Antenna Negative Gain

[madebyagra]

I have make a planar toothed log periodic antenna with broadband characteristic, but in the gain measurement ( i used two antenna method) and use the formula (in balanis antenna book), unfortunately i found the gain value (in dB) is negative, what the meaning of negative value? is it an attenuation? or a loss?

thanks b4

 

[RF_Jim]

I have make a planar toothed log periodic antenna with broadband characteristic, but in the gain measurement ( i used two antenna method) and use the formula (in balanis antenna book), unfortunately i found the gain value (in dB) is negative, what the meaning of negative value? is it an attenuation? or a loss?

thanks b4

I check my 'antenna range' equipment and calculations with a pair of dipoles cut for the frequency or frequencies of interest just to double check with simple, easy to replicate test gear ... is your range and measurement equipment checking out okay with basic antennas (like a dipole)? Within a dB or two is what I find the dipoles are 'good for' over any fancier calibrated gain antennas.

Also, what does the 'match' look like looking into the new antenna (S11 measurement) using a VNA?

Jim

 

[elchiquito]

Yes. Negative gain represents Losses. if the gain and power level's of both antennas is less than the losses then you will see a negative gain at your recieving antenna end. from friss formula Power recieved is equal to gain of recieving and trans and recieve antenna along with the transmitted power plus the attentuation losses ,mismatch due to polarization and so on.

For me to make sure as JIM has said i would check the attenuation with a standard gain horn antenna and then it will give me a rough estimation what should be the gain if i was to do the same with the test antenna.

Regards/

 

[madebyagra]

oh yeah thanks for the answer, for information, i measure the antenna not inside an anechoic chamber, is it affect the result?
and i use this antenna for electromagnetic harvesting, and it's capable of lighting a LED

 

[elchiquito]

If its not Inside an anechoic chamber, a number of things will affect the Power recieved.not only free space losses but attenuation from the surroundings as well.i dont know exactly what electromagnetic harvesting is. probably JIM can shed some light on it.

Regards
ChachitoEL

 

[madebyagra]

If its not Inside an anechoic chamber, a number of things will affect the Power recieved.not only free space losses but attenuation from the surroundings as well.i dont know exactly what electromagnetic harvesting is. probably JIM can shed some light on it.

Regards
ChachitoEL

well, i think the enviroment sorrounding is affecting the negative gain, for electromagnetic harvesting, it is simply converting an electromagnetic wave (from antenna) into DC voltage with a rectifier

 

[elchiquito]

SO When you are doing your calculations are you taking into consideration the losses by the cables and stuff. negative gain=attenuation.i exactly sure to tell you what's going wrong. what kind of attenuation did you see between the recieving and transmitting antenna with standard gain antenna.What is your frequency of operation.

Regards

 

[FvM]

If your antenna is lossless by design (in other words not containing resitive elements), gain can be split into directivity and impedance mismatch. If you see negative "gain" in a validated measurement, I would think about serious mismatch in a first place. Antenna s11 measurement is in fact a basic thing and would be typically performed before the more complex free field transmission measurements.

For dipoles and other antennas with symmetrical input, it's rather accurate and not that much distorted by cable effects. The balun circuit is of course part of the antenna input impedance.

 

[flanello]

I wouldn't differ the gain between loss or attentuation. It's a factor of your antenna.
The gain is a positive value. Only the Gain in dB can be negative as Gain_dB = 10*log(Gain), thus a negative Gain_dB means a Gain<1.

What antenna gain do you expect? There are lots of antennas with negative gain by design.
When you getting results far of that what you expect, you should check your measurement technique with standard dipoles as Jim said

 

[FvM]

The gain is a positive value. Only the Gain in dB can be negative

Of course. But it's already clearly mentioned in the original post, so everyone has taken this understanding as granted, I presume.

 

[DeboraHarry]

If your antenna is lossless by design (in other words not containing resitive elements), gain can be split into directivity and impedance mismatch.

I would disagree with that. The gain is generally defined in terms of the power absorbed by the antenna, not the power received at the antenna terminals. If an antenna has an impedance of 50 Ohms, and you use a 50 Ohm system, all the power will be absorbed by the antenna. But if you change the characteristic impedance of the cable to 1 Ohm, and the transmitter to 1 Ohm, far less power would be abosbed by the antenna. However, the antenna is unchanged - only the feed system is changed. Changing the feed system does not change the gain of the antenna.

A dipole has a gain of 2.15 dBi. That does not change if you use 50 Ohm feeder, 600 Ohm feeder.

Of course, if there is a large mis-match, then the antenna will absorb less power, and so radiate less. But the antenna gain is unchanged by this mismatch.

The gain is given by

Gain = directivity * efficiency.

where efficiency is

power radiated by antenna / power absorbed by the antenna.

mismatch losses can be very significant, but they don't change the antenna gain as such.

 

[FvM]

Changing the feed system does not change the gain of the antenna.

It does, as long as gain is referring to a nominal antenna impedance. If we asssume, that the gain is determined by a s21 meaurement, this will be the case. With the measurement method applied by the original poster, mismatch is affecting the observed "gain".

All specifications I'm aware of are based on nominal antenna impedance, usually 50 ohm. If you refer only to transmitted power as when applying Friis formula, there's of course no mismatch factor. In so far, I see your point.

P.S.: Reviewing Balanis, I read

According to the IEEE Standards, “gain does not include losses arising from impedance mismatches (reflection losses) and polarization mismatches (losses).”

So your correction is in fact claiming the commonly used term definition. We need to determine mismatch and gain separately in a measurement.

Thanks a lot!

 

[madebyagra]

SO When you are doing your calculations are you taking into consideration the losses by the cables and stuff. negative gain=attenuation.i exactly sure to tell you what's going wrong. what kind of attenuation did you see between the recieving and transmitting antenna with standard gain antenna.What is your frequency of operation.

Regards

my antena is 400 MHz - 2000 MHz range, i did'nt take attenuation and some loss into account, because it's VSWR and s11 (in measurement) show a good result, maybe there's something wrong in the measurement place

 

[flanello]

As stated, IEEE defines the gain without impedance mismatch and polarization losses. Additionally a realized gain is defined that includes the impedance mismatch loss (but not a polarization mismatch).
Other gain definitions include the loss form a impedance mismatch.

 

[madebyagra]

and this problem is solved, but anyone here can use a network analyzer to measure gain?

thanks

 

[elchiquito]

Sure can. what is it that you wanna do?

Regards

 

[RF_Jim]

and this problem is solved, but anyone here can use a network analyzer to measure gain?

thanks

As long as you have a suitable reference or baseline against which to base the measurement, such as a pair of standard gain 'horns' or reference antennas (in the old days: dipole antennas that could be adjusted as to length to make them resonant at test frequencies) to first calibrate the 'range'.

The one thing to consider, though, is at an outdoor 'range' (vs indoor) a network analyzer could be affected by other RF sources (since network analyzer is a broadband measuring device) and such interferes as paging transmitters, cellular/GSM base stations and subscriber equipment (phone), also LTE, and WiFi gear nowadays ... Agilent used to make a tuned "measuring receiver" for that purpose at outdoor ranges and also for RFI/EMI testing (some spurious can be stronger than others so a 'tuned' receiver is good to have vs broadband Spec An or Net An).

Jim