[SI-LIST] Re: :PCB Trace Antenna

Hi:

Loop antennas are good for low frequency applications when the sizes of the
antennas are much slower than the wavelength. They are normally low
efficiency due to the fact that they are not at resonance, where the power
will be radiated most efficiently.

For an antenna to resonate, it normally requires about 1/2 lambda
(wavelength). A typical example is a dipole antenna. However, many people
are using inverted-F antennas which can reduce the size down to about 1/4
wavelength. This is the best we can do for a resonating antenna with
reasonable efficiency and bandwidth.

For a thin dipole, the input impedance Zin is about 70 ohms at resonance in
free space. However, it will decrease with increased thickness. The
substrate's permittivity will also decrease it. The relationship should be
Zin_free_space / sqrt( Er_eff )

Regards,

----------------------------------------------------------------------------
Jian X. Zheng, Ph.D., Zeland Software, Inc.
48834 Kato Road, 103A, Fremont, CA 94538, U.S.A.
Tel: 510-623-7162, Fax: 510-623-7135
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> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Andrew Burnside
> Sent: Thursday, September 22, 2005 7:31 AM
> To: adsurevv@xxxxxxxxx
> Cc: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: :PCB Trace Antenna
> 
> 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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