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Dipole antenna impedance matching at a frequency other than the resonant frequency

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srijan.rio

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Dipole antenna impedence matching at frequency other than resonant frequency

Hi all,

I have a confusion regarding impedance matching. Suppose I design a planar dipole antenna for 5 GHz and 50 ohm impedance.
Now if I want to use the same antenna at 1 GHz is it theoretically possible to match the input impedance of the same antenna @ 1 GHz and get good results in return loss and other parameters ?
Apologies if I am missing something obvious.
 

Re: Dipole antenna impedence matching at frequency other than resonant frequency

Supposed 5 GHz is the fundamental dipole frequency, the input impedance at 1 GHz will be mostly capacitive with a very small real part (radiation resistance). This means a matching circuit for 1 GHz can be implemented only with high Q respectively very small bandwidth.
 
Re: Dipole antenna impedence matching at frequency other than resonant frequency

Thank you for your reply.
Ok so theoretically its possible then?
That means if I get the impedance result for the dipole with 5 GHz fundamental frequency for other different frequencies(say 1 GHz) by HFSS simulation, and set the terminal port impedance to that value, should the antenna resonate with same results at that particular frequency( 1GHz). Like should i get the same dip in return loss or it will be different.
 

Re: Dipole antenna impedence matching at frequency other than resonant frequency

srijan.rio said:
Thank you for your reply.
Ok so theoretically its possible then?
That means if I get the impedance result for the dipole with 5 GHz fundamental frequency for other different frequencies(say 1 GHz) by HFSS simulation, and set the terminal port impedance to that value, should the antenna resonate with same results at that particular frequency( 1GHz). Like should i get the same dip in return loss or it will be different.
Click to expand...

not likely

impedance matching is one thing, resonance is another. Its performance is likely to be dreadful

D
 
Re: Dipole antenna impedence matching at frequency other than resonant frequency

you CAN have a non resonating radiating structure. Like a small loop that is driven by a current source. It will be very inefficient though, but will work very broadband simply because you are not resonating it.

with a loop, you can just make it out of thick conductor and pump a LOT of RF current into it. For a electrically-short dipole, you would have to feed it with a very high RF voltage....which would probably be much less practical, and possibly dangerous to the user.
 
Re: Dipole antenna impedence matching at frequency other than resonant frequency

It all depends on the Q: if the antenna is low Q by design then it will have a small resonance even at 1 GHz. Of course efficiency will suffer but that is the price for having a broad band antenna.
 
Re: Dipole antenna impedence matching at frequency other than resonant frequency

If all you want is a match put an attenuator in series with the antenna. A -5 dB pad will result in a -10 dB or better s11. That is theoretically and practically possible but then as was mentioned above you will need a sense of humor about the performance otherwise.

Do you have a sense of humor?
 

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