NEC-LIST: Is a Compact Loop a Loop...?

Re: Is a compact loop a loop or a small folded dipole?

1)
I am not familiar with "compact loop" antennas, and do not have QST
and ARRL publications available right now. NEC certainly does not
assume a uniform current on a loop. I was recently surprised at how
much the free-space pattern computed by NEC for a loop at ka=0.16
differed from an ideal magnetic dipole. The current is nearly
uniform, but has enough variation to fill in the null on axis. Most
books I found did assume a uniform current. Kraus "Antennas, 2nd
Edition" said that a uniform current was assumed to be obtained with
phase shifters or multiple sources. RWP King's chapter in Collin and
Zucker, "Antenna Theory, Pt. I" considers the actual loop current.
That analysis was in good agreement with the NEC pattern, although it
is a lot easier to run NEC than to evaluate the integrals in the
analytic treatment.

The problem with a loop is that as frequency is reduced the current
does approach being uniform, independent of where you put the source.
That means that rows and columns in the inverse matrix resulting from
the MM solution are nearly identical. Hence the matrix becomes
ill-conditioned and numerical errors get magnified. Apparently the
compact loop has a load opposite the source, and that may make things
better. With a big enough load the loop starts to look like a bent
dipole, since the current must get small at the load.

It is always good to use the double precision code when modeling small
loops. With double precision, the solution for a loop in free space
should hold up down to ka of around 0.0002. You can run the frequency
down with some multiplying factor (FR1...) and see where the current
fails to follow the low frequency behavior of I = a*f**2 + j*b/f.
Plotting on a log-log scale makes this apparent. For a loop over
ground, you can compare with the same loop in free space and decide if
the change in impedance due to the ground looks reasonable. The
average gain computed over the upper hemisphere (divided by 2) will
give the far-field radiation efficiency. You can check to see if that
is reasonable (0 < ? < 1). Removing any ohmic loss, setting ground
conductivity to zero and integrating average gain over the whole
sphere (including dielectric ground) should give 1.0. That check can
give an indication of the accuracy for a given loop height and size.

2)
I have not considered a loop as a small folded dipole. On a folded
dipole much below resonance the transmission-line mode would dominate
over the common mode of current, so that would look like a squashed
loop, but it seems like it would not be a good antenna. Seems like
you would consider it a folded dipole only when it was operating in
dipole mode. Maybe the "compact loop" is different than what I am
considering.

Jerry Burke
LLNL
Received on Fri Jun 19 1998 - 10:56:19 EDT