Understanding Frequency Range

Understanding Frequency Range

The following article serves to build an understanding about the antenna parameter known as Frequency Range. An antennas frequency range is one of its most fundamental characteristics, which describes the range of frequencies that the antenna is capable of operating on with an acceptable level of efficiency. Defining a set range is also important so all other parameters can be reported consistently. For example, quite often an antenna is capable of operating outside the set range, but its gain, VSWR, and radiation patterns may be volatile or non-constant. The width of a frequency range is also referred to as bandwidth.

An antenna may have multiple ranges, particularly in the case of cellular antennas and combination designs. Modern multiband cellular antennas advertise (for the sake of simplicity) as operating 700 to 2700 MHz when in actual fact the datasheet will show two or three frequency ranges: 698 to 960 MHz, 1710 to 2170 MHz, and 2300 to 2700 MHz. Designing an antenna to operate from 700 MHz all the way through to 2700 MHz is generally not possible, or at least not possible with high efficiency.

Antenna elements must be physically dimensioned to match a chosen wavelength often the centre frequency of the desired band. The wider the bandwidth means the less tuned the element, trading off a parameter such as VSWR/Return Loss, or Gain. To resolve this design limitation, a wideband antenna will attach multiple highly tuned antenna elements to a combiner/multiplexer, achieving wide bandwidth without compromising other performance parameters. This type of design is common in panel, sectoral, and LPDA antenna types.

Many antenna designs do not contain combiners/multiplexers, and as such declared frequency range can reveal a lot about the expected performance. For example, the design of a Yagi antenna is physically limited to one frequency range. The length of each antenna element must be measured to match one centre frequency at which performance will be greatest. Immediately either side of the centre frequency performance will be good, and slowly deteriorate the further from this central frequency the radio operates. The image below depicts the sweep of a highly tuned antenna element.

antenna tuning to centre of frequency range

Definition of Frequency Range

The operating bandwidth of the antenna is defined by a continuous range of frequencies designated by two limiting frequencies fstart and fstop

Specification Definition

When disclosing frequency range on test reports and datasheets, Halberd Bastion follow the BASTA reporting recommendations. BASTA define the reporting of frequency range per the following (adapted) specification. As BASTA is limited in scope to base station antennas, specification definition has been broadened to cover all antenna types.

  • May consist of one or multiple frequency bands, with each band separated by a comma.
  • The range of each band is specified in kHz, MHz, or GHz.
  • fstart and fstop must be separated by either hyphen or 'to', per IEEE requirements.

Testing & Reporting

Frequency range is generally set during electrical engineering design and used as the basis from which all other parameters are designed around. Operating frequency range can be tested by first setting a performance threshold for VSWR/RL, gain, efficiency, and critical parameters. These parameters are then tested and the frequencies through which the antenna meets all performance targets can be accepted as the frequency range. Likewise if a datasheet has been provided during the testing process, the declared range can be validated accordingly.

The term Frequency Band is discouraged on datasheets as the term band is commonly used to refer to radio bands, such as: UHF Band, X-Band, Ku-Band, C-Band, and so on.

References

  1. NGMN, "Recommendations on Base Station Antenna Standards", NGMN Alliance, N-P-BASTA v9.6, Jan. 2013.