Blog: Nulling Loop Antenna
FCL has completed a project based around a steerable loop antenna, operating as an interferometer and used for its direction finding (DF) capability. This led to some very interesting research work in the fields of medium frequency (MF) antenna design, ionospheric and tropospheric propagation and the frequency planning of terrestrial broadcast networks, both analog and digital. The operating frequency range included the MF band of 300 kHz to 3 MHz and the antenna was designed using elements comprising two mutually coupled wire loops on an insulating structure. The whole assembly was steerable in azimuth using a geared stepper motor and included a feedback system to accurately determine the azimuth position in real time. One of the loops was tuned to the operating frequency using a high capacitance varactor diode and the other was used for coupling the received signal to a communications receiver via the RF feeder cable. The varactor diode was tuned using a DC voltage applied to the RF feeder cable. The whole assembly was interfaced to a PC via a PCI parallel input output (PIO) card. Unfortunately the resolution of the integral ADC, being only 10 bits, was insufficient to track the deepest nulls of the measured antenna radiation patterns. A dedicated 16 bit ADC was fitted instead to convert the AGC voltage from the receiver into a PC readable form, and an 8 bit DAC to generate the varactor tune voltage. At frequencies across the operating band, the AGC voltage was calibrated with a logarithmic (dBuV) scale against a signal generator source, power meter and spectrum analyzer. A calibration factor was built into the software to compensate for changes in the radiation pattern itself and impedance over what is a multi-octave operating frequency range. The PC driver application was written initially around Visual Basic 6 but this was subsequently upgraded to C++. The C++ application provided compatibility with the Microchip® development system Integrated Development Environment (IDE) in line with another planned upgrade.
To verify the system performance, the PC application included a manual option to allow control of the receiver frequency, the varactor diode bias voltage and the antenna rotation whilst monitoring the AGC. This gave a very useful measure of the antenna radiation pattern at various frequencies and the azimuth bearings of the emission sources. One prototype was built to verify a principle and to demonstrate that two or more similar units could be installed at precisely known locations to improve the intelligence received from the DF. Allowing for quantisation noise, the theoretical signal to noise ratio (SNR) achievable from a 16 bit ADC is 98 dB and that from a 10 bit ADC is 62 dB. However, after allowing for practical configurations, the achievable SNR values can probably be reduced by a few dB. This value is often called the effective number of bits (ENOB). The nulls of the radiation pattern at some frequencies were more than 50 dB below the peak.
The objective of an interferometer is to add two identical signals from the same source in anti-phase, thereby causing an apparent cancellation or null. The position of a null of this type can normally be determined more precisely than looking for a peak from the same source. This works well with an antenna provided there are no other sources of any significance at the same frequency that could interfere with the null and give false readings. Once such a frequency is found, a reasonable approximation of the antenna radiation pattern can be obtained by scanning it in azimuth.