Microstrip ground plane removed

Hi,

A recent interview question. Can a transmission line be formed if the ground plane is removed from a microstrip. In other words, you have only a dielectric and a small width conductor trace on top. Can field propagate? If yes, which modes would propagate? Can Quasi-TEM propagate in such a scenario?

The field will propagate but the impedance of the line will increase also.

the field can propagate. However, transmission loss is large, especially when the permittivity is low.
besides, TEM mode can not be built as dual/multi conductors are necessary for TEM mode. Instead, I think the transmission wave is surface wave.

My answer would have been "it depends on what you mean by a transmission line".

At the point where the transmission line changes from standard microstrip to a single conductor on a dielectric slab, the following will happen.

1) There will be a reflection of much of the incident power

2) There will be radiation into free space of much of the remaining power

3) A small amount of the power may continue along the conductor, either as a standing wave (like on a wire antenna) or trapped in the dielectric layer as a thin image guide.

I agree with all above.
Removing the ground plane from a microstrip is similar or equivalent to removing the conductive sheath from a coaxial cable.
Such structures do radiate but certain modes do exist in the dielectric layer between the metal-dielectric and air-dielectric boundaries.
In coaxial structures this phenomenon was investigated by Goubau, and utilized in many transmission systems. The secret of achieving a reasonably low loss is in using "launchers", impedance-transforming devices between the ordinary coaxial line and the Goubau line which is the same coax without a sheath. The impedance transformers were usually cones similar to conical horns.

I think in principle the Goubau line can be similarly used in microstrip as the basic mode is also TEM. Experiments are needed.

In microwave region we have many better antennas I think but such "ground-less microstrip" might find some use for short-range communication or sensing.

Hello Forum,

I am a beginner and got interested in the topic of this thread.

I always thought, naively, that a circuit powered by a DC or RF generator with a transmission line and load needed two conductors to close the circuit.
One conductor being the signal conductor while the other the return conductor.

Is the return conductor always necessary in a circuit? If so, why? The voltage is a concept that needs to be applied at two points (conductors).
Does current in a circuit always need to form a closed circuit? Why?
With two conductors we can then have two currents, equal and opposite. But what do we do with a transmission line made of only one conductor? Both equal and opposite currents exist on conductor: zero net current...maybe?

How does a single-conductor transmission line like the Goubau line work?
Is the longitudinal current inside it equal to zero? If it is zero, why so?
How about the voltage on this line?
All I know the field propagates as a surface wave wrapped outside the single metal wire. I tried to find simple conceptual explanation but not luck so far.

(Microwave waveguide are composed of only one conductor too. But the field (modes) are trapped inside the guide and propagate by reflecting off the guide walls. There is then propagation without any conductors, free space propagation (free space can be viewed as an infinite spherical waveguide).

Dear friend:
It is time to take a good textbook on electromagnetism and read it.
Electromagnetic wave propagation has many modes and needs no medium. This is why we can see the Sun and stars.
DC circuits are only one tiny part of a wide frequency spectrum that extends up to cosmic rays. WE like radio and microwaves as the wavelength is practical and comparable to our human body.
You can learn that EM waves can propagate as TEM mode in two-conductor lines and other modes can propagate through a hollow metallic tube; another kind of modes need only one conductor, and another kind of modes propagate along a boundary of two dielectrics...
It is a wonderful world. Start reading about it!

Hello jiripolivka,
thanks for the reply. You are right, I need to find a good textbook. So far I have a lot of fragmented knowledge about this topic. I read about how different types of transmission lines use multiple conductors and can support TEM, TM, TE, and other hybrid modes. I read about closed waveguides too...
I read about DC circuit theory. But I am so far unable to find an introductory book that talks about single-wire transmission lines and modes propagating outside the conductors line (like the Goubau line). Do you have any recommendation?
It seems that in alllll these apparently different situations (transmission lines, DC circuits, etc...) we can use the concept of current. I can't get rid of the DC theory idea that current needs to form, one way or the other, a closed loop. Is that true?
thanks!

I think that better than thinking in "current" terms, I found easier to think in "charges" terms. You can imagine small charges like electrons or "balls" moving in conductors (where they do not face much resistance) or in dielectrics (where they move like linked to springs), etc.

For instance, the modern UWB antennas are difficult to understand by "currents" approach but "charges" work much better.