negative gain antenna

[kae_jolie]

I have these questions regarding negative gain antenna:

1. Is a negative gain antenna useful in any way? If so, what are real-life uses and applications of it?

2. Does negative gain antenna mean the antenna does not radiate at all or radiate some?

3. Is it ok for receive antennas to have zero or negative gain? I read somewhere that cell phone antennas have negative gain.


Please don't give me the definition of negative gain as I am aware of it already as "the gain is less than that of an isotropic antenna". I need some real-life examples of its use and how it can be useful when it has no gain?

Thank y'all.

 

[pragash]

I have these questions regarding negative gain antenna:

1. Is a negative gain antenna useful in any way? If so, what are real-life uses and applications of it?

its not easy to design high gain antenna for application requires smaller size. PIFA antenna which commonly used for mobile phone application also has negative gain in most of the directions. it doesnt mean its not useful!
2. Does negative gain antenna mean the antenna does not radiate at all or radiate some?
[/QUOTE]
negative gain means the antenna has high losses at a particular direction. your transmit power will get attenuated as much as negative gain in that particular direction.

3. Is it ok for receive antennas to have zero or negative gain? I read somewhere that cell phone antennas have negative gain.

[/QUOTE]
usually cell phone/mobile phone antennas (PIFA) will have positive gain as well as negative gain. nothing wrong in that as long as your antenna/system meets regulatory requirements (certification and etc)

 

[Warpspeed]

I suppose any basic dipole antenna "could" have less performance that the theoretical ideal, and still be a perfectly useful practical antenna.

Not every antenna has gain, and if it does, it is only going to be in the preferred direction.

Lack of gain only means lack of efficiency. Many antennas, especially at longer wavelengths are physically much shorter than the ideal, and they work fine for both transmitting and receiving. Antennas mounted on vehicles and aircraft are typical examples of electrically short antennas.

 

[kae_jolie]

but say you had an antenna that had negative gain across the desired bandwidth, then it will have no use, correct?

So, if gain vs frequency plot for antenna has negative gain for all the frequencies, then this antenna has no use at all. If it still does, please elaborate.

 

[hagster]

Negative gain does not mean negative radiated power. It just means that less power is radiated than input.

For clarification. Gain is only negative when using a dB scale. If you use linear units then it just means the gain somewhere between zero and 1.

 

[kae_jolie]

Thanks, if I have an antenna with gain of -4 dB (equivalent to gain =0.4 unitless), what good is it for? Give some examples.

 

[Feitoza]

Thanks, if I have an antenna with gain of -4 dB (equivalent to gain =0.4 unitless), what good is it for? Give some examples.

That depends on the application. For example, if you want to design an antenna for a cell-phone tower application, you will need a omni-directional antenna. Hence, the gain will probably be small. A good way to estimate this is using the Friis transmission equation:

Pr = Pt*Gt*Gr*(lambda/4*pi*r)²,

where Pr is the received power, Gt and Gr are the Tx and Rx antenna gains. It ignores multipath and other effects but is a good way to check in a first order. If it doesn't fit your specifications, try to use a substrate with smaller loss tangent.

Another example of what good is it for: in your application you are obligated to use an electrically small antenna, that have intrinsically small efficiency; but you could compensate it with higher gain transmitting antenna, or maybe the loss in range is not that critical for you, in a way that this low efficiency antenna still suits your design (because is more critical that it is low-profile than efficient).

 

[Feitoza]

Thanks, if I have an antenna with gain of -4 dB (equivalent to gain =0.4 unitless), what good is it for? Give some examples.

Another simple example are old AM radios: that small loop antennas (remember that wavelength @ AM frequencies are gigantic) presents very low efficiency, but the transmitted powers are in the order of kW or MW, so it generally works. In general, receiving antennas have more degrees of freedom in efficiency. It doesn't stand for Tx antennas, think what a waste of power if you input MegaWatts to an inefficient antenna (remember that efficiency is roughly a measure of what is dissipated over what is radiated)

 

[kae_jolie]

Thanks, Feitoza....

So, this begs for one more question: If both low and high gain antennas can send and receive signals, then what is the difference between the two? High gain antenna's beam is stronger so the signal is received and transmitted better?

From all the above explanations, I am gathering that low gain antennas would be more suitable for receive antenna and high gain antenna would be more suitable for a transmit antenna? Is this a correct assumption?

I just don't see how a low gain or negative gain antenna could be used for transmission. Its signal beam concentration is not strong enough. Is this correct?

 

[Aussie Susan]

Perhaps you should explain what you are trying to do and why you think a 'negative gain antenna' is what is required as part of the solution.
There are several concepts being mixed up here - beam width is one thing but the efficiency of the antenna in turning the input power into radiated power is another.
Susan

 

[Warpspeed]

I was once given the task of very quickly erecting a suitable vertical transmitting antenna for 392 Khz.
The wavelength of that works out to 765 metres.
The best I could do was about 17 metres, plus a loading coil and capacity top hat.

The radiation efficiency must have been very low, but it worked just fine for the purpose.

A full length antenna is not always possible, so something well down in performance is all that can be used for both transmitting and receiving.

 

[kae_jolie]

I am designing a microstrip antenna and I changed the substrate permittivity and got sub-zero gain throughout the bandwidth. I really don't have an application for it. I am just showing that as antenna substrate permittivity increases, the antenna gain decreases. I will need to present my results and worry someone may ask me what is this antenna good for if its gain is below zero db across the bandwidth. I can't really think of a good answer. My mentor never cares about the gain much. (He always say: it's just the gain, who cares...) Apparently a lot of people in the field at the conferences I attend care and they always ask me where is the gain plot. I really need to get to he bottom of this. I really am starting to be convinced that my mentor is correct not caring about the gain. I think it only matters if you want the antenna to have gain and directivity in a certain direction like with satellite antennas. But for antennas like PIFA and patch antennas, I don't see why gain even matters.

 

[Warpspeed]

The answer to that, the gain has been sacrificed for some more desirable feature.
It's usually for diminished size.
While unavoidable resistive losses are excusable, huge impedance mismatches are less so.

Your mentor is right not to worry too much.
When you consider total path loss between transmitter output terminals, and receiver input terminals, a few db lost in the two transmission lines and two antenna are nothing compared to the path loss through free space.

 

[vfone]

Today antenna gain in mobile devices is going backwards, due to higher integration and multi-band topology. Most of the embedded antennas in mobile devices have negative (average) gain in dB.
I mean average gain, because there is possible to get some positive gain in particular directions, mainly due to antenna layout/placement reasons.

 

[Aussie Susan]

Given what you said about changing the substrate permittivity, I would have thought your answer to the question would be that this shows that the substrate does have an impact and that you need to design the antenna taking this into account.
It seems that you are not *wanting* a negative gain antenna (which is the impression I got from your original posting) but want to understand why the permittivity is giving you one.
Susan

 

[kae_jolie]

more questions related to the topic of gain:

1. does higher gain mean that the signal is stronger in some direction so it propagates further, that is why satellite antennas need to have higher gain?

2. how far can the signal from a low gain (5dB) antenna propagate before it gets weak?

3. when is having 5 dB gain over a bigger range of frequencies (large bandwidth) advantageous? what applications require that?

4. is the freq vs gain plot usually plotted at broadside direction only?

 

[Warpspeed]

This guy built a low gain coat hangar antenna for his car.



Is it totally useless ?
Hell no, he can get heavy metal rock bands just great, pretty damned loud too. And he can receive the news and weather reports just fine.

How many db down is it ?
He never had it tested, but he really does not care if its a zillion db down.

It works just great on his old rust bucket car, and he could not be happier. after some punk snapped off the factory original super high gain antenna.

 

[kae_jolie]

when scanning phased arrays, blind spots are considered when the gain is zero, so there are applications where you can't afford the gain to be zero or negative. I believe these are radars and satellite antennas.
If someone could tackle my 4 questions above one by one, I would really appreciate it.

 

[Warpspeed]

1/ Higher gain only comes from concentrating the energy in one direction at the expense of other directions.

2/ Any increased distance increases attenuation.
So the answer is no distance at all before it gets weaker.

3/ Some applications require wider bandwidth, either because the signal itself has wide bandwidth, or there could be many different frequency channels to operate in.

4/ Frequency versus gain is always plotted in the optimum direction.

 

[kae_jolie]

So, what does "concentrating the energy in one direction at the expense of other directions" achieve?
To me, it means that it makes the signal "stronger" in one direction.
Define "stronger". To me, stronger means it can travel further with less attenuation when compared to a low gain antenna. That is why satellite transmit antennas must have high gain.....is this all correct what I just said?

low gain antennas are mostly receive antennas....is this correct?