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lee85
 Post subject: how to increase the efficiency
Posted: Tue Sep 02, 2008 11:02 pm 
 
Captain
User avatar

Joined: Fri Dec 14, 2007 11:23 am
Posts: 9
Location: malaysia
i have design monopole antenna for 915mhz
however the efficiency only 38.7%. so many ways i have done and
change other design but i can't improve the efficiency.

i have limitation size for gnd plane and also size for monopole antenna
what should i do?please help me

regards,
lee85


 
   
 
nubbage
 Post subject:
Posted: Wed Sep 03, 2008 9:30 am 
 
General
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Joined: Fri Feb 17, 2006 12:07 pm
Posts: 236
Location: London UK
Hello lee
How are you measuring the efficiency?
What are you using as a comparison reference to define 100% efficiency?
What gain, polarization and bandwidth do you need?


 
   
 
lee85
 Post subject:
Posted: Wed Sep 03, 2008 11:06 pm 
 
Captain
User avatar

Joined: Fri Dec 14, 2007 11:23 am
Posts: 9
Location: malaysia
1. i measure the efficiency using the chamber

2. what do u mean comparison reference to define 100%

3. the gain 1dB it is linear polarization and the bandwidth 25MHz


 
   
 
nubbage
 Post subject:
Posted: Thu Sep 04, 2008 7:00 am 
 
General
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Joined: Fri Feb 17, 2006 12:07 pm
Posts: 236
Location: London UK
Hi lee
The 1dB gain, linear polar and the 25MHz (about 2.5%) bandwidth are all achievable with a variety of designs, such as normal mode helical quarter wave etc.
It was just you used the term "efficiency" and find that 38% is unacceptable. I wondered what your reference model is for 100% so I can see what basis you are using for acceptable performance.
When you say you are using a chamber, that does not describe the actual method. Are you plotting the E and H plane far-field patterns and integrating them in software as part of a suite, like Scientific Atlanta or similar, then accepting the given answer? Or are you comparing the radiated power with the input power to the antenna and computing the answer as a percentage?
As a rule of thumb, the ground-plane dimensions (critical for compact antennas) should be about lambda/3 by lambda/3 at least in order to achieve high efficiency. Anything smaller will greatly reduce the efficiency.
Have you checked the reflection coefficient and/or S parameters?




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