was of the opinion that Aperture had units of Area, but your words give the units as power. As I was using it, Power density * aperture = power. For a circular aperture of diameter=Lambda, the area would be: A=(Pi/4)*lambda^2 with units of M^2. Then power,W from a wave of power density with units W/M^2 would be as I had it before, with Pi in the numerator. Well, back to the refereces. Where do I find IEEE definitions? Bill

Joined: 03 Jan 2007
Posts: 205

 Posted: Tue Jun 16, 2009 5:09 pm    Post subject: units of area The ratio of power [in watts] to power density [in watts/meter^2] does give the units of area [meter^2]. This is how I was defining it. Remember that the physical area of an antenna (Ap) is not equal to the effective aperture (Ae) or how much power the antenna captures.
W4BSG
Antenna-Theory.com Newbie

Joined: 15 Jun 2009
Posts: 3
Location: Alabama

 Posted: Tue Jun 16, 2009 5:55 pm    Post subject: I realize that physical and effective areas are different, but they ARE related. Over a fair range of sizes, the relationship is scalable. However, in most circular functions, Pi has a fixed function, and the expressions I am puzzled by are not familiar. What is the path of derivation in this area that carries Pi from the numerator to the denominator? Bill- Puzzled.
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