ANTENNA CONSIDERATIONS (CONT.)
COMMON ANTENNA STYLES
There are literally hundreds of antenna styles that can be successfully employed with the
KH Series. Following is a brief discussion of the three styles most commonly utilized in
compact RF designs. Additional antenna information can be found in Linx application notes
#00100, #00126, #00140 and #00500. Linx also offers a broad line of antennas and
connectors that offer outstanding performance and cost-effectiveness.
A receiver antenna should give its optimum performance at the frequency or in
the band for which the receiver was designed, and capture as little as possible
of other off-frequency signals. The efficiency of the receiver’s antenna is critical
to maximizing range-performance. Unlike the transmitter antenna, where legal
operation may mandate a reduction in antenna efficiency or attenuation, the
receiver’s antenna should be optimized as much as is practical.
It is usually best to utilize a basic quarter-wave whip for your initial concept
evaluation. Once the prototype product is operating satisfactorily, a production
antenna should be selected to meet the cost, size and cosmetic requirements of
the product. To gain a better understanding of the considerations involved in the
design and selection of antennas, please review application note #00500
“Antennas: Design, Application, Performance".
Whip Style
A
whip-style monopole antenna provides outstanding overall
performance and stability. A low-cost whip can be easily fabricated from
wire or rod, but most product designers opt for the improved
performance and cosmetic appeal of a professionally made model. To
meet this need, Linx offers a wide variety of straight and reduced-height
whip-style antennas in permanent and connectorized mounting styles.
The following notes should help in optimizing antenna performance:
1. Proximity to objects such as a user’s hand or body, or metal objects will cause
an antenna to detune. For this reason the antenna shaft and tip should be
positioned as far away from such objects as possible.
The wavelength of the operational frequency determines an antenna's
overall length. Since a full wavelength is often quite long, a partial 1/4-
wave antenna is normally employed. Its size and natural radiation
resistance make it well matched to Linx modules. The proper length for
a 1/4-wave antenna can be easily found using the formula below. It is
also possible to reduce the overall height of the antenna by using a
helical winding. This decreases the antenna's bandwidth but is an
excellent way to minimize the antenna's physical size for compact
applications.
2. Optimum performance will be obtained from a 1/4- or 1/2-wave straight whip
mounted at a right angle to the groundplane. In many cases this isn’t desirable
for practical or ergonomic reasons; thus, an alternative antenna style such as
a helical, loop, patch, or base-loaded whip may be utilized.
1/4-wave wire lengths
for KH frequencies:
Where:
234
315Mhz
418Mhz
433Mhz
=
=
=
8.9"
6.7"
6.5"
3. If an internal antenna is to be used, keep it away from other metal
components, particularly large items like transformers, batteries, and PCB
tracks and groundplanes. In many cases, the space around the antenna is as
important as the antenna itself.
L = length in feet of quarter-wave length
L =
F
F = operating frequency in megahertz
MHz
4. In many antenna designs, particularly 1/4-wave whips, the groundplane acts
as a counterpoise, forming, in essence, a 1/2-wave dipole. For this reason
adequate groundplane area is essential. The groundplane can be a metal
case or ground-fill areas on a circuit board. Ideally, it should have a surface
area the overall length of the 1/4-wave radiating element. This is often not
practical due to size and configuration constraints. In these instances a
designer must make the best use of the area available to create as much
groundplane in proximity to the base of the antenna as possible. When the
antenna is remotely located or the antenna is not in close proximity to a circuit
board plane or grounded metal case, a small metal plate may be fabricated to
maximize antenna performance.
Helical Style
A helical antenna is precisely formed from wire or rod. A helical antenna
is a good choice for low-cost products requiring average range-
performance and internal concealment. A helical can detune badly in
proximity to other objects and its bandwidth is quite narrow so care must
be exercised in layout and placement.
Loop Style
A loop- or trace-style antenna is normally printed directly on a product's
PCB. This makes it the most cost-effective of antenna styles. There are
a variety of shapes and layout styles that can be utilized. The element
can be made self-resonant or externally resonated with discrete
components. Despite its cost advantages, PCB antenna styles are
generally inefficient and useful only for short-range applications. Loop-
style antennas are also very sensitive to changes in layout or substrate
dielectric, which can introduce consistency issues into the production
process. In addition, printed styles initially are difficult to engineer,
requiring the use of expensive equipment, including a network analyzer.
An improperly designed loop will have a high SWR at the desired
frequency that can introduce substantial instability in the RF stages.
5. Remove the antenna as far as possible from potential interference sources.
Any frequency of sufficient amplitude to enter the receiver’s front end will
reduce system range and can even prevent reception entirely. Switching
power supplies, oscillators, even relays can also be significant sources of
potential interference. The single best weapon against such problems is
attention to placement and layout. Filter the module’s power supply with a
high-frequency bypass capacitor. Place adequate groundplane under
potential sources of noise. Shield noisy board areas whenever practical.
Linx offers a low-cost planar antenna called the “SPLATCH,” which is an
excellent alternative to the sometimes problematic PCB trace style. This
tiny antenna mounts directly to a product's PCB and requires no testing
or tuning. Its design is stable even in compact applications and it
provides excellent performance in light of its compact size.
6. In some applications it is advantageous to place the receiver and its antenna
away from the main equipment. This avoids interference problems and allows
the antenna to be oriented for optimum RF performance. Always use 50
coax, such as RG-174, for the remote feed.
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