Imagine for a second that the 'platform' is a large parabolic dish and is being efficiently illuminated by your antenna.  Obviously the platform dimension (dish diameter) becomes the antenna aperture.

On the other hand, if the antenna is not illuminating the structure at all (directional antenna, aimed away), then the structure wouldn't matter a bit.

For low directivity antennas on complex structures (like aircraft), the antenna characteristics (gain, and I suppose aperture too) will be a complex function of direction (over the sphere).  Note - the aperature will not be a nice clean number; it will be a very complex pattern over the sphere.

There's some nice, but expensive, SW called ALDAS that can model the installed performance of low directivity antennas on structures, but I don't recall 'aperture' as such being part of the package.

http://www.maasdesign.co.uk/aldas.html

Thanks VB1BLL,

Your insight is correct.  The problem is with low directional antennas which illuminate part of the aircraft.  Since I am illuminating, say the wing tips, I believe that the wing tips sould be included in the aperture number.  

For example, an antenna having a stand-alone aperture of say 5 cm., could, in reality, have an apperture of 30 meters (the distance to the wing tip).

Your thoughts would be appreciated.

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In the example you give, you should remember that the wing tips will not be illuminated with very much energy (because they are fairly far away).  The amount of energy at the wing tip (available for reflection) is probably so low as to be of no consequence.  By way of example - imagine the distant mountains; do you include them as well ?

On the other hand, the entire wing surface presents a very large, mostly flat, surface area that is closer - it will have an effect in certain directions.  Remember that these reflections will depend on the angles and thus all antenna parameters (gain, aperture, etc.) will become complex functions of direction.  If you were to calculate aperature in every direction, then you would probably want to present your data in the form of a colourful Mercator projection of the sphere onto a flat graphic.

By the way, I have no idea where one would set the cut-off point in terms of deciding if a weak reflection were to be included or not.  Probably around -20dB, or perhaps based on combined phase (?).

This is an age old issue with antenna engineers.  If you ask 7 antenna engineers you get 4 different answers. This can be a very complex issue depending on the platform. If one looks at the equation for gain you'll find it contains a term for the antenna apperature.  This aperature term can be "effective", "scattering" or "physical".  See the book "Antennas" by John Kraus (second edition pg 29+).

Research "antenna effective height".  An antenna is a function of it's environment.  But how much effect that environment has is dependent on many factors. One major factor is the antenna type.  Is it ground plane dependent  or independent type of antenna?  What is the frequency? The material of the platform (dielectric, metal or both?).

Research antenna software scattering programs and you may stumble across information.