NEC-LIST: Is a Compact Loop a Loop or a Small Folded Dipole (???)

Jerry,

Your comment, copied below struck a chord.

>There are also demonstrated cases where NEC is not accurate for
>antennas near ground. Small loops near ground are to be avoided. As
>a loop becomes electrically small the NEC impedance matrix becomes
>increasingly ill-conditioned. This can be a problem without ground,
>but with the limited accuracy of the table-lookup used for the field
>due to ground, the error can easily overwhelm the solution. A loop
>smaller than resonant is bad, and the minimum size will depend on the
>distance of the loop from the ground. The problem is obvious when
>input resistance goes negative, but I would not trust any results for
>a small loop near ground.

I am involved in a debate, prompted by an
attention-to-the-eye-catching paper, at least by its title, by
Prof. Mike Underhill, G3LHZ and M.J. Blewett, G4VRN, entitled
"Magnetic Loop or Small Folded Dipole?" [reference IEE Seventh
International Conference on HF Radio Systems and Techniques,
Conference Publication No. 441, pp. 216-225, 7-10 July 1997].

In my view compact loops behave like loops, and numerical modelling
confirms this, since the radiation resistance rigorously calculated (I
said) using NEC is that of a small loop. I have made a detailed
analysis of such antenna types about 5-years ago [reference "An Update
on Compact Transmitting Loops", QST, November 1993, pp. 35-42]. In
that article I made reference to several commercially available
compact transmitting loops for the radio amateur, but the loops I
evaluated, by numerical modelling, by experiment and by operational
experience, are manufactured and marketed by Magnetische
Kurzwellenantenna "AMA", Christian Kaeferlein, DK5CZ,
Weinbergstrasse 5, D-64285 Darmstadt, Germany. His AMA series of
electrically small loops have been available since 1983. AMA stands
for Abstimmbare Magnetische Antenna --- tunable magnetic antennas.
The electrical design for these loops was worked out by Hans Wurta,
DL2FA. These DK5CZ type AMA loops are available for the bands 160M to
10M. Four sizes are available, ranging in size from 0.8 to 3.4
metres.

More recently, since I am now licensed to use NEC-4, I can more
accurately (I said) numerically model antennas; and, in the summer of
1996, I accurately measured the power radiated and so determined the
radiation efficiency of a 1.7 m diameter loop at a frequency 3.852 MHz
(the AMA-11, conductor aluminum, diameter 32 mm).

The radiation resistance Rr for the AMA-11 loop at 3.852 MHz (loop in
free space, conductor loss zero) according to NEC-4 (I use the EZNEC
pro version, with the NEC-4D (double precision) machine, available
from Roy Lewallen, W7EL) is 4.356 milli-ohms. The conductor loss
resistance is 41.6 milli-ohms. The antenna system resistance (base of
loop 1.41 m over good ground (estimated ground at field site (10 mS/m,
13)) according to EZNEC-4D is Ras = 79.75 milli-ohms, and so the
radiation efficiency Rr/Ras (100) is equal 5.5 percent. We measured a
radiation efficiency of 4.7 percent for this loop (difference - 0.6
dB). Note: we also correctly predict the tuning range for the loops.

We determined the radiation efficiency as follows (reference
Communications Quarterly, Fall 1998 issue (on press)). For a short HF
mobile reference antenna we measured the field strength at 100, 200,
300, 400 and 500 m, and determined the best fit curve to these data.
We could therefore deduce the near-field ground wave much more
accurately than we could by a measurement at a single site. To
determine the power radiated Pr we need to know the unattenuated field
strength Eu in mV/m at 1000 m, since

        Pr = (300/Eu) squared watts

We determined Eu from the field strength at 100 m, attenuated by 20
dB, since the unattenuated inverse distance field strength decreases
by 1/distance --- for our reference antenna the radiation efficiency
was 5.13 percent. The field intensity for the AMA-11 loop was less by
- 0.74 dB, and so the radiation efficiency of this antenna is 4.7
percent.

Conclusion: The tuning-coupling efficiency for the AMA loop is
therefore rather good. And, NEC-4D is doing a good job.

The author's response was to point out:

1) That the antenna's bandwidth determined by SWR is greater that the
antenna's bandwidth determined by the Q-factor of the tuned loop.

My counter argument is that this is EXACTLY what we expect. Since the
RF tuned power amplifier behaves as if it were a source of power
having a source (or output) lossless impedance of 50-ohms (this is
another topic of debate, see Belrose et. al., Communications
Quarterly, Fall 1997 issue), transferring power to the tuned
conjugately matched loop (input impedance 50-ohms), the operational
bandwidth (SWR measurement) is in the ideal case twice the antenna's
bandwidth --- but in actual practice (my VLF/LF experience) the
operational bandwidth is 1.5 - 2 times the antenna's bandwidth.

The operational bandwidth of the 1.7 m diameter loop is 1.7 times the
antenna's bandwidth computed by NEC-4D.

2) A second comment by the authors is "that it is too bad that NEC
does not accurately model loop antennas, since NEC assumes a uniform
current distribution around the loop". This was assumed by looking at
the space wave patterns I have calculated.

This is nonsense, and it is very easy to see that NEC does NOT assume
this. If you change the tuning capacitor from the top-side, away from
the feed, to the point of feed you can see that this changes the
impedance, and even more importantly the space-wave pattern.

Conclusion wrt to the present discussion: I am happy with the
(apparent) ability of NEC-4D to model loops, at least for the case
where the loop diameter is as small as 0.02 wavelengths.

There are three (perhaps) controversial subjects discussed in this note:

1) Whether NEC correctly models small loops;

2) Whether a small loop behaves like a loop or small folded dipole;

Comment: John Kraus, W8JK, in his classic book "Antennas", published
in 1950, analyzes a small round loop as a square loop, treated as four
short linear short dipoles --- but if one calculates a radiation
resistance (as did Underhill and Blewett) different from that for a
small loop one in trouble!!!

and,

3) Conjugate matching and source impedance of tuned RF power
    amplifiers.

Comment Jerry? Comments, anyone?

Regards, Jack

_____________________________________________
John S. (Jack) Belrose, PhD Cantab, VE2CV
Senior Radioscientist
Radio Sciences Branch
Communications Research Centre
PO Box 11490 Stn. H
OTTAWA ON K2H 8S2
CANADA
TEL 613-998-2308
FAX 613-998-4077
e-mail <john.belrose_at_crc.ca>
_____________________________________________
Received on Fri Jun 19 1998 - 10:56:19 EDT