

Near Field to Near Field Gain Calculation  Pyramidal Horn 
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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


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 

Joined: Fri Feb 17, 2006 12:07 pm
Posts: 236 Location: London UK

Hi In the nearfield the concept of "gain"
in the usual sense, breaks down. Gain comes
about by focussing or vector addition along
one spacevector of voltages having different
phase along that vector. As a rule of thumb
in the nearfield I think I am right in saying
that the loss from horn A to horn B will be
22dB at one wavelength 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 nearfield will
just be a splurge of the sum of vectors, without
the "main beam and sidelobe" structure of a
farfield pattern.
_________________ At bottom, life is
all about Sucking in and blowing out.





confused_fella

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


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 

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.
_________________ At bottom, life is
all about Sucking in and blowing out.


Posted 11/12/2012



