LUNA Direction Finding System

This project is a correlative interferometry direction finding system for a sounding rocket using a vertically-orientated five-antenna circular array. By rotating the traditional correlative interferometry system, the system gets improved elevation range at the cost of limited azimuth. Each antenna implements an RF frontend with custom filter, and LNA. The system works by comparing the voltage phase between each antenna and a central reference antenna. These phase values are then correlated with a calibration dataset. The maximum correlation will be the radio wave’s direction of arrival.

Link Budget

The radio source is a Light APRS, which transmits at 144 MHz and 1W. The DF antenna must work for rocket launches up to 30,000 ft. With a safety factor of 1.25, the free path loss is -99 dB. The receiving antenna is a dipole with 2.15 dBi of gain. I also account for -5 dB of loss on each end.

$30 d B m - 0.5 d B - 99 d B + 2.15 d B i - 0.5 d B = - 67.85 d B m$

The measured noise floor is at −95 dB, so the signal will be 22.15 dB over the noise floor.

Parts List

AD8302

This is the log-amp phase comparison IC. It takes in the reference RF signal and an RF signal from one of the antennas and converts it to an analog voltage.

MCP3461R

The MCP3461R is a high speed and cheap ADC that is synchronized with other ADCs. These ADCs are connected in a SPI line to quickly read each set of five readings.

BFP740F

The BFP740F is a RF transistor with a noise factor of 0.8 dB and 30 dB of transducer gain. The transistor is implemented in a 50 ohm impedance matched inductively degenerated LNA.

DC Biasing

For the transistor to start conducting there must be approximatly 0.7 to 0.8 volts between the base and emitter. By adding a DC voltage to the input and output, the transistor can be biased. This puts the transistor into an always on state, where the small RF signal can be amplified.

When adding a DC bias, you must be careful to keep the RF and DC seperate. This can be achived with a capacitor to block DC from the RF input and output and an inductive RF choke to prevent the RF signal leaking into the DC supply.

For the LNA, I implemented the BFP740F with $I_{C} = 6 m A$ , and $V_{C E} = 3 V$ .

$\begin{array}{rc} h_{f e} = & 250 \\ I_{B} = & I_{C} / h_{f e} \\ = & 0.000024 \\ R_{B} = & \frac{3.3 - 0.8}{I_{B}} \\ = & 104 k Ω \\ R_{C} = & \frac{3.3 - V_{C E}}{I_{C}} \\ = & 50 Ω \end{array}$

Simulation

After optimizing the LNA for impedance match, noise factor, gain, and stability, the LNA gives the following performance.