Validation of antenna performance
Usually a switched calibration signal is injected into the signal path of a receiver for estimating the total power available at the terminals of the antenna (the antenna temperature) and the noise power due to the receiver system (the receiver temperature).
Normally, the antenna output is fed to a balun for converting their balanced output to unbalanced output. The electrical connection between the antenna output terminals and the balun input is established through pressure contact alone, using a bolt and nut arrangement as shown in the Fig 1. Unless the bolt holds the antenna terminal firmly against the balun input, the antenna signal is bound to undergo a finite amount of transmission loss at the connection. Thus the receiver produces less than what is expected when the antenna gain is maximum. It often becomes difficult to know that there is a loss of signal owing to improper connection at the feeding point of the antenna, since the sky signal also keeps varying as a function of time.
In order to avoid this problem, a calibration setup was made to measure the absolute transmission loss in the signal path. A CW signal is radiated from a known distance towards the antennas and the receiver response is compared with the expectation as a check on the quality of the electrical/mechanical contact at the feeding point. The prediction of the received CW power at the antenna terminals can be easily made using the Frii’s transmission equation.
Bolt head Pcb for balun
Balun EoR antenna
Balun
Bolt and Nut for holding the antenna terminals
Fig. 1. Bolt and nut arrangement for making electrical connections of the antenna
Fig. 2 shows the experimental setup used for monitoring the gain of the EoR antenna. It consists of a standard calibrated half-wave dipole of ETS Lindgren (Model No. 3121 D) that is placed at equal distances from each of EoR antennas and with the same polarization as that of the antennas; the dipole is mounted 3 m above the ground. A standard signal generator from Agilent Technologies (Model No.8257D) is connected to the calibrated antenna through a balun for transmitting CW signals at various frequencies. The two EoR antenna outputs are connected to receiver systems for estimating the signal received at their terminals.
If a continuous wave (CW) is transmitted through the calibrated half wave dipole, then the net signal power available at the EoR antenna terminals situated at a distance of ‘R’ from the radiating dipole is calculated using the Friis equation for transmission as given by
Calibrated half wave dipole antenna EoR Ant - 2 Transmitter
B a l u n
EoR Ant - 1
R=5m
Ant – 1 Output Ant – 2 Output 30 dB
Signal Generator Front End 1 Front End 2
Digital Receiver
Spectrum output
Fig. 2. Experimental setup used for monitoring the gain of the EoR antenna λ Pr = Pt GtGr ( ------) (4*3.14*R)2
where Pr is the power available at the EoR antenna Pt is the power fed to the transmitter Gt is the gain of the transmitting antenna Gr is the gain of the receiving antenna λ is thewavelength of the transmitted signal R is the distance between the transmitting and receiving antennas
If baluns used in both transmitter and receiver have finite amount of insertion loss ( Lt & Lr), then the above equation gets modified to
λ Pr = Pt GtGr ( ------) LtLr (4*3.14*R)2
The power available Pr can be expressed in terms of temperature using the relationship
Pr = kTrB
where k is the Boltzmann constant = 1.38e-23 m2kg s-2 K-1 Tr is the temperature of the blackbody producing same power as Pr B is the bandwidth over which measurement is made = 85.4 KHz
Thus the equivalent temperature Tr corresponding to Pr is given by
Tr = Pr/(kB) K
Signals of various frequencies having a constant power level of -100 dBm were transmitted through the calibrated antenna. The signal generator was set to provide a power level of -70 dBm and a 30 dB attenuator was used between the signal generator and dipole; this was done in order to have a stable power from the signal generator. The power available at each of the EoR antennas is calculated using the relationships shown above. The equivalent antenna temperatures corresponding to the received CW powers at each of the EoR antennas were estimated using the calibration signal available in the receivers. Table 1 lists the available power in terms of temperature along with their expectation values; this is done at a set of frequencies. The measurements made match closely the expected values with a maximum error of about 10%, indicating good electrical contact between the antenna terminals and the balun input and good understanding of the system performance and calibration. This in turn validates the expectations for gain and transmission losses for the antennas. It is assumed that the gain of the EoR antennas is constant throughout the frequency range of operation since their beamwidth and beam pattern is identical at all those frequencies. While predicting the value of power available at the antenna input, only balun loss is considered in case of antenna 2, whereas for the antenna 1, both the balun and phase switch losses have been considered.
Sl. Frequenc Loss of Loss of Loss of Gain of Gain Expected Measured N y- MHz Balun of Balun of Phase Tx. of antenna temp. antenna o. Tx Rx switch – Antenna Rx. at the Rx. –K Temperature - K antenna- antenna – dB - dBi Ant. - Lt -dB Lr - dB dBi FE – 1 FE - 2 FE – 1 FE - 2 1. 131.25 -1.15 -0.83 -0.8 1.6 2.15 140.86 169.7 115.5 159.0 2. 116.1254 883 -1.53 -0.83 -0.8 0.9 2.15 140.32 169.06 140.0 178.0 3. 146.2036 13 -0.5 -0.83 -0.8 1.3 2.15 123.05 148.25 118.5 150.0 4. 161.2426 758 -0.22 -0.83 -0.8 1.8 2.15 121.06 145.86 121.5 152.0 Table 1.