A simple antenna is
described that is easily made from coaxial cable and is useful for
troubleshooting purposes. It is balanced and does not require a
balun to be fed from coaxial cable. Such an antenna has many uses
around the lab.
: A simple dipole antenna can be made from the
stripped end of coaxial cable. This type of antenna is used in some
wireless products and can be used to expose equipment to simulated
fields from wireless devices. Figure 1 shows an example of such an antenna.
It is made by stripping the cable to expose a quarter of a wavelength
of center conductor while a quarter wavelength of the shield is folded
back over outer covering of the coax cable.
To understand how the antenna works, Figure 2 shows the details of
the antenna. In Figure 2, the shield is folded back over itself for a
quarter of a wavelength. The folded shield is insulated from the
original shield underneath by the outer jacket of the coax. This is
done in practice by taking a pointed
object, such a small bladed screwdriver, and unbraiding the exposed
that it is composed of parallel wires. The wires are then folded back
as shown in Figure 2 of my September 2004 Technical Tidbit, Mobile Phone Response to EMI from Small Metal ESD
. The antenna in Figure 1 has been improved by wrapping the shield wires with copper foil tape
to insure a continuous surface.
Drawing of Coaxial Antenna Showing Current Flow
red arrows in Figure 2 show the direction of the currents that flow. Inside of
the coax, the center conductor current is matched by an equal current
returning on the inside surface of the shield. For convenience, I have
shown the shield current as a single line of arrows inside of the
shield. In fact, the current is spread equally over the inside surface
of the shield. When the currents reach the end of the shield, the
center conductor current continues out onto the exposed center
conductor. Since the exposed center conductor is one quarter of a
wavelength long, a standing wave exists on it causing
a maximum of current as the center conductor exits the shield and zero
current at its end.
The shield current folds back and flows on the outside of the copper
foil covering the shield wires. Note that this current is flowing in
the opposite direction as the current on the inside surface of the
original coax shield.
Here's where it gets interesting. Skin effect causes the two surfaces
of a thin metal sheet, such as a cable shield, to be quite
independent at high frequencies as far as current flow is concerned.
The outside surface of the shield of
the original cable forms a "center conductor" with respect to
the inside of the folded back shield with its copper foil covering.
This parasitic coax is
one quarter of a wavelength long and is shorted at the end where the
center conductor exits. This makes the impedance looking into the other
where the folded shield stops nearly infinite. Thus as far as the
current flowing on the outside of the folded shield is concerned, there
is no inside surface of the folded shield at all! The impedance looking
there is nearly infinite, so the folded shield ends appearing to have
no inner surface and thus the folded shield has the same
standing wave on it as does the extended center conductor. The current
on the folded shield is maximum at the folded end and zero at the other
end. It does not enter the parasitic coax formed by the original cable
shield and the inside surface of the folded back shield.
If the antenna is precisely cut there will be no common mode current
flowing back on the outside of the cable towards the source. Just to be safe, the
cable in Figure 1 has several ferrite cores placed on it. This allows
for some error in cutting the length of the folded back shield.
The antenna in Figure 1 was cut for the upper mobile phone band, around
1800 MHz. As shown in Figure 1, it is a little longer than needed (it
easier to trim an antenna shorter than to make it longer). To
calculate one quarter wavelength divide 300 by the frequency in MHz and
multiply the result by 1/4. The result is then multiplied
by 0.8. This is an estimated factor because wave travels
slower on the cable than in free space. The antenna is then trimmed to
the exact length by measuring its reflected power at the frequency of
interest. When properly tuned, the reflected power will be minimized.
One can use this antenna to subject a device to the expected field of a
wireless device such as a mobile phone. Just feed the antenna the
correct power level (600 mW for a mobile phone at its maximum transmit
power). The input impedance of this antenna is close to 70 Ohms at resonance so if 50 Ohm
cable is used, allow for the mismatch reducing the actual transmitted
power. Holding this antenna close to another device will show if a
nearby mobile phone will cause problems. For the specific case of mobile phones, one antenna will
need to be made for each frequency band to be tested.
A few final cautions are in order. If you are using an antenna like the
one described in this article for troubleshooting radiated immunity
problems, be careful not to cause interference or use so much power as
to be dangerous. Performing the test in a shielded chamber is
desirable. When I am troubleshooting with this antenna, it is usually
done at very close ranges to simulate proximity to a mobile phone. An
anechoic chamber may not be needed for this case since the path length
for reflections off the walls is so much greater than the few
centimeter or less distance between the antenna to the product. A plain
shielded chamber may be adequate.
Don't forget safety! Power levels above WiFi and mobile phone levels can be dangerous under some conditions.
A simple antenna can be made from coaxial cable. This antenna is useful
in subjecting electronic devices to simulated radiation from a nearby
wireless device such as a mobile phone.
tutorial on this subject, covering background as well as more technical details, is available at:
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