Ahmad Zain | Universiti Sains Islam Malaysia (USIM) (original) (raw)
Papers by Ahmad Zain
Vertical ionospheric soundings were carried out in the equatorial zone above Pant Raja, district ... more Vertical ionospheric soundings were carried out in the equatorial zone above Pant Raja, district of Batu Pahat, Johor, Malaysia (lat. 1° 86' N, long. 103° 8' E) for the winter months of December 2004, January and February 2005. The vertical digital sounder is used to measure and to determine the critical frequency, maximum usable frequency and electron density at any given time. The presence of the F3-layer is quite prominent compared with mid- and high-latitude ionosphere. The greatest occurrence of the F3 layer during the winter of 2004 is in the period between 1100 LT to 1300 LT (13.3%) while the least is between 0700 LT to 0900 LT (1.2%). The mean critical frequency for the F3 layer is about 8 MHz (foF3) while the mean virtual height for the F3 layer is about 600 km (h'F3). The maximum virtual height of the F3 layer (h'F3) recorded is about 900 km on the 7th February 2005 at 1800 LT while the minimum is about 420 km on the 5th February 2005 at 1300 LT.
... the Parit Raja values of hmax increase from the dawn value of ~270 km to a plateau at 500 5... more ... the Parit Raja values of hmax increase from the dawn value of ~270 km to a plateau at 500 550 km between 1200 and 1700 LT. The hmax value reaches a high level by about 1300 LT and then falls such that at midnight it is about 250 km lower than at noon. The peak can be ...
This paper presents radiation emission measurements from a mobile phone. Measurements are conduct... more This paper presents radiation emission measurements from a mobile phone. Measurements are conducted during the mobile when it is in standby, receive, and transmit mode based on the CISPR and EMC standards. The results obtained are in the form of electrical field strength observed in three different domains, i.e., frequency, time, and radiation distribution along the mobile phone structure. The mobile phones used are grouped according to the placement of the antenna either internal or external. It is found that the radiated emission is 65% higher during transmission rather than on standby. Radiation from the phone with the external antenna can be further reduced using an EWI radiation filter.
American Journal of Engineering and Applied Sciences, 2009
Problem Statement: Ionospheric scintillations, which cause significant effects on satellite signa... more Problem Statement: Ionospheric scintillations, which cause significant effects on satellite signals for communication and navigation, often takes place in equatorial region such as Malaysia. However, this disturbance are not fully understand due to few studies performed. This research reports the study and monitoring activity on Total Electron Content (TEC) and ionospheric scintillation in Malaysia using GPS measurements. Approach: One dual-frequency GPS receiver was positioned at the main station in Parit Raja, West Malaysia (1.86° N, 103.8° E) and Sipitang, East Malaysia (5.10° N, 115.56° E) respectively. Dual-frequency GPS data collected during the one-month ionospheric experimental campaign was used for TEC and scintillation computation and analysis. The TEC with 15 sec interval were computed from combined L1 and L2 code-pseudorange and carrier phase measurements. Whereas, the scintillation parameter S4 index was computed as a standard deviation of the received signal power normalized to average signal power every 60 sec on L1. A corrected S4 (without noise effects) was also computed and used in the analysis. Results: It was found that the daily maxima vTEC for Parit Raja (PR) ranged from 38-100 TECU, which is generally higher than those of Sipitang, which ranged from 30-42 TECU. However, a general consistency for both stations can be seen during the 1 month campaign period. Conclusions/Recommendations: In conclusion, these results show good agreement in the existence of the equatorial anomaly observed during moderate solar flux conditions and undisturbed geomagnetic condition. This will contributes to the knowledge of equatorial ionosphere and help in space weather condition. However, to better understand and characterize the ionosphere over Malaysia, more campaigns should be encouraged.
This paper presents a thorough investigation of the effects of substrate thickness on the perform... more This paper presents a thorough investigation of the effects of substrate thickness on the performance of infinite reflectarrays. The reflectarrays designed in the X-band frequency range are investigated in terms of electric field intensity, reflection loss and reflection phase performance. It has been demonstrated that the electric field intensity decreases from 3.42e5V/m to 2.34e5V/m as the thickness of the substrate is increased from 0.2mm to 2.0mm. The measured reflection loss has been shown to decrease from 15dB to 2.25dB as the substrate thickness is changed from 0.127mm to 0.381mm. Furthermore the effect of substrate thickness on the reflection phase performance has also been demonstrated by waveguide scattering parameter measurements.
There has been much interest recently in developing reconfigurable reflectarray antenna for tunab... more There has been much interest recently in developing reconfigurable reflectarray antenna for tunable microwave applications. Liquid crystal (LC) has been given much attention due to its dielectric anisotropy property which allows the change in the frequency and hence the phase of reflection. In this paper, the phase tunability of the periodic array using K15 nematic liquid crystal as the dielectric
The ionosphere over Malaysia is unique because of her location near the equator where the existen... more The ionosphere over Malaysia is unique because of her location near the equator where the existence of the equatorial anomaly, electrojet, and fountain effect have made it good for studies. As part of a rejuvenation process in investigating the equatorial ionosphere in Malaysia, a short duration ionospheric experimental campaign was carried out at Fraser's Hill (lat. 3° 42.9' N, long. 101° 44.2' E, at an altitude of 1298 m) from August 1 to 18, 2005. Two sets of equipment were installed; a digital ionosonde and a dualfrequency GPS receiver. This paper reports on the measurements obtained from the ionosonde. The system consists of the sounding equipment using a delta antenna for transmitting and a dipole antenna for receiving. The observable F-layer for this site can be seen in the late evenings around 18:00 UTC. The observed critical frequency for this layer varies from 4 MHz to 6 MHz. The lower critical frequency can be seen between 21:00 and 00:00 UTC. Other observed phenomena were spread-F and sporadic-E.
GPS is of great importance in precise positioning, however, the accuracy is marked by error sourc... more GPS is of great importance in precise positioning, however, the accuracy is marked by error sources, i.e. ionospheric effects. The signal information is delayed and the carrier phase experiences an advance due to the dispersive character of the ionosphere. The delay can be more than 100 meters in the worst case scenario and tends to increase with increasing solar activity. This error can be corrected by processing the GPS data if it is known. This paper describes the possibility of using statistical methods to forecast the ionospheric delay which shows repeatable patterns in time series. The statistical method used is the Holt-Winter method due to its ability to forecast time series with repeated trends and seasonal patterns. Based on the TEC data collected during a period of a month, ionospheric delay forecast is generated for the following month, which is then compared with the real data. Results show that there is a 6% error between the forecast and the real ionospheric delay and the error correction for the delay can be more than 90%.
Abstract The ionosphere is the topside of the atmosphere that contains high concentration of ion... more Abstract The ionosphere is the topside of the atmosphere that contains high concentration of ions and can reflect radio waves due to refraction. The ionospheric region of interest is located above the equatorial region at longitude 1° 52' and latitude 103° 48' (Parit Raja, Johor). ...
Abstract This paper describes an analysis of ionospheric models compared with real data during ... more Abstract This paper describes an analysis of ionospheric models compared with real data during the occurrence of travelling ionospheric disturbances. GPS data was taken from Parit Raja Station (1o 52' N, 103o 06' E) in Southern Malaysia. Comparison was made from collected data ...
This paper analyzes and compares TEC measurement at GPS stations belonging to UKM installed at Hu... more This paper analyzes and compares TEC measurement at GPS stations belonging to UKM installed at Husafell, Iceland (64° 40' N, -21° 1' E) in the high-latitude and UKM (2° 55' N, 101° 46' E) in the equatorial latitude. A high performance dual-frequency GPS receiver with chock-ring antenna was used in this study. GPS data for the whole year of 2009 were used in this analysis. BGS's Bernese Processing Engine (BPE)'s precise point positioning (PPP) mode was used to generate TEC at the single station with the zero-difference technique for differencing level of choice. Comparison of the maximum value of TEC between Husafell, Iceland for high-latitude and UKM, Malaysia for the equatorial region was carried out. Results show that the maximum value of TEC at high-latitude reaches its maximum at post local noon which is similar to that over the equatorial region. TEC values reach their maximum at 13:00 UTC at Husafell while at UKM it is at 06:00 UTC (14:00 LT). It is observed that TEC at UKM's equatorial station is approximately twice compared to the Husafell station.
Abstract Radio waves such as Global Positioning System (GPS) signals propagate through the ionosp... more Abstract Radio waves such as Global Positioning System (GPS) signals propagate through the ionosphere and experience several effects, one of which is an additional group delay that is approximately proportional to the total number of free electrons encountered by the ...
Vertical ionospheric soundings were carried out in the equatorial zone above Pant Raja, district ... more Vertical ionospheric soundings were carried out in the equatorial zone above Pant Raja, district of Batu Pahat, Johor, Malaysia (lat. 1° 86' N, long. 103° 8' E) for the winter months of December 2004, January and February 2005. The vertical digital sounder is used to measure and to determine the critical frequency, maximum usable frequency and electron density at any given time. The presence of the F3-layer is quite prominent compared with mid- and high-latitude ionosphere. The greatest occurrence of the F3 layer during the winter of 2004 is in the period between 1100 LT to 1300 LT (13.3%) while the least is between 0700 LT to 0900 LT (1.2%). The mean critical frequency for the F3 layer is about 8 MHz (foF3) while the mean virtual height for the F3 layer is about 600 km (h'F3). The maximum virtual height of the F3 layer (h'F3) recorded is about 900 km on the 7th February 2005 at 1800 LT while the minimum is about 420 km on the 5th February 2005 at 1300 LT.
... the Parit Raja values of hmax increase from the dawn value of ~270 km to a plateau at 500 5... more ... the Parit Raja values of hmax increase from the dawn value of ~270 km to a plateau at 500 550 km between 1200 and 1700 LT. The hmax value reaches a high level by about 1300 LT and then falls such that at midnight it is about 250 km lower than at noon. The peak can be ...
This paper presents radiation emission measurements from a mobile phone. Measurements are conduct... more This paper presents radiation emission measurements from a mobile phone. Measurements are conducted during the mobile when it is in standby, receive, and transmit mode based on the CISPR and EMC standards. The results obtained are in the form of electrical field strength observed in three different domains, i.e., frequency, time, and radiation distribution along the mobile phone structure. The mobile phones used are grouped according to the placement of the antenna either internal or external. It is found that the radiated emission is 65% higher during transmission rather than on standby. Radiation from the phone with the external antenna can be further reduced using an EWI radiation filter.
American Journal of Engineering and Applied Sciences, 2009
Problem Statement: Ionospheric scintillations, which cause significant effects on satellite signa... more Problem Statement: Ionospheric scintillations, which cause significant effects on satellite signals for communication and navigation, often takes place in equatorial region such as Malaysia. However, this disturbance are not fully understand due to few studies performed. This research reports the study and monitoring activity on Total Electron Content (TEC) and ionospheric scintillation in Malaysia using GPS measurements. Approach: One dual-frequency GPS receiver was positioned at the main station in Parit Raja, West Malaysia (1.86° N, 103.8° E) and Sipitang, East Malaysia (5.10° N, 115.56° E) respectively. Dual-frequency GPS data collected during the one-month ionospheric experimental campaign was used for TEC and scintillation computation and analysis. The TEC with 15 sec interval were computed from combined L1 and L2 code-pseudorange and carrier phase measurements. Whereas, the scintillation parameter S4 index was computed as a standard deviation of the received signal power normalized to average signal power every 60 sec on L1. A corrected S4 (without noise effects) was also computed and used in the analysis. Results: It was found that the daily maxima vTEC for Parit Raja (PR) ranged from 38-100 TECU, which is generally higher than those of Sipitang, which ranged from 30-42 TECU. However, a general consistency for both stations can be seen during the 1 month campaign period. Conclusions/Recommendations: In conclusion, these results show good agreement in the existence of the equatorial anomaly observed during moderate solar flux conditions and undisturbed geomagnetic condition. This will contributes to the knowledge of equatorial ionosphere and help in space weather condition. However, to better understand and characterize the ionosphere over Malaysia, more campaigns should be encouraged.
This paper presents a thorough investigation of the effects of substrate thickness on the perform... more This paper presents a thorough investigation of the effects of substrate thickness on the performance of infinite reflectarrays. The reflectarrays designed in the X-band frequency range are investigated in terms of electric field intensity, reflection loss and reflection phase performance. It has been demonstrated that the electric field intensity decreases from 3.42e5V/m to 2.34e5V/m as the thickness of the substrate is increased from 0.2mm to 2.0mm. The measured reflection loss has been shown to decrease from 15dB to 2.25dB as the substrate thickness is changed from 0.127mm to 0.381mm. Furthermore the effect of substrate thickness on the reflection phase performance has also been demonstrated by waveguide scattering parameter measurements.
There has been much interest recently in developing reconfigurable reflectarray antenna for tunab... more There has been much interest recently in developing reconfigurable reflectarray antenna for tunable microwave applications. Liquid crystal (LC) has been given much attention due to its dielectric anisotropy property which allows the change in the frequency and hence the phase of reflection. In this paper, the phase tunability of the periodic array using K15 nematic liquid crystal as the dielectric
The ionosphere over Malaysia is unique because of her location near the equator where the existen... more The ionosphere over Malaysia is unique because of her location near the equator where the existence of the equatorial anomaly, electrojet, and fountain effect have made it good for studies. As part of a rejuvenation process in investigating the equatorial ionosphere in Malaysia, a short duration ionospheric experimental campaign was carried out at Fraser's Hill (lat. 3° 42.9' N, long. 101° 44.2' E, at an altitude of 1298 m) from August 1 to 18, 2005. Two sets of equipment were installed; a digital ionosonde and a dualfrequency GPS receiver. This paper reports on the measurements obtained from the ionosonde. The system consists of the sounding equipment using a delta antenna for transmitting and a dipole antenna for receiving. The observable F-layer for this site can be seen in the late evenings around 18:00 UTC. The observed critical frequency for this layer varies from 4 MHz to 6 MHz. The lower critical frequency can be seen between 21:00 and 00:00 UTC. Other observed phenomena were spread-F and sporadic-E.
GPS is of great importance in precise positioning, however, the accuracy is marked by error sourc... more GPS is of great importance in precise positioning, however, the accuracy is marked by error sources, i.e. ionospheric effects. The signal information is delayed and the carrier phase experiences an advance due to the dispersive character of the ionosphere. The delay can be more than 100 meters in the worst case scenario and tends to increase with increasing solar activity. This error can be corrected by processing the GPS data if it is known. This paper describes the possibility of using statistical methods to forecast the ionospheric delay which shows repeatable patterns in time series. The statistical method used is the Holt-Winter method due to its ability to forecast time series with repeated trends and seasonal patterns. Based on the TEC data collected during a period of a month, ionospheric delay forecast is generated for the following month, which is then compared with the real data. Results show that there is a 6% error between the forecast and the real ionospheric delay and the error correction for the delay can be more than 90%.
Abstract The ionosphere is the topside of the atmosphere that contains high concentration of ion... more Abstract The ionosphere is the topside of the atmosphere that contains high concentration of ions and can reflect radio waves due to refraction. The ionospheric region of interest is located above the equatorial region at longitude 1° 52' and latitude 103° 48' (Parit Raja, Johor). ...
Abstract This paper describes an analysis of ionospheric models compared with real data during ... more Abstract This paper describes an analysis of ionospheric models compared with real data during the occurrence of travelling ionospheric disturbances. GPS data was taken from Parit Raja Station (1o 52' N, 103o 06' E) in Southern Malaysia. Comparison was made from collected data ...
This paper analyzes and compares TEC measurement at GPS stations belonging to UKM installed at Hu... more This paper analyzes and compares TEC measurement at GPS stations belonging to UKM installed at Husafell, Iceland (64° 40' N, -21° 1' E) in the high-latitude and UKM (2° 55' N, 101° 46' E) in the equatorial latitude. A high performance dual-frequency GPS receiver with chock-ring antenna was used in this study. GPS data for the whole year of 2009 were used in this analysis. BGS's Bernese Processing Engine (BPE)'s precise point positioning (PPP) mode was used to generate TEC at the single station with the zero-difference technique for differencing level of choice. Comparison of the maximum value of TEC between Husafell, Iceland for high-latitude and UKM, Malaysia for the equatorial region was carried out. Results show that the maximum value of TEC at high-latitude reaches its maximum at post local noon which is similar to that over the equatorial region. TEC values reach their maximum at 13:00 UTC at Husafell while at UKM it is at 06:00 UTC (14:00 LT). It is observed that TEC at UKM's equatorial station is approximately twice compared to the Husafell station.
Abstract Radio waves such as Global Positioning System (GPS) signals propagate through the ionosp... more Abstract Radio waves such as Global Positioning System (GPS) signals propagate through the ionosphere and experience several effects, one of which is an additional group delay that is approximately proportional to the total number of free electrons encountered by the ...