Design considerations when developing antennas

This article originally appeared in the November '21 magazine issue of Electronic Specifier Design – see ES's Magazine Archives for more featured publications.

On top of the ground plane, the position of the antenna on the PCB is also significant: designers should ensure that emissions generators such as SDIO (secure digital input/output), USB, and LCD circuits or metallic objects do not lie in the antenna’s path. Finally, the casing for the device can affect the antenna’s ability to radiate efficiently.

Designers measure antenna performance in terms of radiated efficiency and gain. Other objects close by can alter the electromagnetic fields of the antenna’s radiation pattern and cause the antenna’s efficiency to be too far dropped or gained.

The level of efficiency also depends on how well the antenna was matched, and if there are losses along the RF feed trace indicating how well the antenna feed line design transfers that energy.

If the new design is to be used on the carrier networks, it will need to be submitted for formal network and regulatory testing and approval. It may fail testing if the antenna doesn’t perform correctly or if the device creates radio interference.

Following is Antenova’s checklist to help designers build a well-optimised device.

1. Select the most appropriate antenna

Of course, designers should start by selecting a high-quality antenna that fits well in the design. Some of the options are surface mount device (SMD) antennas, external antennas, or flexible antennas, which help overcome certain design issues.

SMD antennas are designed to work in certain positions on a PCB, usually on an edge or a corner, and they often require a ground plane beneath them.

FPC (flexible printed circuit) antennas are not placed on the PCB, so they don’t take up any board space, but the exact angle at which they are mounted and the routing of the coax cable will be crucial to their performance.

The manufacturer’s datasheet should explain how to place a particular antenna in a design and in the PCB stack-up to allow it to radiate effectively.

2. Allow the correct ground plane length

For an embedded antenna, part of the surface of the PCB is used as the second radiator for a balanced antenna system. The length of the ground plane is directly related to the wavelength of the signal, and this must be of the specified length to allow the antenna to operate correctly.

This is usually explained in detail in the datasheet of the antenna. If the PCB is very small, the RF aspect of the design becomes more challenging.

3. Other components

Antennas themselves are passive components that can receive energy and will pick up the noise radiated from other noisy sources. That noise can degrade the received signal: it is therefore best to keep the antenna well away from certain components, such as LCDs, power circuits or batteries.

4. Transmission line design

The transmission line is the copper trace line that carries energy to and from the antenna. The transmission line ideally carries 100% of the power to the antenna, although in reality this is difficult to achieve. This is partly due to the absorptive loss of the materials used, sharp bends in the trace, and reflected energy from impedance mismatch.

If the trace is not designed properly, there can be a very high level of resistance and this can cause a reduction of the signal energy caused by that energy coupling to ground – instead of transferring to the antenna.

The resultant loss can be as high as 50%. The impedance value for an antenna’s trace line is usually an impedance of 50 ohms, and the other RF elements (such as the radio) in the design will also be 50 ohms.

The old way to calculate the impedance of an antenna was to use Smith Charts, which show the correct value for impedance in relation to frequency and voltage standing wave ratio (VSWR).

Today, however, there are software calculators (such as Antenova’s online one here) that give instant results for grounded co-planar waveguide (or GCPW) and other trace systems – basing the calculation on the thickness of the PCB, the thickness of the copper, and the dielectric constant of the PCB substrate.

Both the thickness and dielectric constant of the PCB play a key role in tuning the return loss for the antenna.

5. VSWR and antenna matching

Voltage standing wave ratio is an important value that helps designers to build a successful wireless design. The VSWR is a measurement of the return loss from the antenna: in other words, the energy that is being transferred through or being reflected back along the transmission line. Both kinds of loss are detrimental to the performance of the antenna.

The dimensions and length of the trace and PCB stack should be calculated to minimise the VSWR as far as possible. A low VSWR value means that the antenna is receiving more power. This is preferable: designers generally consider a VSWR value that is smaller than two to indicate a well-matched antenna.

6. Tuning the antenna

One way to optimise the performance of an antenna is to tune it for the region or country where it is going to operate. For example, the 4G frequency range is a wide one, ranging from 698MHz to 2,690MHz, but each different world region uses just a small portion of this band, and an antenna can only operate on one frequency at a time.

This means that when a product is to be used in one geographical region, it can be tuned to operate in a narrower section of the frequency band. This will boost the performance of the antenna.

Another technique is to add an active tuning network into the design. This is effectively an additional RF switching circuit – namely a PI matching circuit – which is placed close to the antenna feed point, to fine tune the antenna and boost performance. To design a matching circuit, designers will usually need some assistance from an RF specialist.

7. Co-existence of multiple antennas

Often devices contain more than one antenna operating at different frequencies on the same PCB, but at close proximity. This makes the design more complicated. If the antenna is a receive-only system, such as a GPS receiver, it could be de-sensed by a nearby transmit antenna (such as a 4G radio), which will impact the performance of the GPS system. It is best to separate these antenna systems. This can be achieved by placing them the right distance apart, or by placing the antennas so that they are orthogonal to each other – or otherwise by notching the ground plane to remove the ground currents shared between the antennas. This is where the design becomes more complex – and it will pay dividends if you ask for an antenna specialist’s advice! For more guidance on antenna designs, you can visit antenova.com and its advice page: ask. antenova.com