Re: NEC-LIST: Small Loop Antennas Close To The Ground

Chris Teixeira wrote:
>
>I am modeling an application that involves electrically small loop
>antennas close to the ground with NEC-4.1. Because I am new to the
>world of NEC, I decided to do some validation test cases for my own
>benefit. I have a question about an unusual observed NEC result for a
>very small loop close to the ground....

Chris,

The interpretation of measured/and simulated data for of small compact
loops is a somewhat controversial subject. I have modelled,
remodelled and remodelled compact loops, recently (using NEC-4) and in
the past (using MININEC and NEC-2). I never experienced difficulty.

For your interest I copy below correspondence, in response to the
claim that a small loop behaves like a small folded dipole, rather
than like a small loop.

73, Jack, VE2CV

COMPACT LOOPS

Background

Is a Compact Loop a Loop or a Small Folded Dipole?

I was interested to read in Pat Hawker, G3VA's TT Column in the
November 1997 issue of RadCom, which brought my attention to the
eye-catching paper, at least by its title, by G3LHZ and 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 is
that of a small loop. I made a detailed analysis of such antenna
types about 7-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 Kaferlein, 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 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).

In Conclusion [see below]: A small compact loop behaves like a loop,
and the method of tuning and matching, resonating by a capacitor, and
magnetically coupling power to the loop by means of a small coupling
loop, is an efficient way to resonate and match the loop. To keep
conductor loss low the diameter of the loop's conductor must be large
(the conductor diameter of the DK5CZ AMA loops is 32 mm, the loop is
made out of a formed (almost a perfect circle) aluminum tube).

Radiation Resistance

The impedance of a compact loop is more complicated than given by
simple formula. I think G3LHZ will agree with that. The loop's
impedance depends on frequency; loop diameter; conductor loss; ground
induced loss particularly when operating on the lower frequency bands
(since loops employed for 160M, 80M and 40M are typically vertically
oriented and installed at quite a low height over ground); and,
importantly, on the fact that the loop is tuned and fed in some way,
since this alters the current distribution on the antenna.

Q-Factor

A principle point made by Mike Underhill, G3LHZ (reference TT February
1998) is that the Q values measured by SWR and antenna bandwidth
methods are much lower than predicted by the traditional loop
radiation formula. This is to be expected. Let us examine this
difference.

As already noted the impedance of a compact loop is more complicated
than given by simple formula.

According to NEC-4D, for a 1.7 m diameter vertical loop (an AMA-2),
conductor diameter 32 mm an aluminum tube, at a height of 2 m over
average ground, the untuned loop impedance at 7, 10 and 14 MHz is 0.22
+ j 204.2 ohms, 0.707 + j 314.5 ohms and 3.19 + j 518.9 ohms
respectively. When tuned the resonant loop impedance is 0.175 ohms,
0.388 ohms and 0.888 ohms respectively. Traditional loop radiation
formula will not predict these differences.

Continuing, the loop's bandwidth can therefore be calculated, since Q
= X/R and BW = f/Q. For 7, 10 and 14 MHz the loop's bandwidth is 6,
12.3 and 24 kHz.

Now, when we measure bandwidth by measuring SWR, we must remember that
the effect of the out-put impedance of the RF power amplifier must be
taken into account, since for maximum power transfer the output or
source impedance is the complex conjugate of the antenna's impedance
(reference Belrose, VE2CV, Maxwell, W2DU and Rauch, W8JI,
Communications Quarterly, Fall 1997, pp. 25-40) --- and while not
representing a loss, the source impedance (for narrow band antenna
systems) increases the bandwidth of the antenna system. The
operational bandwidth can theoretically be double the antenna
bandwidth, but in actual practice the realized operational bandwidth
is somewhat less (in my experience the increase in BW is by a factor
somewhat > 1.5).

The measured bandwidths for the AMA-2 1.7 m loop at 7, 10 and 14 MHz
are 10, 20.6 and 41.3 kHz respectively. The operational bandwidth is
therefore greater than the antenna's bandwidth by a factor of about
1.7. The loops operational Q is lower than the antenna Q by this
same factor.

Field Strength Measurements

As noted in a QST November 1993 article, author VE2CV evaluated
several compact loops, by numerical modelling, by experimental
measurement and by operational experience, loops manufactured by
Christian K=E4ferlein, DK5CZ, his AMA series.

More recently, since we were set up to measure the radiation
efficiency of HF mobile antennas, see Communications Quarterly article
Fall 1998 issue, we measured (on our calibrated test site) the
radiation efficiency of an AMA 8 loop (a 1.7 m diameter loop).

We had during the course of this measurement program, measured the
ground wave field strength for various mobile whips at 100 m, 200 m,
300 m, 400 m and 500 m, and so determined the unattenuated field
strength at 1 km, from which the power radiated can be determined, and
hence the radiation efficiency determined.

For the loop we determined a radiation efficiency of 4.7 percent, by
comparison of the ground wave field strength with that for the
reference mobile whip, for a frequency of 3.852 MHz.

The radiation resistance, according to NEC-4D (we use EZNEC pro
available from Roy Lewallen, W7EL) of an AMA-8, a 1.7 m diameter loop
(conductor diameter 32 mm) at 3.852 MHz is equal to 4.356 milli ohms
(tuned loop in free space, conductor loss zero) . The conductor loss
(aluminum conductor) is 41.6 milli ohms. The antenna system
resistance (base of loop 1.41 m over good ground (estimated ground at
field site s =3D 10 mS/m; e =3D 13) according to NEC-4D is Ras =3D
79.75 milli ohms, and so the radiation efficiency (Rr/Ras) (100) is
equal to 5.5 percent (difference 0.6 dB), which is a good agreement
considering the accuracy with which one can measure field strength,
validating NEC-4D and revealing that the efficiency of the method of
coupling to the low impedance loop is rather good.

Conclusion

G3LHZ concludes that "it is a pity that NEC fails to model a tuned
loop correctly". He concludes that either the assumption is made that
current around the loop is exactly uniform or that any out-of-balance
currents do not radiate. But NEC assumes nothing of the sort. NEC
calculates the resultant current around the loop in amplitude phase.
In my view NEC-4D accurately predicts the performance (impedance,
bandwidth, gain and pattern) of compact loops, and no handwaving based
on traditional formula can prove otherwise.

All of the above refers to a loop effectively fed at the base, with
the tuning capacitor at the top. Feed and tune and match the loop at
the same point, and you will find a significant difference --- e.g.
the method of feed and tuning originally devised by Patterson ---
since this alters the current distribution on the loop --- this cannot
be anticipated or demonstrated by traditional formula.

If the compact loop (perimeter/wave length < 0.25) looks like a loop,
behaves essentially like a loop, it must be a loop. But the radiation
characteristics must be rigorously calculated.

There is in my view no controversy --- G3LHZ and G4VRN are wrong in
there view that a compact loop does not radiate (essentially) like a
loop. The patterns that I have calculated (a whole set of them appear
in publications by Chris Kaferlein) and everything I have said about
loops is correct.

_____________________________________________
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 613-998-2779
FAX 613-998-4077
e-mail <john.belrose_at_crc.ca>
_____________________________________________
Received on Mon Mar 13 2000 - 06:27:21 EST