FreeLists / si-list / [SI-LIST] Re: PCB Trace Antenna

["Zabinski, Patrick J." <zabinski.patrick@xxxxxxxx>]

Boopathy,

From a general sense, antennas are most efficient when
their resonant frequency matches that of the desired 
center frequency.  Accordingly, you'll likely want the
antenna to resonate at 13.5 MHz.

In contrast, you mention a need for a 50 ohm impedance.
My guess is that there's an amplifier connected to
the antenna that is specified to operate in a 50
ohm environment, which you interpret as a need for
a 50 ohm impedance of the trace.

While it makes logical sense to make the antenna look
like 50 ohms with respect to the amplifier, be sure not
to terminate the antenna with a 50 ohm resistive termination.
Also, the 50 ohm environment does not necessitate
the need for 50 ohm transmission lines.
If you do so, the antenna structure will not resonate,
and the antenna's efficiency will drop dramatically.

In place of a 50 ohm transmission lines and 
resistive termination, it's most often
best to use RF analysis tools (e.g., simulation tools or
simple Smith charts) to add tuning stubs and/or 
LC-type impedance-matching networks.  I cannon
readily explain this process in simple/short
e-mail, but instead suggest you search
google or IEEE papers for "impedance matching,"
"impedance tuning," or "tuning stubs."

Outside of impedance matching, you'll need to tune
the overall antenna structure to resonate at 13.5 MHz.
If you use a simple, straight microstrip, the structure
will be about 18 feet (6 meters) long in common FR4
dieletric.  My guess is that your board will not be
this long.  :o)

There are a few common ways of obtaining resonance
with much shorter structures.  One common way is to
use loop antennas (per suggestion by Istvan),
where a trace is routed in a series of spiral-loops
on the board.  In effect, the structure will act
as an inductor with a small parasitic capacitance.
If you add enough loops/coils, you'll be able to get 
to 13.5 MHz.  

Another option is use a combination of a loop structure
with a capacitive load on the antenna's near end (i.e.,
next to the amplifier).  By using an EM tool, you
can estimate the effective inductance of the loop
structure, then use simple resonance formulas (e.g.,
F = sqrt(1/2*pi*L*C) to determine what capacitance
you need.  By using a combination of the physical
loop and lumped capacitance, the overall structure can
be much smaller.

One point to note: as you reduce the physical size of
the antenna structure, the antenna's apeture will be 
reduced, which will impact the effective gain
of the antenna.  If your communications link has minimal
margin, I suggest you make the loop as physically
large as you have space for (check out "Friis transmission
formula" for details/ideas).

Regards,
Pat


> Dear Istvan,
>    I need to Create an PCB trace antenna of 
> F=13.5Mhz, Impedence=50 ohms, L=1.2uH.
> Now I need to determine PCB trace length (end to end), width 
> of the trace
> etc. Is there is any calculator available to determine it?.
> 
> Right Now we have a plan to complete the placement & routing 
> of the board
> which will lie inside the loop antenna. Is it a right design 
> to do it?? 
> 
> Otherwise, I need to Make two separate PCB (one for component 
> & another one
> for loop antenna). 
> 
> Regards,
> Boopathy J
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