RF Module External Antenna Design

Antenna Length Table

INFORMATION: The table below has listed several common RF transmitter frequencies and their matching antenna length, the best antenna length is calculated by divide the wavelength by four.

NOTE: Best antenna length equals to a quarter wavelength, the values for commonly used frequencies are listed above for your quick reference.

Basic Antenna – Whip

An antenna can be defined as any wire, or conductor, that carries a pulsing or alternating current. Such a current will generate an electromagnetic field around the wire and that field will pulse and vary as the electric current does. If another wire is placed nearby, the electromagnetic field lines that cross this wire will induce an electric current that is a copy of the original current, only weaker. If the wire is relatively long, in terms of wavelength, it will radiate much of that field over long distance.

 

The simplest antenna is whip. This is a quarter wavelength wire that stands above a ground plane. The most common examples are found on automobiles and are used for broadcast radio, CB and amateur radio, and even for cellular phones. This design goes back to the 1890’s when Marconi set out to prove that radio signals could travel long distance. To achieve the goal, he had to stretch a long wire above the ground. Due to the low frequencies, thus a long wavelength, the wire had to be long. He also found that the wire worked better when it was high above ground.

All antennas, like any electronic component, have at least two connection points. In the case of the whip, there must be a connection to ground, even if the ground plane area is nothing more than circuit traces and a battery. The whip and ground plane combined to form a complete circuit. The electromagnetic field is set up between the whip and the ground plane, with current flowing through the field, thus completing the circuit. Ideally, a ground plane should spread out at least a quarter wavelength, or more, around the base of the whip. The ground plane can be made smaller, but it will affect the performance of the whip antenna. The ground plane area must be considered when designing an antenna.

A quarter-wave whip is not a compact antenna. At 1 MHz, in the AM Broadcast band, one quarter of the wavelength is about 246 feet, or 75 meters. At 100 MHz, in the FM Broadcast Band, it is nearly 30 inches (75 cm). This dimension continues to shrink at higher frequencies, being nearly 3 inches (7.5 cm) at 1000 MHz. A simple formula for the quarter-wave (in cm) is 7500 divided by the freq. (in MHz), or for inches 2952 / freq. (in MHz). This formula is only a starting point since the length may actually be shorter if the whip is overly thick or wide, has any kind of coating, or is not fed close to ground. It may need to be longer if the ground plane is too small.

The length of the antenna should be measured from the point where it leaves close proximity to ground, or from the transmitter output. If a whip is mounted on a box, and connected to the transmitter with plain wire, that wire becomes part of the antenna. To avoid tuning the antenna improperly, coaxial cable should be used to connect to an external antenna. On a circuit board, the equivalent to coax is a trace that runs over a ground plane (ground plane on the backside). The above are examples of transmission lines, whose purpose is to efficiently transfer power from one place to another with minimum loss. Do not try to run an antenna line too close to ground, it becomes more of a transmission line than an antenna. Fortunately for those who need a small remote device, a transmission line left open-ended will radiate some energy.

Compact Antenna – Short Whip

A simple alternative to the whip is to make it shorter than a quarter wavelength and add an inductor near the base of the whip to compensate for the resulting capacitive reactant. The inductor can be made by coiling up part of the whip itself. This type of antenna can have performance nearly equal to that of a full size whip.

The short whip design is optimized for undersized ground planes. When tested on the edge of a small board, gain was only 3 to 4 dB less than a fully sized whip and ground plane.

 

Compact Antenna – Helical

This is similar to a spiral that is not flattened. Start with a piece of wire that is 2 or 3 times longer than a whip and wind it into a coil. The number of turns on the coil will depend on wire size, coil diameter, and turn spacing. The coil will need to be cut to resonate, and can be fine tuned by stretching the length of the coil. If the coil is wound tightly enough, it may be shorter than one-tenth of a wavelength. This antenna tunes sharply, requiring care in tuning. The real part of the antenna impedance is less than 20 ohms, and depends on the size of the coil and its orientation to ground.

 

For 433.9 MHz, we wound 14 turns of 22 gauge wire around a 0.25 inch (6 mm) form. When tuned, its length was just under one inch. The proximity of this coil to ground makes a big difference in performance. When the coil runs near and parallel to ground, maximum gain is only -18 dBd. When the loose end of the coil was pulled away from ground, as shown in the alternate version of drawing, the gain increased to -5.5 dBd, and the null became deeper.

The big problem with this antenna is the mechanical construction and it’s bulky size. It can be easily de-tuned by nearby objects, including hand, so it may not be good for hand-held use.

Enclosures

An antenna should not be located inside a conductive, or metal enclosure. Care should be taken to keep the antenna away from metal surfaces. If a conductive area is large in terms of wavelength (one half wave or more), it can act as a reflector and cause the antenna not to radiate in some directions. If a metal box is used as enclosure, an external antenna is required.

Testing and Tuning

Antennas may seem to be a mystical art. Unlike many electronic devices, any change in nearby materials or dimensions can affect antenna performance. Trying to build a published design does not guarantee results. Testing an antenna design is necessary, tuning is usually required.

A network analyzer is normally used to test the impedance or VSWR of the antenna. Some antennas that have an impedance near 50 ohms can be tuned by looking at return loss or a VSWR display. Low impedance antennas may require the use of a Smith Chart display to get good results. In this case, the antenna should be tuned to a point near the pure resistance line.

There are other options, such as a spectrum analyzer with a tracking generator, which can be used with a directional coupler. The coupler will feed power to the antenna while feeding the reflected power from the antenna back to the analyzer. The coupler must have an isolation between the Generator and RF Input Port of 20 dB or more. Calibration is done by noting the power readings with a 50 ohm load connected and then unconnected. Using this technique, return loss can be measured. If the antenna is near 50 ohms, the return loss back to the RF input port will be high, due to the antenna absorbing most of the power. A good antenna will show as a dip on the screen at the correct frequency. A dip of only 3 or 4 dB (about 5:1 VSWR) is normal for a low impedance antenna measured on a 50 ohm analyzer. A dip of 9 dB (about 2:1 VSWR) or more indicates a well-matched antenna in a 50 ohm system. If the dip is not centered at the right frequency, the antenna length or tuning needs to be adjusted.

Antenna measurements of any kind are tricky since the antenna is affected by nearby objects, including the size and shape of the circuit board, and even by the cable connections to the network analyzer. Pass your hand close to the antenna and the dip should move around a little. If it does not, the antenna may not be connected properly. Antennas which are ground plane sensitive may see all additional wires as an extension of that ground. Try wrapping your hand around the cable that goes to the analyzer. If the measurement changes much, you may need to try a different tactic. One possibility to minimize RF currents on the cable is to put a few good high frequency ferrite toroid or some absorptive material over the cable.

The best way to fine tune a remote transmitter antenna is by using the transmitter itself. Put an antenna on a spectrum analyzer and try to keep other large metal objects out of the way. Find a place to locate the transmitter that is away from metal and a few feet away from the analyzer. Always locate the transmitter in the exact same spot when testing. If you have a desk that is wood, mark its position with a pencil or tape. If hand held, hold it in your hand just above the marking on the desk. Be sure to position your hand and the rest of your body the same way during each test. Take a reading of the power level, and tune the antenna to achieve maximum radiated power. The same thing can be done for a receiver. Transmit a signal to it, and adjust the antenna to receive the lowest signal level from the generator.

Common problems with antennas usually involve insufficient free space around the antenna. The antenna can not run close to ground or any other trace without affecting the antenna performance. This includes traces on the other side of the board, batteries, or any other metal object.

Receiver performance can be degraded by digital circuits. Digital switching is very fast and creates high frequency noise that can cause interference. Keep receiver antennas away from digital circuit traces. Try to keep digital traces short, and run them over a ground plane to help confine the electromagnetic field that is generated by the digital pulses. If an external antenna is used, then use a coaxial cable.

A transmission line for G-10 material which is 0.06 inch thick requires a trace width of one-tenth inch, half of which for a 0.03 inch thick board. This results in a 50 ohm transmission line that will carry RF with minimum loss and interference.

High static voltage may damage sensitive semiconductors or SAW resonator. We recommend placing an inductor between the antenna and ground to short out any static voltages. For the 400 MHz region, a value near 200 nH is a good choice. At 916 MHz, a more appropriate value may be 100 nH.

Terminology

Wavelength – Important for determination of antenna length, this is the distance that the radio wave travels during one complete cycle of the wave. This length is inversely proportional to the frequency and may be calculated by wavelength in cm = 30,000 / frequency in MHz.

Ground plane – A solid conductive area that is an important part of RF design techniques. These are usually used in transmitter and receiver circuits. An example is where most of the traces will be routed on the topside of the board, and the bottom will be a mostly solid copper area. The ground plane helps to reduce stray reactants and radiation. Of course, the antenna line needs to run away from the ground plane.

dB (decibel) – A logarithmic scale used to show power gain or loss in an RF circuit. +3 dB is twice the power, while -3 dB is half the power. It takes 6 dB to double or halve the radiating distance, due to the inverse square law.

Antenna Characteristics

Gain – An antenna that radiates poorly has low gain. Antenna gain is a measure of how strongly the antenna radiates compared to a reference antenna, such as a dipole. A dipole is similar to a whip, but the ground plane is replaced with another quarter-wave wire. Overall performance is about the same. An antenna that is 6 dB less than a dipole is -6 dBd. This antenna would offer one half the range, or distance, of the dipole. Compact antennas are often less efficient than a dipole, and therefore, tend to have negative gain.

Radiation Pattern – Radiation is maximum when broadside, or perpendicular to a wire, so a vertical whip is ideal for communication in any direction except straight up. The radiation pattern, perpendicular to the whip, can be described as omnidirectional. There is a null, or signal minimum, at the end of the whip. With a less than ideal antenna, such as a bent or tilted whip, this null may move and partly disappear. It is important to know the radiation pattern of the antenna, in order to ensure that a null is not present in the desired direction of communication.

Polarization – It is important that other antennas in the same communication system be oriented in the same way, that is, have the same polarization. A horizontally polarized antenna will not usually communicate very effectively with a vertical whip. In the real environment, metal objects and the ground will cause reflections, and may cause both horizontal and vertical polarized signals to be present.

Impedance – Another important consideration is how well a transmitter can transfer power into an antenna. If the antenna tuning circuit on a transmitter (or receiver) is designed for a 50 ohm load, the antenna should, of course, have an impedance near 50 ohms for best results. A whip over a flat ground plane has an impedance near 35 ohms, which is close enough. The impedance changes if the whip is not properly tuned or bent down, or if a hand or other object is placed close to it. The impedance becomes lower as the antenna is bent closer to ground. When the
whip is tilted 45 degrees, the impedance is less than 20 ohms. When the whip is bent horizontal to one-tenth of wavelength above ground, the impedance approaches 10 ohms. The resulting impedance mismatch, a 5:1 ratio (VSWR) will contribute an additional loss of 2.6 dB.