Knudsen's array

Knudsen’s array

The Knudsen station is still under construction.


Knudsen Station serves as UCLA’s primary Earth Station for ELFIN, with VHF (2m) uplink and UHF (70cm) downlink antennas. It provides a higher gain than the traditional OSCAR-class amateur radio station to allow ELFIN’s transmitter to be operated at a lower level.

The Knudsen station supports other university nanosatellite missions. This helps demonstrate technical capability, improves and maintains our proficiency, and fosters relationships within the nanosatellite and amateur radio communities that will be beneficial for ELFIN’s launch and early orbit operations.

Main features of the Knudsen Station include:



UHF/VHF Antenna 436CP42UG (4x) / 2MCP8A (2x)
UHF/VHF Gain (Est) 23.9dBi / 9.2dBi
Polarization Circular
Preamplifier LNA 70 with mast housing (1x, after a 70cm 4 port power divider)
Noise Floor 0.35dB +/- 0.05
Gain 21.0 +/- 1.0 TBD
Amplifier Mirage B-2518-G
Output Power 160 W
Rotor AlphaSpid RAS
Vertical Load 250kg
Torque 158 N m
Break Torque 1582 N m
Coax LMR-400 (UHF) / LMR-200 (VHF) / LMR-900 (run to shed)
dB/100m @ 150MHz 5.0 / 2.2
dB/100m @ 450MHz 8.9 / 3.8
Run Length (Est) 10m / 15m


We have two arrays mounted on top of a RT-1832 tower and controlled with an AlphaSpid RAS rotor. One is a quad array of 436CP42UG cross polarized UHF Yagis, arranged in 2×2 fashion with 6 ft spacing. The uplink is handled by a pair of 2MCP8A cross polarized VHF yagis, one each for RHCP and RHCP. The vertical mast is a hollow aluminum rod, while the cross braces are hollow fiberglass rods.

The preamps will not initially be switchable because the satellite uplink is on VHF, so there is no need for the added complexity for ELFIN. However, adding a switching circuit is a possible upscope to allow UHF uplinks to support other missions. The system uses LMR-400 coax cable from antennas to the port divider (70M-4PORT. From the port dividers at the base of the tower, we will use a thicker coax (LMR-900) for the ~6m run to the shed.

Knudsen Rotor

The RAS rotor, aluminum mast, and fiberglass crossbeam

Rotor Control System

The RAS uses a rotary encoder for position information, which is inherently a relative motion measurement that needs periodic zeroing. We sought to improve the control and precision of our antenna array. Using a magnetometer and triple-axis accelerometer, we can get a direction in 3D space. We plan to use LSM303s on an I2C bus attached to Arduino Pro Minis on the T-braces for data collection. This may be relayed to an Arduino Mega host rotor controller in the shed with an LCD and buttons for manual control. Data (TTL serial) and power will travel over shielded cat5. Code will be based on K3NG’s existing design, but may have to be modified for a different setup.

Our final design plans for multiple LSM303s to check the relative alignment of the overall array. The array direction will be calculated to be an average of these individual readings to achieve higher overall accuracy. Additionally, the readings will tell us if the antenna alignment is out of specification and needs to be fixed.

By using this custom system, we hope to gain:

  • Absolute pointing relative to the earth rather than pointing relative to the alignment of the rotor
  • Varaible rotation speeds of the rotors (reduces jerky motion of a start-stop system that causes wear on the system)
  • Azimuth and elevation slewing simultaneously
  • Array alignment data


On the UHF (downlink) side, we are using a USRP N210 with a TVRX2 daughterboard, fed by an additional LNA70 preamp.

On the transmit site, we’re using an Icom IC910-H radio as the transmitter, with a Kantronics KPC-9612+ hardware TNC. From the Kantronics, data will be routed to a fanless industrial computer and then piped back to the Mission Operation Center. Both 120VAC and 13.8VDC (Anderson Powerpole) is available, and a backup power supply sized for 24 hours is also planned.


We’re using an Arrow Brentwood 5’x4′ shed for our main rooftop electronics. The shed sits atop two plastic pallets to protect the roof’s weather membrane,  allow runoff water to flow underneath and also increase ventilation in the shed. There is a roof fan to increase ventilation, and the shed has a reflective painting to increase the albedo. These measures are designed to reduce overall temperatures in the shed and improve the life of equipment stored there.

Block Diagram