# Why can't you put a plane under a trace antenna?

I'm working with RFID for the first time. All the app notes I've read show a separate region of PCB allocated for the trace antenna, with no planes above/below. I want to understand why this is, and if it's strictly necessary. I understand that the plane will shield the antenna from one direction. For my application, this is ok.

I can fathom the following scenario: 1) An electromagnetic wave is incident upon a trace antenna with a copper plane below it 2) The distance between the plane and antenna is half the wavelength (not realistic for RFID, but this is a thought experiment) 3) The field seen at the antenna is 0, because the incident wave and reflected wave are 180 deg out of phase.

So, in this scenario, there's no changing field and no current induced in the trace antenna, so nothing is received, but this is only because the separation between the copper plane and antenna is lamba/2. When this is not the case, I would imagine there should be current induced in the antenna.

I'm particularly interested in this because I want to save board space. Specifically, I want to put components and signals on top, and the antenna on the bottom, with a copper plane in between.

If I cannot do this, can anyone explain, with precise language and in EM fundamentals (no handwaving armchair engineer stuff please!) why this will not work?

Thanks

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If you really want to do that, your best bet would be to get an RF simulator. waves hands wildly from my engineering armchair – Daniel Jan 5 at 21:19
The incident and reflected waves are also 180 degrees out of phase if the distance between the antenna and ground plane is approximately 0. – The Photon Jan 5 at 21:20
You can put a plane under a trace antenna. First, mount the antenna on the roof of the hangar. Then, taxi in the Boeing 787 Dreamliner or what have you. – Kaz Jan 8 at 0:00

An antenna converts a low impedance transmission line (from the chip or also via a coax or stripline) to a high impedance at the end of the antenna. The high impedance is to suit the impedance of free space (377 or 120$\pi$ ohms). An antenna does this gently and in doing so creates a standing wave along its length in order to allow the current to gently fall to zero at the end (such as in a quarter wave monopole) whilst the voltage reaches a maximum thus, the correct ratio of E field to H field is created (a ratio which defines the impedance of free space i.e. 377 ohms).

Generally, the near-field extends to one wavelength and anything in the vicinity can prevent the above happening effectively. If you just have a track above an earth plane you are going to create a sudden discontinuity (at the end) and get a total reflection of power back to the transmitter with no power being emitted into space. That's not an EM antenna.

I'm working with RFID for the first time.

Given this, you may actually be using an "antenna" where the dominant field is magnetic and there are plenty of things you can do that you cannot do if you were using an EM transmission to transmit power. So, quit the generalized hand waving in your question and get specific - what operating frequency and what antenna design have you targeted?

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It's not that the plane will block transmission in one direction, but that it will absorb much of the power you put into the antenna, and also greatly change the antenna characteristics.

The plane is in the near field, which means the antenna can "see" it. It will cause the electrical properties of the antenna to be quite different than when there is no plane.

The plane will also act like a receiving antenna. Depending on the size, coupling, shape, and orientation of the plane, it may re-radiate some of the power it takes from the antenna, but it will also certainly dissipate some as heat. Even if it re-radiated all of the power, the pattern would be hard to know up front, and the currents running thru the plane and their associated offset voltages would not likely be good for the rest of the circuitry.

So leave the clear area around the antenna as specified. It's better for the antenna, better for the rest of the circuitry, and the result will be more predictable.

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I suppose another factor is if the antenna is meant to capture voltage directly via an E field (units of V/m) or a current via a changing B field. I do not know which is the case for RFID. But I want to understand what this plane does that is so troublesome. "Changing antenna characteristics" may be true, but it is a coarse statement that doesn't illuminate any mechanisms in action. I am interested in the details. Also, cell phones put nfc antennas under circuitry. I'm under similar space constraints. – user2864293 Jan 5 at 22:07
Antenna are tuned to receive at a certain frequency best. By adding a plane underneath you will change that frequency. In fact, you change what type of antenna it is. the antenna you're likely looking at that doesn't have a plane is probably an inverted-F. If you place a plane on one side of it, it becomes a horrid micro-strip/ inverted-F antenna abomination, the calculations for which would be untenable. – Dave Jan 6 at 1:14

Yes you can put a plane under a trace antenna. There is nothing preventing you from doing it. If you do it and your design "works" for your intended application, you are done. However, if it does not perform to your satisfaction, perhaps following a proven design might do it, saving the time, expense, and effort to design your own.

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