Comprehensive Evaluation of an Antenna for TV White Space Devices

Abstract: In this paper, an ultra-high frequency (UHF) antenna for TV white space devices (TVWSDs) is designed, fabricated and systematically tested. In addition to its radiation efficiency validation using a novel measurement approach, system performance of the antenna is comprehensively studied by integrating it onto different platforms, including a wideband spectrum monitor and a wireless communication client, and testing under realistic scenarios. The performance comparison with a commercial antenna is also performed to demonstrate the effectiveness and robustness of the in house-developed antenna. Overall practical and real-time performance evaluations show that the in-house developed antenna is a strong contender for TVWSDs.

Typical antenna properties including reflection coefficients, gains and radiation patterns are commonly measured in an anechoic chamber where reflections of electromagnetic waves are absorbed. Quantifying the loss of an antenna itself, radiation efficiency is another key property of an antenna, and it is a dominating factor for antenna performance in multipath environments [7].
However, measuring the radiation efficiency in an anechoic chamber is often difficult and inaccurate [8]. The traditional Wheeler Cap method measures narrowband radiation efficiency. By removing the need of any assumption on the antenna equivalent circuit, Johnston improved the efficiency measurement to ultra-wide range of frequencies, but this method is based on a variation of the physical dimension of the cap [9]. Reverberation chamber has been used in recent years for easy and accurate radiation efficiency measurement [10].

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The Journal of Engineering
This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication in an issue of the journal. To cite the paper please use the doi provided on the Digital Library page. In this paper, radiation efficiency is measured in a reverberation chamber using the modified two-antenna method [11], and duplicate antennas under test are placed in the chamber at the same time as Fig. 2(a) shows. The radiation efficiency can then be calculated based on (1), in which ω is the angular frequency, ߬ ோ is the chamber decay time and ‫ܥ‬ ோ is the chamber constant ‫ܥ‬ ோ ൌ 16ߨ ଶ ܸ/ߣ ଷ (ܸ is the volume of the chamber and λ is the wavelength). ൏• represents the average value of S-parameters, and ܵ ,௦ is the stirred part of S-parameters.
The radiation efficiency of the in-house developed antenna over the UHF TV spectrum is plotted in Fig. 2(b) together with its realised gain which is measured in an anechoic chamber using a common reference antenna methods [12]. According to Fig. 2(b), ranging from 1.4dBi to 1.9dBi, the realised gain reaches 1.7dBi in average over 470-790MHz. The radiation efficiency is between 76.2% and 92.4%, and hence most power delivered to the antenna is capable of being radiated. Both gain Page 4 of 11

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The Journal of Engineering This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication in an issue of the journal. To cite the paper please use the doi provided on the Digital Library page. and radiation efficiency are uniform over the UHF TV spectrum, and the measurement results indicate that the in-house developed antenna provides stable and efficient services for TVWSDs.

III. Field trails
The in-house developed antenna is then integrated with wideband and narrowband systems, respectively to assess its performance in different practical applications.

Wideband spectrum sensing
To verify the in-house developed antenna is able to operate over the entire UHF TV spectrum, it is connected with a RFeye system provided by CRFS for intelligent spectrum monitoring [13]. The This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication in an issue of the journal. To cite the paper please use the doi provided on the Digital Library page.

Fig. 4 Testing environments for spectrum detection and TVWS communication.
An antenna prototype is connected to the RFeye node as shown in Fig. 3(a). On the RFeysSite software, the start and end detection frequencies are set to 470MHz and 790MHz, respectively, and 320kHz (0.1% of the bandwidth of the UHF TV spectrum) is chosen for resolution bandwidth. When monitoring the spectrum, the whole system is placed at Room 353 of the Engineering Building in the Queen Mary University of London (QMUL) indicated on the testing environments given in Fig. 4. The measured data is export from the USB memory stick and plotted in Fig. 5. Using -90dBm as a signal intensity threshold, occupation statues of the UHF TV channels are given in Fig. 6.

IET Review Copy Only
The Journal of Engineering This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication in an issue of the journal. To cite the paper please use the doi provided on the Digital Library page.  According to Fig. 5 and Fig. 6, the UHF TV channels from 36 to 60 are vacant, which may be resulted from few signal transmission above 600MHz or poor system performance of the in-house

IET Review Copy Only
The Journal of Engineering This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication in an issue of the journal. To cite the paper please use the doi provided on the Digital Library page. developed antenna at high frequencies. Therefore, the spectrum is monitored again at the same location with same settings, but the antenna connected to the RFeye node is changed to a commercial one provided by the CRFS as illustrated in Fig. 3(b). The detected signal is also plotted on

Narrowband wireless communication
A TVWS spectrum license has been granted by the Ofcom for real-time wireless transmissions in the Mile End campus of QMUL, and a communication link on the TVWS is set up as illustrated in Fig.   4. The base station and its sector antenna pointing at northwest are placed on the third floor in building A. The base station is connected to the Ethernet to acquire transmission information and its transmitting power is 23dBm. Maximum gain of the directional sector antenna is 11dBi and its feeding cable has 1dBi loss. Client with the in-house developed antenna is on the second floor in building B. Distance between the base station and client is 127.5m, and there are two blocking buildings, some trees and a busy road between them. During communication, signals undergo reflection, fading and multipath interference due to people, office facilities, walls and trees.

IET Review Copy Only
The Journal of Engineering This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication in an issue of the journal. To cite the paper please use the doi provided on the Digital Library page.