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Printed Antenna - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement


rf_sd77
 Post subject: printed antenna
Posted: Fri Jun 11, 2004 2:23 pm 
 
Captain

Joined: Fri Jun 11, 2004 2:17 pm
Posts: 5
I am fairly new into antenna design as I have been mainly a RF guy for 4 years working on TX and RX circuits for cellular products and have ventured into the 2.4GHZ area now.

Question is I am trying to figure out the main advantages of having an antenna at 200ohms versus a 50ohm for 2.4Ghz.

From the tx or rx path I have a balun that converts the signal from 50ohm to 200ohm (this is from previous designer who left and i took over and he is overi n japan now). I have worked with cellular antenna and have always matched to 50ohms, just curiuos why for a 2.4GHz antenna it would be 200ohms.

any insight on starting this desing would be greatful as I am the only RF guy here and I am getting some antenna design books next week. Thanks for responses.


 
   
 
K5DVW
 Post subject:
Posted: Fri Jun 11, 2004 2:51 pm 
The only reason that I can come up with is... that impedance (200 ohms) is just what that specific antenna type looks like. As you know, not all antennas are 50 ohms driving impedance, infact most arent, so you have to match to them. It just turns out that some standard kinds of antennas, like monopoles have a pretty low impedance to start with and can be used in 50 ohm systems without matching. Some patch antennas can look like 200 ohms due to the physics of the antenna.

In other words, there is no advantage to having a 200 ohm termination impedance specifically, but the antenna impedance is more driven by the type antenna and what kind of performance you need from an antenna pattern point of view. Matching is often secondary.

If you have a resistive 200 ohms, then consider yourself lucky, that's an easy match.

Good luck,

Darrin


 
  
 
rf_sd77
 Post subject: thanks
Posted: Fri Jun 11, 2004 5:50 pm 
 
Captain

Joined: Fri Jun 11, 2004 2:17 pm
Posts: 5
thanks for the insight, I had the feeling that was the reasoning, just was not sure.

I appreciate the quick response.


 
   
 
Chris
 Post subject:
Posted: Wed Jul 07, 2004 3:05 pm 
Also, just to add to the excellent comments, most antennas if not all, have reactance so the actaul impedance you are considering needs to take into account R + jX for matching to your system or calculating radiated power. If X is not minimized then radiated power is reduced. Of course there are other variables to consider in antenna design.



Posted  11/12/2012

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