low gain and high bandwidth microstrip antenna

[kae_jolie]

I have a microstrip antenna which shows high bandwidth of 70%, but very low gain near 0dB at broadside. Is this a practical or useful antenna? In other words, how good is bandwidth if antenna can't direct the power?

Should I aim for at least 5dB gain when designing a microstrip antenna?

I appreciate your answers.

 

[E Kafeman]

0 dB gain compared to what? If it is a relative wavelength small antenna with a gain of 0 dBi for a bandwidth of 70% relative to center frequency, is it not bad at all.
As comparison, a physical big antenna, type a three element log periodic antenna, is in that range or somewhat better.

 

[flanello]

0 dB gain is the same as 0 dBi!

This depends on the needs of your application.
If the 0dB is acceptable and the 70% bandwidth is needed then you have a good antenna for your device, if not you need to improve the antenna.

 

[kae_jolie]

Thanks, flanello. Now the question is when is 0dB acceptable? Isn't the gain tied with radiation, meaning if you have a 0dB antenna in the broadside direction, this means that it is not radiating in the broadside direction. Is this correct understanding? If so, why would I have an antenna that does not radiate in the desired direction?

Thanks.

 

[khaled.ayman]

Hi Kae,

You would have to look at the whole scenario from the link budget perspective, so having 0dBi doesn't necessarily mean you have no radiation. Remember the link budget equation states the following:

A link budget equation including all these effects, expressed logarithmically, might look like this:

PRX = PTX + PTX + GTX + GRX - LTX - LFS - LP - LRX

Where:
PRX = received power (dBm)
PTX = transmitter output power (dBm)
GTX = transmitter antenna gain (dBi)
GRX = receiver antenna gain (dBi)
LTX = transmit feeder and associated losses (feeder, connectors, etc.) (dB)
LFS = free space loss or path loss (dB)
LP = miscellaneous signal propagation losses (these include fading margin, polarization mismatch, losses associated with medium through which signal is travelling, other losses...) (dB)
LRX = receiver feeder and associated losses (feeder, connectors, etc.) (d)B

So basically here you already have some power transmitted by your device... I hope I tackled this point correctly

 

[kae_jolie]

Khaled,

Gain = efficiency x directivity. Efficiency is ratio of power radiated to power incident. With a gain of 0dB, power radiated would be equal to 0. I am not sure how from the link budget equation we could have any radiation.

From a physical explanation, if antenna can't direct power efficiently in a certain direction, what purpose does it have? 0dB gain tells me it can't direct power efficiently.

Thanks.

 

[khaled.ayman]

Hi Kae,

Your transmitter puts out out a fixed amount of power, the antenna does not change that, it’s only distributing that power.

In your example is there a certain value for directivity?

Hypothetically when we get 0 dBi gain it's representing the radiation pattern of an isotropic antenna, right?

 

[kae_jolie]

There is no certain value for directivity. I'm under impression, though, that I need to have at least 5dB gain.

You are correct about the isotopic antenna. In some designs, I even get negative gain values.

 

[khaled.ayman]

Have you tried increasing the size of your ground plane, in HFSS & CST you have the option to model an infinite ground plane, maybe that could help...

Good luck!

 

[E Kafeman]

0 dB gain is the same as 0 dBi!

Sum of an antenna omnidirectional radiation efficiency loss is always more then 0 dB.
Antenna, speaker or lightsource gain, or less loss, in any particular direction is relative something.
Gain expressed in dB[unitless] is not a shortcut for dBi.

If an antenna gain is just specified in dB from a manufacturer, be sure to ask if it is dBi or
dBd. If they cannot tell you or do not know the difference, then you should consider
buying from another vendor!
From:
https://wireless.fcc.gov/outreach/2004broadbandforum/comments/YDI_understandingdb.pdf

 

[RCinFLA]

There is directivity and there is antenna loss. You can create a very directive antenna (usually high gain) but with a lot of loss so it yields 0 dbi in the maximum lobe direction.

One example of a desired low directivity antenna is for GPS receiving antenna. Ideally, you want a half spheroid pattern to cover the sky above you from horizon to horizon. Would be 3 dbi if there was 100% antenna efficiency but with real antenna losses a 0 dbi to 1 dbi is quite common.

 

[flanello]

kae_jolie: Do you know the efficiency and/or the directivity of your antenna? If the efficiency is high you have an antenna that radiates in almost all directions. However, a microstrip antenna usually radiates only in the top direction. Thus microstrip antennas usually have a high gain (and directivity). For example, if your antenna radiates only into the upper space it has a directivity of 3dB and thus also a gain of 3dB - if the efficiency is 100%.

E Kafeman:
I was referring the IEEE definition of gain - IEEE Standard Definitions of Terms for Antennas:
There, gain is the ratio of radiation intensity radiated into a given direction to the radiation intensity if the accepted power of the antenna is radiated isotropically.
Thus, gain is an unit less number that can be expressed in dB. And according to this definition, gain in dB is the same as gain in dBi.
You are right, some antenna manufactures also include the matching of the antenna in the gain, thus you have to check if gain refers only to the accepted power of the antenna or includes also the matching!

 

[E Kafeman]

There are many ways to read a bible....

There, gain is the ratio of radiation intensity radiated into a given direction to the radiation intensity if the accepted power of the antenna is radiated isotropically.

IEEE does continue this text fragment with definition of gain measurement method for a directive antenna but nowhere says that you can express gain ratio without reference.

Gain is a numerical multiplicator relative something. As a pure number can it be expressed in dB or even binary if that format is preferred.
Lets call this number for X. If it is a multiplicator relative an isotropic antenna do we get the formula X*i=actual antenna radiation. Unit is [X*i] and if X is expressed in dB do we get [dB*i]. It is according to simple mathematical rules.

"dB isotropic gain" is accepted as a way to express dBi, as a reference is given.
An isotropic ideal radiator is not the only reference allowed to be used as antenna gain reference.
dBiC is for some antenna designers a better describing reference unit, dBd is fully ok, and common as a dipole often is used as a reference in many antenna measurement locations.
Absolute gain, is another possibility to use as long as you specify "absolute gain" (defined by IEEE).

Even dB[any specified antenna] is ok, but dB[nothing] without any given reference is exactly what it says. A relative reference to nothing.

It is possible to write a formulas for calculation of antenna gain and other parameters without using a single unit or reference. G for gain and f for frequency gives a unitless result in a symbolic calculation. Gain is then just a symbolic letter related to other letters with a numeric ratio.
In a such formula can both gain and frequency numeric parts be written in dB.
This type of symbolic formulas is often used in antenna books, describing radiation sphere intensity and impedance.

Can that be cause of the misunderstanding that dB can be used as multiplicator without reference?

As IEEE paper seems to be trustworthy, a rather resent IEEE document is Std 802.22. It can be downloaded for free here:
**broken link removed**

This document do not define antenna gain but the expression is used in a lot formulas. Always with the reference included.

Page 97: This RSSI shall be measured in dBm and shall be normalized for a 0 dBi antenna gain and 0 dB coupling and cable loss.
Page 355: EIRPMAX(dBm) = maximum transmit power (dBm) + maximum antenna gain for the specified channel (dBi).
Page 358: This pattern was developed assuming a typical antenna gain of 12 dBi.

It do not exist any IEEE document that express antenna gain in db, without reference.

 

[jeeudr]

Khaled,

Gain = efficiency x directivity. Efficiency is ratio of power radiated to power incident. With a gain of 0dB, power radiated would be equal to 0.

Thanks.

This sentence is not accurate. The gain is only a measure of the distribution of energy around the antenna.

In efficiency x directivity, remember that in dB (log scale) you add. in linear scales, you multiply.

 

[flanello]

Yes, gain is a multiplicator, and with the IEEE definition this multiplicator is based on an isotropic radiator: " if the accepted power of the antenna is radiated isotropically". Thus this multiplicator has its intrinsic reference and can be stated without an additional reference.
If you base the gain on an other reference you have to indicate the reference you use.
However, it is good to use dBi instead of dB, to make clear that you use an isotropic radiator as reference.