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Near Field to Near Field Gain Calculation - Pyramidal Horn - RF Cafe Forums

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confused_fella
 Post subject: Near field to near field gain calculation - pyramidal horn
Posted: Fri Feb 20, 2009 7:20 pm 
 
Lieutenant

Joined: Fri Feb 20, 2009 7:14 pm
Posts: 2
Hey guys I was wondering if someone could help me out with the near field to near field gain calculations. Let's say I took data for two gain horns, one as a receiver and one as a transmitter, at one foot. From this data, can I use some sort of calculation and get the gain information at 2ft, 3ft, or at any distance given that I am still inside the near field region?


 
   
 
nubbage
 Post subject: Re: Near field to near field gain calculation - pyramidal horn
Posted: Sat Feb 28, 2009 4:51 am 
 
General
User avatar

Joined: Fri Feb 17, 2006 12:07 pm
Posts: 236
Location: London UK
Hi
In the near-field the concept of "gain" in the usual sense, breaks down. Gain comes about by focussing or vector addition along one space-vector of voltages having different phase along that vector.
As a rule of thumb in the near-field I think I am right in saying that the loss from horn A to horn B will be 22dB at one wave-length separation and will then decrease as an inverse cubic law. Thus doubling to two wavelengths drops the level to -22dB - 10*LOG(8)
In the vicinity of the horns the polar pattern in the near-field will just be a splurge of the sum of vectors, without the "main beam and sidelobe" structure of a far-field pattern.

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confused_fella
 Post subject: Re: Near field to near field gain calculation - pyramidal horn
Posted: Tue Mar 03, 2009 5:45 pm 
 
Lieutenant

Joined: Fri Feb 20, 2009 7:14 pm
Posts: 2
Oh hey thanks for answering! I'm still not quite understanding what exactly is going on here. I believe you are explaining in full detail the characteristics of the gain horns and calculating the loss through vector addition. You'll understand more of what I mean once I explain.

The way I've been doing the experiments is that I've been using a network analyzer to capture the data using one pyramidal horn as transmit and one as a receive. The distance is set at one meter. I then assume that the gain of each horn is free space loss minus half of the value (at each frequency) taken from the network analyzer. Then by using the equations from Chu and Semplak " Gain of Electromagnetic Horns" I calculate the correction factors and project the near field gain I just calculated (at one meter) to far field gain.

Chu and Semplak's equations for correction works only if you are correcting to far field from near field. If i were in near field and I want to get the gain at another near field position, what process would I use? Let's say I was at one foot, and using the simple method of extracting the free space loss, I obtained the gain. Is there a way I can use this information to get the near field gain at two feet?

I'm guessing you were trying to answer my question with the previous post, but I'm not exactly as educated as you take me to be. So if you can elaborate a little or give a few more examples, that would be great. I know in the near field there's an array of vectors coming out from the horn, but I don't know how to sum them up. I'm assuming whatever the network analyzer picks up on the receiver horn does this...

Can you please help me?


 
   
 
nubbage
 Post subject: Re: Near field to near field gain calculation - pyramidal horn
Posted: Wed Mar 18, 2009 6:49 am 
 
General
User avatar

Joined: Fri Feb 17, 2006 12:07 pm
Posts: 236
Location: London UK
Hi
The VNA does perform the vector summation on its receive port, and will present the summation as a decibel level compared to the transmit port, if you select this presentation. On mine (HP8410B) I only have the S parameter set, so I would choose S21 display in dB. This level will show (assuming S11 is better than -10dB) the gain of the send and receive horns, together with the coupling loss. All the textbooks I have looked at say the near field energy falls off as the inverse cube of distance. Thus going from one foot to two feet will decrease the level by 9 dB (10*LOG[2 cubed]).
You would have to screen the area against reflections using RAM absorbed.
I am not familiar with Chu and Semplak's paper/method, so I cannot comment. Hope that helps some.

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