why is the gain of the small antenna so small?

**joshuacp** · 24th July 2005, 14:31

Hello, everyone:
Nowdays, I'm designing a small antenna that operates at the frequency of 398MHz,linear polarization and the gain is 0dBi. I have searched lots lots of papers relating to the aspect of compact antennas, in which I find that the C-patch antennas can satisfy the size requirement. I followed a paper which is in the attachment. The paper has given most the dimensions of the antenna, except the location of the probe. According to the dimension and scale showed by Fig.3 and Fig.5 in the paper, I got the approximate location. With the dimensions by the paper and some changes of the location of the probe, I got good reflection coefficient results, and the resonant frequencies are approximately the same with the results given by the paper. But the gain performances(about -10dBi and -20dBi) of both the antennas are so poor that the antennas couldn't be used in actual systems while the directivities are still very good. Could you please tell me the reason of the poor gain performance? Is it because of the surface wave? Because the paper gives the reflection coefficient performances without the gain performances, I wonder whether this kind of C-patch antennas has been used in actual systems.
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In the attachment, there are the paper and two geo files performed in IE3D 10. The dielectric constant of the substrate is 4.3, and the height is 1.59mm.
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Added after 6 minutes:

Because the network in our Lab. today is not good. I'm sorry that I couldn't attach the paper with the post. The title's name is
Compact microstrip antennas for personal mobile communication.
You can find it in the IEEE datebase.

**armsgroup** · 24th July 2005, 14:33

max gain of the antenna will occure in case of half wave lenght and for ur freq will be nearly 38cm so we lose gain when designing with lenght lower than half wave

**vfone** · 25th July 2005, 01:34

Here you can get some ideas how to make a small antenna more efficient:
http://www.w8ji.com/radiation_resistance.htm

**joshuacp** · 25th July 2005, 11:46

Because the antenna efficiency is very low, about 1% by the simulation results of IE3D 10. It's that 1% of the input energy is radiated, then where does the other 99% of the input energy go? Could anyone tell me? I followed the file, Compact microstrip antennas for personal mobile communication, which can be found in the IEEE database, the antenna has been reported, but if the efficiency is so poor, then could this kind of antenna be used in actual systems?

**House_Cat** · 26th July 2005, 04:50

The energy not radiated by the antenna is dissipated in heating the transmission line and series components between the final amplifier and the antenna, such as baluns and coils.

Any energy not radiated ends up as potentially damaging heat.

**joshuacp** · 28th July 2005, 01:57

the attached files

**asdfaaa** · 28th July 2005, 04:46

Q factor of antennat is related to the longest dimension. you may refer to those papers for more details,

Quality factor of general ideal antennas Fante, R.; IEEE Transactions on Antennas and Propagation [legacy, pre - 1988] Volume:17, Issue:2, Page:151 - 155, Mar 1969

The complex Poynting theorem: reactive power, radiative Q, and limitations on electrically small antennas Grimes, D.M.; Grimes, C.A.; IEEE International Symposium on Electromagnetic Compatibility, 1995. Symposium Record. 1995 Pages:97 - 101, 14-18 Aug. 1995

Regards,

**wusen** · 29th July 2005, 08:07

Originally Posted by joshuacp

Because the antenna efficiency is very low, about 1% by the simulation results of IE3D 10. It's that 1% of the input energy is radiated, then where does the other 99% of the input energy go? Could anyone tell me? I followed the file, Compact microstrip antennas for personal mobile communication, which can be found in the IEEE database, the antenna has been reported, but if the efficiency is so poor, then could this kind of antenna be used in actual systems?

The other 99% of the input energy is consumed due to dielectric loss, metallic loss and surface wave loss. It is especially serious in the electrical small antenna.

you can try to change the parameters described below and adjust the feed point and the size of slot at the same time to optimize the vswr.
first, change the Loss Tangent from 0.02 to 0, the radiation efficiency may increase to 3%~7%;
then, chang the Real part of Conductivity from 4.9e7 to 4.9e20, it may increase to 10% further;
last, change the Epsr from 4.3 to 1, you will get a 100% radiation efficient.

**joshuacp** · 31st July 2005, 02:07

The content of my discussion is in the attachment.

**joshuacp** · 31st July 2005, 03:34

Hello Wusen, thank you very much. With your advice, I've tried to use the air as the substrate with the height of h=10mm. The size is the same as that in the attached file above. By slightly changing the position of the probe. A good gain result had been abtained. The simulation results obtained using IE3D is: f=1.6GHz, Incident Power is 0.01W, Input power is 0.00941473W, radiated power is 0.00891789 W, radiation efficiency is 94.7227% and antenna efficiency is 89.1789%.

So to design an antenna with these sepcifications:
1) size of 70*70*5mm( 0.09 by0.09 by0.0066 ),
2) center frequency is 433MHz:
3) linear polarization
4) gain>0dBi
5)1% of bandwidth(1-4MHz)
I will try to make a tradeoff between the size and the gain.

I will give my sincere thanks to all the people who took part in the discussion and visited this post.

why is the gain of the small antenna so small?

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