NEC-LIST: FW: Low-Q

Dale

Thanks for your comments and I understand you are not claiming to have
solved all the practical problems. Where you distinguish field coupling
from element coupling there is a need for further clarification on two
matters.

The first is that the practical bandwidth of an antenna nearly always means
the band within which the input impedance meets some criterion (e.g. 2:1
swr) so that I do not think it is possible to detach bandwidth from input
impedance. Taking the electric-magnetic dipole pair, you have shown that
each element on its own might have a high Q, while placed close together and
fed in phase-quadrature the pair exhibit low Q. I am not sure whether you
are saying this automatically means that the input-impedance bandwidth of
the pair is also increased. You have shown that the stored energy is less
and that means low Q according to your argument, but does this imply a
higher input-impedance bandwidth? In other words I have not seen in your
papers the vital link between low Q and high bandwidth when your extended
definition of Q based on stored energy is in use (it might be there and I
missed it).

The reason I question that is that if there is low coupling between the
electric-magnetic dipole pair, that means there is little mutual impedance
between them. If that is true, then there can be little change in the input
impedance of either element when they are brought close together; therefore
the input-impedance bandwidth won't change much either. That seems to show
that element coupling as well as field coupling is important in the
practical application of the low-Q concept.

The second point is the orthogonality of a pair of adjacent parallel
dipoles, in which the two field distributions are nearly parallel at all
points further away than about five times the separation. In this case
there is strong field coupling all round, and strong element coupling also.
I did not understand in what sense you said these systems were mutually
orthogonal.

Thank you for your patience,

Best regards,

Alan Boswell
  
  
     

-----Original Message-----
From: Dale M Grimes [mailto:dmg6_at_psu.edu]
Sent: 27 January 2003 16:00
To: alan.boswell_at_baesystems.com
Subject: This morning's comments

-- 
Alan,
	<<Thank you for your reply and the further clarification.  In 
the antenna community I think the emphasis is slightly different from 
what you said, and it is that Q is accepted as a way of measuring 
bandwidth.  The classic
"small" antenna problem is VLF transmission where extremely large but
electrically small antennas provide very small bandwidths, a Q of 100 being
typical for an antenna supported on many large masts.  The end user needs
bandwidth, for obvious reasons (but not at the price of reduced
efficiency).>>
	Agreed, and here we diverge from the antenna community. 
Although a better antenna would be nice, our primary objective is to 
better understand limit conditions on EM fields.  Hence we can accept 
no constraints on overlapping modes.  If an antenna works we, or 
others, can worry about how to implement it later.
	<<A practical antenna has to be driveable by a single source, 
so when there are two or more ports there will usually be a 
power-splitter network and
there is then a measureable bandwidth at the single input port, although I
understand that is a complication that would be rather unwelcome in an
already complicated analysis.  Where the elements are fed by separate
synchronised sources, the issue remains how well each element is matched to
its source when the frequency is changed.>>
	Agreed. However, again our primary objective was to study the 
phenomena  not to design a specific antenna.  If an implementation 
appears promising a driving network can be synthesized.  We didn't 
have the resources to do more.
	<<<You mentioned a phased array.  In designing phased arrays 
the problem of mutual coupling between elements is very real, and the 
difficult trick is to
maintain a good input match under all conditions of scan angle.  This is a
case where the dipoles are usually parallel, and they couple to one another
so that the input impedance of each dipole depends strongly on what is going
on in the other dipoles.  I do not think something similar occurs between an
x-directed magnetic dipole and an x- or y-directed electric dipole (both
antennas assumed at the origin) because the fields distributions do not
allow it, either in the near field region nor the far field.  I was
therefore questioning the operation of your 2-element antenna because
without mutual coupling I could not see the input impedance of either
element being affected by the current in the other element, and if the input
impedance does not change, the Q (in one sense) does not change either, even
though the stored field energy might change.  I understand there is some
necessary asymmetry in your NEC model but it would be a pity if the coupling
arises only as a result of the asymmetry.  I would welcome any clarification
you have on this point.>>>
	Of course, you are again correct about mutual coupling 
between elements of a phased array.  However, we are interested in 
field overlap, not element-to-element coupling.  That is not our 
point.  A simple phased array of two dipole elements is interesting 
because each dipole retains its own orthogonal power and energy (the 
output power depends upon inter-element coupling but, again, we don't 
care about that.) The entire point in bringing this up is that 
coupling exists through the external fields. This external  coupling 
shifts the location both of far and near energy fields.  Collin uses 
power and energy orthogonality as the reason all stored energy should 
be counted in the numerator of the Q expression.  Our point is that a 
simple phased array is an obvious and accepted case where the element 
properties remain orthogonal but the position of the energy is 
shifted.  Is there an equivalent energy shift upon modulation?  We 
set up to measure it and found the answer to be yes.
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