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Gain of a small Antenna

 
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gelunmak
Antenna Theory Regular


Joined: 05 Jun 2009
Posts: 17
Location: Hong Kong

PostPosted: Mon Aug 03, 2009 9:12 am    Post subject: Gain of a small Antenna Reply with quote

Let say i've a small antenna which usually is a multi-pol antenna with high cross-pol (i.e. say neither pure-vertical nor pure-horizontal pol.) across the bandwidth.

How would you describe the gain of this antenna after a complete 3D measurement? (i.e. Will you take Gain_total or Gain_theta or Gain_phi or Gain_max-pol?)

Thanks
-gelunmak Cool
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bigSteve
Antenna Wizard


Joined: 14 Mar 2009
Posts: 265

PostPosted: Tue Aug 04, 2009 4:18 am    Post subject: Gain of a non-standard polarized antenna Reply with quote

Gelunmak,

Pure linear polarization, as is the case for dipole antennas, doesn't always happen with real antennas. For instance, mobile or cell phone antennas are usually crammed into a small section of the device. As a result, their polarization typically does not have a single component; in fact it varies between theta and phi-polarized all over the radiation pattern, and with frequency as well.

As a result, radiation patterns are typically measured as the magnitude of the gain. This is particularly the case for hand held devices, since they do not have a fixed orientation, it wouldn't make sense to design them for a fixed polarization.

Note that you can still view patterns as a Gain_theta and Gain_phi and Gain_total radiation patterns. Sometimes looking at the individual components can shed light on how exactly the antenna is radiating. So even in mixed-polarized antennas where the polarization isn't so important, it can sometimes be useful to look at the individual patterns.
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gelunmak
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PostPosted: Tue Aug 04, 2009 6:20 am    Post subject: Reply with quote

I pretty much understand the performance of such small antenna with multi-pol. And so i realize they are different from those standard-size or large antenna (like base station array) in term of gain.

Like for standard single dipole with gain 2.15 dBi. I believe the Gain_Total are pretty much the same as Gain_Theta becasue the cross-pol (gain_phi) are very low.

Another example is a Vertica-pol patch antenna array with low-cross pol, again, we can choose Gain_Total to describe the antenna gain.

Therefore, we can simple pick Gain_total to describe the gain of such large or standard size antenna.

My Question was can i still pick Gain_total to describe the gain of small antenna?

My answer is "Yes, we should choose Gain_total" even though people may argue that the small antenna is multi-pol over the sphere.

Actually, I believe it's more meaningful to use "Efficiency" to describe the performance of a low-gain small-antenna and use "Gain" for high-gain large-antenna. Agree !? Cool
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admin
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Joined: 03 Jan 2007
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PostPosted: Wed Aug 05, 2009 3:50 am    Post subject: antenna efficiency Reply with quote

Efficiency is a great parameter because it describes how well an antenna is behaving like an antenna. An ideal antenna would convert all the power at its terminals to radiated power, an efficiency of 100%.

So yes, if you don't care about having the power go in aparticular direction, efficiency is a great metric to use.
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gelunmak
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Joined: 05 Jun 2009
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Location: Hong Kong

PostPosted: Wed Aug 05, 2009 8:37 am    Post subject: Reply with quote

Yup, Efficiency is great. So we should target for high-efficiency small-antenna instead of high-gain small antenna (assuming we're targeting Omni-pattern, which is always true for small-antenna.)

Bty, if people insist to ask for the "gain" of small antenna, will you just simply pick the Gain_total as the "gain" of the antenna?

Theoretically every antennas, no matter what size, what kind, what polarization, ... , must have their own value of "gain"! (Just like every antennas must have efficiency between 0% - 100%). So i suppose there's only one value for my question (either Gain_total or Gain_max pol. after measurement)

Another supporting point is when we look at the definations or equations of Gain, Efficiency, and Directivity. It's all related to power, not polarization.

Will you go for my view ... Gain_total?
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admin
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PostPosted: Wed Aug 05, 2009 5:38 pm    Post subject: gain Reply with quote

Yes, gain should be measured as total gain unless specifically stated otherwise, so I agree.
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gelunmak
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Joined: 05 Jun 2009
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PostPosted: Thu Aug 06, 2009 6:27 am    Post subject: Reply with quote

Thanks for Admin's reply and i like the statement "gain should be measured as total gain unless specifically stated otherwise".

So for the case of circular polarization antenna, it's either LH or RH, it means the polarization is specifically stated, so we choose Gain_RHCP (say for GPS antenna) instead of Gain_total.

Thanks
- gelunmak
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