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Receive Aperture Efficiency in Nonuniform Field

 
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tripelo
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Location: Dallas, TX

PostPosted: Sat Aug 21, 2010 1:24 am    Post subject: Receive Aperture Efficiency in Nonuniform Field Reply with quote

Looking for leads related to the effectiveness of receive antenna apertures in nonuniform fields.

Nonuniform field - meaning that received signal field strength has variation within the receive antenna aperture.

Many types of antennas synthesize an aperture using phased arrays of elements. In a nonuniform field, all elements excited by the field re-radiate, but seems the elements in a weak part of the field may re-radiate proportionately more energy than they intercept from the field (due to element coupling methods), thus reducing aperture efficiency. At close to microwave frequencies, or above, this effect might be somewhat mitigated by various means of coupling of the array elements; for example, the use of isolators, etc. But isolators may not be practical to implement at lower frequencies.

Seems that antennas achieving aperture size (gain) primarily via passive means (such as reflectors) might fare better in nonuniform fields. In this case with a reflective derived aperture, there may be less array elements to re-radiate energy (conceivably could only be a single element).

If apertures formed via reflective means are more efficient in receiving energy in nonuniform fields, then what about apertures formed passively via directors (as in Yagi antennas)?

Does anyone know of a reference related to this?

Do some antenna configurations for low frequencies, say HF up through 1GHz, better utilize received energy in nonuniform fields?
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bigSteve
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PostPosted: Sat Aug 21, 2010 6:32 pm    Post subject: Reply with quote

It's entirely dependent on the nonuniformity of the field. If you know the distribution of the field exactly, you'll be optimal with an array, because you can tune all the received energy to be back in phase, for coherent combining.

If you throw a horn on there or another type of high gain antenna, the received energy is more or less the integral of the power distribution across the aperture. How well you receive the energy is entirely dependent on the non-unifrom power distribution.
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tripelo
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PostPosted: Sat Aug 21, 2010 11:42 pm    Post subject: Reply with quote

Thank you for your reply.

bigSteve wrote:
It's entirely dependent on the nonuniformity of the field.

Yes.

bigSteve wrote:
If you know the distribution of the field exactly, you'll be optimal with an array, because you can tune all the received energy to be back in phase, for coherent combining.

Yes.

Suppose the variation in the field across the aperture is unknown, or varying in some unknown way as a function of time.

bigSteve wrote:
If you throw a horn on there or another type of high gain antenna, the received energy is more or less the integral of the power distribution across the aperture.

Good point. It seems reasonable that a horn type of antenna integrates the energy across the aperture whether the received field is constant or varying across the antenna aperture..

It also seems that most antenna types in which the aperture is of a reflector type ( such as horn, parabolic, etc.) would integrate the energy regardless of variation.

Whereas, at least most arrays (regardless of array element weighting) would sum the energy in some manner such that the total would be less than the integral of the energy, because of increased re-radiation losses.

bigSteve wrote:
How well you receive the energy is entirely dependent on the non-unifrom power distribution.

Yes.

If one knew the distribution, then one could set the array element weights (tapering, or such) to match the field in an optimum way (say for S/N ratio).

If one does not know how the received signal varies across an aperture;

It seems the best one could do is to integrate the energy, such as within an aperture similar to that of a horn (as you mention).

Re-stating the question:

Which types of receive antennas best integrate energy when a received field of unknown non-uniformity is present across the aperture?

It seems that using antennas capable of integrating the total energy are the best one could do. Tho many types of antennas appear not to be efficient integrators when exposed to nonuniform field strength across the aperture.

I think, but am not certain, that reflective-derived apertures are more efficient integrators of signal strength in a nonuniform field than unmatched apertures synthesized from arrays of elements (in the cases that come to mind).

How do large apertures (high gain antennas) synthesized by Yagi directors fare?

Do they integrate the energy?

Or, do the directive elements in the weaker part of a diffracted variable strength field, re-radiate a disproportional amount of energy, thus lowering aperture efficiency?
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bigSteve
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PostPosted: Mon Aug 23, 2010 3:42 am    Post subject: Reply with quote

The integral of the energy isn't necessarily a good thing. Really the horn integrates the e-field across it's aperture. So if the e-field has a sinusoidal distribution across the aperture, then the received power is zero. This is why horns have a peak position normal to the plane of the horn, and nulls in certain directions where the E-field distribution of the corresponding off-axis plane wave integrates to zero.

I think for you to understand the question, you should consider mobile phones. These work in highly non-uniform field regions, because there is hand on them, and a head next to them, and massive multipath, moving user, etc etc. Now for cell phones with diversity antennas, they don't just sum the two signals together (effectively integrating them), because the received voltage wave is just as likely to be in phase as out of phase. So you need some intelligent method of combining the signals coherently, or just use switched diversity - chose the element with the higher strength.

There's really no way to no whether a dish or a Yagi will work better without having a good idea of the actual channel model you are using to describe the non-uniformity of the field.

Also, I'm not sure if re-radiation is a big deal. Particularly on Yagis, the efficiency can be 80-90%, so re-radiation is down at least 10dB from the received signal. If the antenna has a high efficiency, re-radiation is negligible.
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