Why are the efficiencies of my patch antenna so poor?

[lonely_wolf_0123]

I have created an 2.4GHz patch antenna (in CST MW Studio). But I have a problem with efficiency. As the picture of S11 and Z below. I supposed that my Z is good enough for matching 50 Ohm port and S11 also below -10dB. But then why my radiation efficiency is so poor? I have known that radiation efficiency could be calculated by: er = 1-|s11|^2.
with: er: radiation efficiency
s11: reflection coefficient
If that the case, my expected er should be nearly 99%?
Thank you in advance.
P/s: I intended to put this patch to 2x2 array antenna, and its efficiency is also poor. I think I need to figure out from the initial patch first.

 

[volker@muehlhaus]

I have known that radiation efficiency could be calculated by: er = 1-|s11|^2.
with: er: radiation efficiency

This is wrong. Radiation effiency is a measure for loss (dissipation) inside the antenna, regardless of S11.

Something in your antenna modell introduces loss. Typically, substrate loss from poor substrate materials like FR4 is the biggest effect, and copper loss has only a minor effect.

 

[FvM]

According to report, you have about 0.1 dB reflection loss and 5.3 dB antenna (conductive and dielectric) losses. 70 % of input power is burned in the antenna.

 

[emmamia12]

You're correct that S11 shows good matching (about -37 dB) and the impedance looks close to 50 Ω, so most of the power is being delivered to the antenna. However, radiation efficiency isn't just about matching—it also depends on how much of that accepted power is actually radiated vs. lost inside the antenna.

The formula you mention, er = 1 - |S11|^2, tells you how much power is accepted from the feedline (i.e., how little is reflected), but it doesn't account for losses inside the antenna (like dielectric or conductor losses).

Common reasons for low radiation efficiency even with good S11:

• High-loss substrate (e.g., FR4): absorbs power as heat.
• Conductor/ohmic losses: in thin/poorly conducting materials.
• Design issues: like ground plane currents or losses in the feed.

Suggestion: In CST, check the simulation results for radiation efficiency vs. total efficiency. You can also temporarily set the materials to be ideal/lossless—if efficiency jumps, you’ll know losses are the cause.


So yes, your patch is well matched, but probably not well radiating.

 

[Andrew77]

Total gain seems 1,4dBi ...to get better radiation efficiency try suspended patch antenna

 

[lonely_wolf_0123]

Thanks all of you for replying me! I have figured out the main factor caused poor efficiency. After I checked, there was a significant loss in my available Fr4 substrate. In case I can't afford a better substrate, what can I do to enhance my radiation results? Thanks you.

 

[vfone]

I believe that currently, on the world market, there are millions of 2.4GHz printed antennas made on FR4 substrate.
Also, if you search the net for 2.4GHz printed antennas on FR4, you will get countless of articles and publications.

 

[volker@muehlhaus]

I believe that currently, on the world market, there are millions of 2.4GHz printed antennas made on FR4 substrate.
Also, if you search the net for 2.4GHz printed antennas on FR4, you will get countless of articles and publications.

So what are you trying to tell us? It has been shown many times that patch antennas on FR4 have this poor efficiency, in simulation and in measurement.
Other antenna types like PIFA are less troubled by FR4 losses, but have less directive pattern.

--- Updated Saturday at 5:20 PM ---

In case I can't afford a better substrate, what can I do to enhance my radiation results?

If you need the directional pattern of the patch, you could create a better substrate by a sandwich from thin FR4 carrier (patch only) and an additional air layer between the PCB and the ground plane. The thin substrate with the patch is then mounted at some distance from the ground plane (suspended patch antenna, see #5 above), creating a mixed dielectric with FR4 + air. Of course, your patch needs to be redesigned then, and mechanics is more complicated then.

If you don't need the directional pattern, look at other antenna types like PIFA. They have better radation efficiency when using FR4, by pattern is very different.

 

[vfone]

I am not telling you anything. Is about facts.
The first publication that came my way shows that a 2.4GHz patch antenna built on FR4, with 6.2dB directivity, gets almost 70% (-1.6dB) of efficiency:

https://ieeexplore.ieee.org/document/7295902

And there are countless of papers like this.