Folded-type rectangular loop antenna with tunable circuit

Abstract

A design for a small and tunable loop antenna is presented. This design enables the higher-order mode resonant frequency to become low while keeping the radiation pattern in good shape. The use of a tunable circuit to utilise higher-order mode for bandwidth enhancement has been investigated. The measured and simulated radiation patterns and voltage standing wave ratio are given.Introduction: For portable reception of digital broadcasting services in the UHF band (470-770 MHz), mobile terminals should have sufficient gain to receive signals; however, the space for a built-in antenna is very small compared to the wavelength in the UHF band. Loop antennas [1] are suitable for a built-in antenna; however, the size of a loop antenna for the UHF band is larger than the size of the mobile terminals. Meander line antennas [2] can have a longer electric length to resonate in the lower UHF band. However, a large number of folds is needed to resonate in the UHF band and a meander line antenna with a low number of folds has a low antenna gain in the lower frequency of the UHF band.This Letter proposes a folded rectangular loop antenna (FRLA) in which the antenna element consists of a long copper wire that is bent and folded a few times at each corner of a rectangular loop. The configuration of the FRLA where the number of folds on the long side of the FRLA is larger than that on the short side is such that the higherorder mode resonant frequency becomes low while keeping the radiation pattern in good shape. In addition, we investigated the use of a tunable circuit with the FRLA to utilise higher-order modes for bandwidth enhancement.

Efficiency‐based approach to quantifying ‘tuneability’ performance in frequency‐agile narrowband antennas

“…Thus, they also need efficient tuning to achieve a very wide operating bandwidths. Recent examples of the aperture tuning approach include the use of RF switches [3,7], varactor diode [4, 5, 8-12, 19, 21], RF MEMS [17] and CMOS [6] tuneable capacitors, PIN diodes [16], tuneable magneto-dielectric [33] and tuneable dielectric substrates (including liquid crystal substrates [18]) as well as liquid metal [13] tuning techniques. Each of these techniques permits the antenna to serve different bands at different times.…”

“…A search of published techniques for tuning compact and electrically small antennas in general terms revealed that tuneability is predominantly viewed in terms of tuneable input response (impedance matching from S 11 ). Of the sample of papers describing tuning techniques referenced [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21], all either reported explicit tuning ranges in percentage terms with respect to a required S 11 threshold or provided the data required to do so. The aforementioned list gives a sample set of 18 papers, of which 15 were published in the last five years, thus the sample set is taken to be representative of contemporary reporting.…”

“…A key metric of interest for such antennas is 'tuneability' or frequency agility, recording the range of frequencies that can be addressed by the tuneable antenna. A survey of the contemporary literature suggests that this assessment is almost always supported by a stated range of measured tuneable input responses S 11 [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. However, this makes no assessment of the radiation efficiency offered over the tuned range, reporting of which over a tuneable range was found to be much less widespread.…”

“…However, this makes no assessment of the radiation efficiency offered over the tuned range, reporting of which over a tuneable range was found to be much less widespread. The survey found that in nearly half of cases no efficiency data were given [7,8,10,13,15,16,[18][19][20], while in others values were given but the method used to measure them was not declared [6,9,17]. Of the papers found to have reported both efficiency values and the method by which these values were obtained (only 40% of those surveyed), none were found to have attempted to distil an efficiency-based approach to recording metrics of frequency agility.…”

Low profile UHF loop antenna prototyped and investigated by circuit modeling

“…For mobile communication of digital broadcasting services in the UHF band (470–770 MHz), portable terminals should have suitable gain to receive signals. In comparison to the wavelength in the UHF band the space require for a built‐in antenna is very small . Even though, the size of a loop antenna in the UHF band is larger than the size of the mobile terminals, they are suitable for a built‐in antenna .…”