[SI-LIST] Re: :PCB Trace Antenna
- From: "Andrew Burnside" <Andrew.Burnside@xxxxxxxxxxxxxxxxxxx>
- To: <adsurevv@xxxxxxxxx>
- Date: Thu, 22 Sep 2005 15:31:20 +0100
Virenda
Of course the dielectric underneath the antenna will effect your readings.
If you think about the path of the current, then there will be associated field
lines.
If you cut these field lines with anything, then you will change the impedance.
You can see an illustration of this if you bring a metal object or your hand
near the antenna, whilst making the measurement. Here you are loading the
antenna. Try and keep the antenna in free(ish) space, whilst making
measurements.
The smartcard antenna described earlier, is normally used as an inductance
loop. This is a near field antenna, so different considerations apply.
A dipole antenna is normally used as a far field antenna. Presumably you know
the operating frequency of the antenna.
As a check measure the traces and apply the velocity factor of the PCB trace.
Theory should actually tell you the centre impedance of the antenna.
In other words, yes the dielectric will affect the measured impedance, as this
is one of the factors that determines the velocity factor.
Bear in mind, that simple dipole impedances tend to be around 70 ohm, whilst
the impedance of your VNA will be 50 ohm.
I am sure that a quick web search will find you lots of articles on antenna
measurement. This is probably not the best place to ask, as the group is
oriented toward digital design.
Andrew
:I hava a similar issue (sort of). I have a printed
:antenna made by an engg. who doesn't work with as
:anymore. I am trying to measure the inductance of
:this antenna (dipole antenna not loop). This antenna
:is on a thin laminate. I am using a VNA to get the
:S11. Now the problem is depending on how and on what
:material i place the antenna i get very different S11
:readings. I have removed the tuning capacitor.
:Question 1: Is this the right way to measure the
:inductance of an antenna? or is there a better way to
:do it?
:Question 2: Should the dielectric underneath the
:antenna affect the inductance readings?
:as always, thanks in advance for your suggestions.
:regards,
:Virendra
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