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vanjer

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how to measure magnetic field strength of a loop antenna
Nov 29^th, 2008, 2:34am

Dear all,

Has anyone know how to measure magnetic field strength of a loop antenna using a spectrum analyzer, i.e. to know the resonance of the antenna ? I've heard that we can create a simple wire loop, connected to an SMA connector, then we can read it from Spectrum Analyzer. But how does it actually work? And if it can, why?

Thank you in advance

Would like to hear from you soon.

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pancho_hideboo

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Re: how to measure magnetic field strength of a loop antenna
Reply #1 - Nov 29^th, 2008, 2:51am

vanjer wrote on Nov 29^th, 2008, 2:34am:

to know the resonance of the antenna ?

If you want to know resonance frequency of antenna, you should use Network Analyzer.

vanjer wrote on Nov 29^th, 2008, 2:34am:

I've heard that we can create a simple wire loop, connected to an SMA connector, then we can read it from Spectrum Analyzer.

This is very common method for evaluating radiation spectum roughly.

vanjer wrote on Nov 29^th, 2008, 2:34am:

But how does it actually work? And if it can, why?

I can't understand a meaning of your question.
What do you confuse ?

Or you confuse an operation of unbalanced feeding loop antenna ?
http://www.emcesd.com/pdf/emc99-w.pdf

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vanjer

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Re: how to measure magnetic field strength of a loop antenna
Reply #2 - Nov 29^th, 2008, 3:04am

thank you for the reply. Sorry if my question sounds dumb.. just a new engineer started working ^^

for the wire loop, the bandwidth of the loop will have effect, right? So we can also measure the bandwidth with VNA can't we? Is it have a wide bandwidth?

One more thing, why does the loop need to be put with 50 ohm resistor in series?

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pancho_hideboo

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Re: how to measure magnetic field strength of a loop antenna
Reply #3 - Nov 29^th, 2008, 3:20am

vanjer wrote on Nov 29^th, 2008, 3:04am:

for the wire loop, the bandwidth of the loop will have effect, right? So we can also measure the bandwidth with VNA can't we? Is it have a wide bandwidth?

I can't understand a meaning of your question.
Explain correctly with measurement system diagram you are thinking.

You have two antennas. The above is which one, DUT antenna(TX) or antenna which is connected to spectrum analyzer(RX)?
In this case, DUT antenna also could be RX which is connected to spectrum analyzer and the other is TX which is connected to signal generator.

If you use network analyzer, there is only one antenna which is DUT.

vanjer wrote on Nov 29^th, 2008, 3:04am:

One more thing, why does the loop need to be put with 50 ohm resistor in series?

Which antenna you mention, DUT or the other ?

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« Last Edit: Nov 29^th, 2008, 11:31pm by pancho_hideboo »

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vanjer

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Re: how to measure magnetic field strength of a loop antenna
Reply #4 - Nov 29^th, 2008, 3:33am

sorry, i mean the wire loop attached with SMA (not DUT).

Is the wire loop can be measured by VNA as well? or, in other word, is the simple wire loop (not DUT) has certain bandwidth which can be measured by the VNA?

I attached the picture of the wire loop that given to me. Why the 50 must be present there?

Once again thank you so much..

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loop_sma.jpg

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pancho_hideboo

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Re: how to measure magnetic field strength of a loop antenna
Reply #5 - Nov 29^th, 2008, 3:36am

vanjer wrote on Nov 29^th, 2008, 3:33am:

Is the wire loop can be measured by VNA as well? or, in other word, is the simple wire loop (not DUT) has certain bandwidth which can be measured by the VNA?

Maybe you can evaluate it since antenna is far from matching(or resonance).

vanjer wrote on Nov 29^th, 2008, 3:33am:

I attached the picture of the wire loop that given to me. Why the 50 must be present there?

Maybe this antenna is used as TX which is connected to signal generator.
Then DUT antenna is connected to spectrum analyzer as RX.
Also this 50Ohm makes Q of antenna lower regardless of this antenna as TX or RX.
Characteristics of antenna is reciprocal regarding TX and RX.

Since small loop wire antenna is far from resonance, it's frequency characteristics is broad enough for evaluating DUT antenna.
This means a gain of small loop wire antenna is very small.
So you can evaluate DUT antenna by small loop wire antenna.

But if you evaluate only resonance frequency of DUT antenna, you should use network analyzer without using small loop wire antenna.

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« Last Edit: Nov 30^th, 2008, 1:47am by pancho_hideboo »

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vanjer

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Re: how to measure magnetic field strength of a loop antenna
Reply #6 - Nov 29^th, 2008, 3:51am

I know what is DUT.

now, I'm just referring to the loop probe. Nothing to do with any other antenna (DUT) at all. sorry for the misunderstanding.

my very initial question is how to measure a magnetic field strength of a loop antenna (DUT). The DUT is also a loop antenna - that's why the misunderstanding happen (it is a tag antenna for near field UHF RFID). Then I asked the possibility of using the loop probe to find the resonance.

Then you answered that the resonance of the DUT can be found by VNA -- resolved

Then it get mixed because then I'm curious about the loop probe. So the subsequent question is all about the loop probe. I just asked whether the loop probe itself has certain bandwidth that can be measured by VNA.

Sorry for misunderstanding once again. Thank you for your pdf file, very useful and I think I can get very useful information from there :D

Thank you!

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