Antennas and Ground Planes

Question

I'm looking at this datasheet for the ANT-433-HETH antenna. In the box labeled "Suggested Board Layout" I see a dimension labeled "Minimum Distance to Ground Plane" of 0.5 inches.

I always thought you should basically have your antenna feed point be directly over (or embedded in for through hole) a ground plane... am I sorely mistaken?

Is it a common practice to have your antenna feed point separated from your ground plane by (at least) some amount?

The idea of a minimum distance to the ground plane also begs the question of what is an "appropriate" distance, because if the ground plane is far enough away then what's the point?

Answer 1

There are many, many different designs for antennas, and some designs are quite unusual. Antennas commonly use a ground plane, but this is not a strict requirement. A loop antenna and a dipole are two examples that don't require a ground plane.

The basic requirements for an antenna are:

1. a good match to the circuit driving it (and almost always resonant at the operating frequency), so that the most power possible can be put into the antenna, and

2. having current flowing along its length, so that the resulting fields radiate that energy into space. (Receiving antennas are just this process in reverse).

Item (2) explains why you can't just stick a small tank circuit on a board and expect it to radiate efficiently.

Item (1) generally comes under the topic of "tuning", where you bring the antenna into resonance or wherever it was designed to be tuned. A dipole antenna is effectively a resonant length of wire broken in the middle to allow the feedpoint to be inserted. A "ground plane" antenna removes half the dipole and substitutes the ground plane for that. The inductance of the radiating element works with the capacitance between it and the ground plane to form the resonant circuit that gives the antenna proper tuning. When used this way, the ground plane may be called a "counterpoise".

A helical antenna coils up the radiator somewhat, to increase the inductance and shorten the length. Shortening the antenna affects its performance, as mentioned earlier.

So far, we've got a coiled radiator sticking up above a ground plane. But they've got a surface-mount version that lies parallel to the board. I can't tell from the data sheet if both ends are connected, but I have to guess that one end is still open...it's just soldered down in order to hold it in place. If you bring this arrangement too close to the ground plane, it will add capacitance to the circuit and detune it a lower frequency. Some of the energy will also be coupled to the ground and be lost, or at least upset the intended radiation pattern.

Answer 2

"I always thought you should basically have your antenna feed point be directly over (or embedded in for through hole) a ground plane"

This is only true for some antennas.

Most generally: Try to keep the antenna as far away as possible from any electrically conducting materials, especially from metal surfaces.

Exception: With each antenna comes a specific field configuration (E-field & H-field). Metal surfaces are fine as long as they are strictly perpendicular to the E-field. The problem with conductive surfaces is that they short-circuit the E-field (force it to 0). As long as the E-field hits the surface strictly perpendicular, the surface is equipotential with respect to the E-field, and the field configuration remains undisturbed.

The exception is most commonly met whenever there is a symmetric property to your antenna. E.g. a complete di-pole has two axes, feed-point in the middle. In the plane perpendicular to the di-pole, right at the feed-point, the E-field happens to be perpendicular to the plane. You can thus replace one axis of the di-pole by a "ground plane", feed-point exactly where the now mono-pole hits the ground plane. This is also happens to be true for some other commonly used antennas.

On the other hand, you can use the effect as part of the antenna design in order to force the E-field into some configuration. This is done e.g. in some directional antennas.

Near-field vs. Far-field: The field of an antenna can be categorized into near-field and far-field. Field-disturbances in the near field are generally catastrophic with respect to the intended antenna performance, field disturbances in the far-field only affect performance in the direction of the disturbance. As to where the near-field ends and the far-field starts is non-obvious: Some antennas are more sensitive than others. As a rule of thumb: Everything 3-5 lambdas away is definitly far-field. Anything closer may or may not interfere with antenna characteristics, modifying its center frequency, directivity, matching, ...

The concrete antenna you are referring to has a helical shape. This thesis on helical antennas aproches helical antennas using two models:

1. folded di-pole (circumference << wavelength): behaves roughly like a di-pole
2. axially radiating helical antenna (circumference ≈ wavelength)

Judging from the radiation diagram, the antenna under consideration is somewhere between those two extremes, at least when mounted perpendicular to the ground plane. In this case the E-field is strictly perpendicular to the ground-plane. The feed-point should be right on the ground plane and the ground plane should optimally extend some centimeters in all directions around the feed-point.

If the antenna is mounted parallel to the ground plane, it will short-circuit the E-filed. The ground plane will profoundly change the near-field configuration and you therefore need to consider it as part of the antenna configuration. In effect, you are now looking at a totally different antenna, which is why the theory in the linked thesis does not apply any more. I bet the antenna will also induce a fair level of HF into the ground plane (normally considered problematic). As you might see from the radiation diagram, the new antenna is also quite directional with practically zero radiation in direction of the ground plane.

I have no idea why it is advantageous to keep a minimal distance between antenna and ground plane. Maybe to contain losses in the ground-plane, but could just as well be due to matching or tuning or directivity or all combined.

Answer 3

Quoting from page 10 of the paper "Improved Performance of a Radio Frequency Identification Tag Antenna on a Metal Ground Plane":

When the metal-antenna separation distance is much less than a quarter of a wavelength, the antenna properties begin to suffer because the reflected wave has a phase shift approaching 180 degrees, and a phase shift of 180 degrees causes total destructive interference with the signal coming directly from the antenna.

Not the same antenna shape (right?) but hopefully still useful info.

Also potentially useful: "The Effects of a Metal Ground Plane on RFID Tag Antennas".

Answer 4

I am not an RF expert either but would like to post my experience as an answer because the comment box seems too crammed.

And yes, that is really weird! With all the antennas I worked with the antenna feed point was always over a ground plane, the RF trace to the antenna meets a certain max distance and thickness.. where it connects to (in my case) pcb printed folded/unfolded antennas, where the antenna i son the edge with no ground plane.

Lots of documents suggest how to tune the impedance to match the frequency but from my experience keeping the RF out close the the printed PCB I can use a balun with no additional tuning components and every thing works well.

I did notice that you are talking about 433mhz. Most of my experience is in 2.4ghz.

It is possible that in sub giga frequencies your feed point does not need need to be over a ground plane at all as long as your coil compensates for the frequency.. which is not that exact any way in these frequencies.

This document from TI , this one too and also this one might help you further understand how to deal with your engineering. It refers to the common frequencies used and how to troubleshoot rf problems.

I cannot offer a definite answer - as the world of RF is very complicated and sensitive. I hope this can help you find your answer though.

Answer 5

Looking at the diagram they are showing you a surface mount layout - the pads are the same distance apart as the length of the coil - and I think that the .5 inch "distance from ground plane" is enough space to accommodate the .35 inch diameter coil - I think the idea is to avoid the entire antenna lying flat against a copper ground layer a fraction of a mm away - they're trying to avoid the parasitic capacitive effects that might cause