Loop Antenna design on FR4

[abdul.1991]

Hi everyone,

I want to ask a simple question on designing a loop antenna. can we make loop antenna on PCB (FR4). And added to that, what are the dimensions of loop antenna (circular or rectangular).? like we take in Patch antenna.

Thanks in advance.

 

[FvM]

Yes you can make a loop antenna on FR4.

Good antenna text books have full chapters about loop antenna design, e.g. my favorite Balananis, Antenna Theory.

There are different types of loop antennas, electrical small (antenna dimensions small compared to wavelength) and electrical large (circumference about one wavelenght). What kind of loop antennna are you going to make?

 

[volker@muehlhaus]

can we make loop antenna on PCB (FR4).

Yes.

And added to that, what are the dimensions of loop antenna (circular or rectangular).? like we take in Patch antenna.

For patch antennas, there is one clearly defined size. For loop antennas, there is not that one size.

Loop antennas are small relative to the wavelength, but you can choose the size as needed. This loop is characterized by its inductance, with some series resistance (losses + radiation resistance). Then, resonance is created by adding a series capacitor, and that L-C resonator is coupled (different coupling methods possible) and matched.

Loops that are very small compared to the wavelength are used at low frequency (RFID at 13.56MHz, wireless charging etc). These don't operate as "normal" antennas creating far field radiation, they are just creating a magnetic near field and couple to other loops by that magnetic field.

As you see, there is not one single "loop antenna" design case. The Balanis book that FvM mentioned will help with the "normal" loop antennas that are designed for far field radiation. For coupled inductor cases (RFID, wireless charging) there is also a lot of literature, but basically that is coupled inductor design.

 

[abdul.1991]

Thanks for your replies.

Actually I want to make a small circular and rectangular loop antenna for 900MHz. I need to know what exact dimensions (diameter/Radius) should I take for that antenna in mm.?

Thanks

 

[volker@muehlhaus]

Actually I want to make a small circular and rectangular loop antenna for 900MHz. I need to know what exact dimensions (diameter/Radius) should I take for that antenna in mm.?

It is important to understand that there is not one correct dimension. I designed a really small 915MHz RFID loop for a customer who needed very small size (consulting project). If you can make it larger, it will be more efficient (better gain = better range). The resonance frequency is always set by a series capacitor, so many different loop sizes are possible.

Have a look at page 11 here:
https://www.pcb-pool.com/download/pp_info_rfid_magicstrap_applicationnote.pdf

 

[abdul.1991]

It is important to understand that there is not one correct dimension. I designed a really small 915MHz RFID loop for a customer who needed very small size (consulting project). If you can make it larger, it will be more efficient (better gain = better range). The resonance frequency is always set by a series capacitor, so many different loop sizes are possible.

Have a look at page 11 here:
https://www.pcb-pool.com/download/pp_info_rfid_magicstrap_applicationnote.pdf

Thanks for your reply,

If possible can you please send or attach hfss simulation file of 900MHz loop antenna.?

 

[volker@muehlhaus]

If possible can you please send or attach hfss simulation file of 900MHz loop antenna.?

No, sorry. I use ADS / EMpro (not HFSS) and I have no free models to give away. It was payed work for clients.

 

[abdul.1991]

Ok,

I face the problem of resonance, I get a good S11 (-30db) at 900MHz, but with a very wide B.W (500MHz to 2000MHz). I want it to be a narrow band only for 825MHz to 915MHz. What should I do.?

 

[FvM]

I face the problem of resonance, I get a good S11 (-30db) at 900MHz, but with a very wide B.W (500MHz to 2000MHz). I want it to be a narrow band only for 825MHz to 915MHz. What should I do.?

First time that I hear a request to intentionally reduce antenna bandwidth. But the reported S11 bandwidth sounds rather dubious, I don't believe that's it's possible with a loop antenna unless you have introduced resistive elements. So most likely your design or analysis failed somehow.

A S11 curve with low reflection factor doesn't tell you unequivocally if it represents radiation resistance (wanted) or loss resistance (unwanted).

Bandwitdh can be easily reduced by lumped or distributed filters, this shouldn't be your concern presently.

 

[volker@muehlhaus]

I face the problem of resonance, I get a good S11 (-30db) at 900MHz, but with a very wide B.W (500MHz to 2000MHz).

As FvM already mentioned, this is not a problem. As long as your transmitter works inside the 825MHz to 915MHz band, only that will be radiated. Having some nice bandwidth is great, because you will be well matched even with manufacturing tolerances.

But a very wideband antenna is suspicious because that often results from high losses in the antenna (conductor or dielectric losses) and means low gain. Check that in your simulation.