Radiation from Structures Usually Thought of as Ground

dj_hdr.jpg (17748 bytes) Radiation from Structures
Usually Thought of as Ground
David Jefferies
E-Mail

ecently, publisher Jack Stone asked me to write about residual radiation from feeds, and other objects in the near field region of an antenna. While this article was gestating, I was further stimulated by a talk by Mike Underhill on compact loop antennas, at the University of Surrey IEE meeting on the 16th January 2002, to speculate about radiation from objects that are supposed to be grounded, are intended to be artificial grounds (radials and the like), or indeed are actually ground in the sense of being damp or wet earth.

Let's start from the traditionalist's idea that it is accelerated charge that radiates, and that all radiation from antenna structures can be traced back to currents formed by the oscillation of charge. So a requirement for radiation from the ground is that it supports current flow, in this scenario.

WHAT IS A GROUND PLANE?
You may ask, how can the ground support currents? Well, I ask "what is a ground plane?". Suppose it is a copper sheet such as might be used for a capacitor plate. Now let us construct what we call "the antenna", which is an identical copper sheet spaced a little way above the ground plane. We have made a capacitor antenna. We feed the top plate with current from the transmitter; this current spreads out and produces time varying charge on the upper sheet that induces equal and opposite time varying charge on the bottom sheet. Yes, currents are flowing on the ground plane as well as on "the antenna". Both will radiate.

fig01.jpg (28643 bytes)

If we replace the upper plate by a thick rod, a monopole, nothing much changes except maybe the sizes of the induced currents and charges. Radiation from the currents on the ground combines with radiation from current on the monopole. A lot depends on where the other side of the feed is returned to ground. It is possible to imagine the RF currents running all the way back through the transmitter power supply to the Utility ground, and then making their way back "the long way home" to the ground at the base of the antenna. This can be a large loop of RF current, very much larger than the physical size of the driver monopole, and this loop can and will contribute to the radiated power and the measured radiation resistance. Thus a short fat monopole, under certain grounding conditions, may be expected to be an efficient radiator, according to these ideas.

fig02.jpg (17796 bytes)

MORE TO THE STORY
That seems to me to be only part of the story. Antennas divide roughly into those that develop predominantly electric fields close to them, and the others which generate magnetic fields in the first instance, but not much E field. Both kinds of antenna may be expected to induce current flow in the ground or grounded structures. For a changing magnetic field induces current in a shorted turn and the changing electric field terminates on surface charge on the ground which is time-varying. As the outgoing propagating wave develops, farther away from these ground current sources and antenna fields, the ratio of E to H field approaches Zo, 377 ohms, the impedance of free space (E is measured in Volts/metre and H in Amps/metre so E/H = Volts/Amps = Ohms). This is a characteristic of an outward propagating radiating wave. If this wave extends to the ground, currents exist on the ground sheet (as in a normal waveguide wall) and we arrive at a "chicken and egg" complex systems problem; do the fields generate the currents or do the currents generate the fields—or do they mutually support each other? If the propagating wave continues to hug the ground, it is called a "ground wave" or "surface wave" . Alternatively, the ground current may decrease away from the antenna and set up an upwardly-directed "sky wave".

Thus in the transition region from near to far field, it may be perfectly reasonable to suggest that the currents injected by the driving antenna into the ground, by near-field induction, also contribute to the radiated power and also to the radiation resistance. The antenna driver certainly doesn't need to be comparable in size to a wavelength. We might imagine a coaxial feed, with the core connected straight to ground at the origin, the sheath unconnected, and the ground returned to the transmitter at some distance away. There appears to be no radiating part of the antenna; the feed is connected directly to ground.

TRY THIS EXPERIMENT
So here is a suggestion for an experiment for antenneX readers. Try this on a copper or aluminium sheet, of dimensions about a half wavelength across, buried at a shallow level in the ground, and with the transmitter ground returned to the edge of the sheet, which is fed by connecting the centre conductor of the coaxial cable feed to the middle of the sheet. Don't connect the coax outer to the sheet at the middle, but return the transmitter ground lead to the edge of the sheet by a different route. Try this on radial ground rods. You could even try this on an equipment or building earth lead that is grounded at the building, or at the electricity supply substation. The important factor to bear in mind is that the RF currents return via a different path to the shield of the coaxial feeder. For this purpose, we use the term "grounded" as a term for connection to physical ground, and the term "earthed" for a connection to a length of wire that is grounded some way away. See if you can get a ham contact at a distance. One might expect such an arrangement to set up a sky wave at an angle of a few tens of degrees to the ground.

fig03.jpg (18778 bytes)

[With compact antennas, balancing the current on the feed is vital. There is so little radiation from the small antenna structure proper, that any small residual current on the outside of the feed, or unbalance current in a parallel wire feed, will be significant to the radiation performance. Thus, a balun that might be entirely adequate for a resonant antenna may not have enough isolation for its compact cousin.]

SMALL LOOPS
Mike Underhill's loops are tuned loops. They have high current and high voltage capacitors in the antenna structure, across which large voltages are developed. The resulting E fields he demonstrates by holding a fluorescent tube close to the antenna. These E fields, I suggest, terminate on charges on all the wiring and grounded structure in the room or adjacent to the site, and the resulting induced currents radiate. This may be why he measures results which are so very much at variance with the accepted theory of small loop antennas. This phenomenon is not a drawback, for we can all use the mechanism to achieve useful radiation from small "antenna spaces".

The ground has sheet resistance, and it has sheet inductance. Even if it is a perfect conductor, its inductance allows it to support RF voltages and current between contacts which are placed a distance apart on it. If it has sheet resistance as well, that is, it is a lossy ground, then the fields together with the currents can penetrate the ground.

It all comes back to a question I raised early on in the life of the antenneX "Theory Forum" when I asked "What is an antenna?" I think now that a functional definition is that any part of the environs of the transmitter which transforms near-field-induced current into radiation must also be considered to be "part of the antenna structure". I would suggest that this mechanism accounts for nearly all of the "better than expected" behaviour of compact antennas as reported in antenneX. Perhaps these antennas are not so compact, after all. Their associated ground planes and connections also matter. -30-

Dr. David J. Jefferies
School of Electronic Engineering, Information Technology and Mathematics
University of Surrey
Guildford GU2 7XH
Surrey, England
D.Jefferies email
Click Here for the Authors' Biography

~ antenneX ~ February 2002 Online Issue #58 ~