The HP 10509A Circular Loop Antenna is a specialized active magnetic-loop receiving antenna developed by Hewlett-Packard in the early 1960s. It was originally supplied with the HP 117A VLF Comparator system for receiving the 60 kHz signal from WWVB, the U.S. time and frequency standard station.
Key Characteristics
- Loop diameter: approximately 43 inches (1.09 m).
- Constructed from heavy-duty aluminium tubing.
- Contains a built-in preamplifier housed in an enclosure at the base of the loop.
- Uses a single coaxial cable carrying both DC power and received RF signals.
- Designed primarily for VLF reception around 60 kHz, although later modifications and replacement preamplifiers have allowed operation across roughly 10–100 kHz.
How It Works
The HP 10509A is a magnetic-field antenna. Unlike a dipole, which responds mainly to the electric field component of a radio wave, a loop antenna responds primarily to the magnetic field component. This often provides better rejection of local electrical noise sources. The loop exhibits the classic figure-of-eight reception pattern with deep nulls perpendicular to the plane of the loop.
For a small magnetic loop, the received signal is proportional to the loop area and magnetic field strength. The loop behaves electrically as an inductive element feeding a high-gain amplifier.
Historical Versions
Several amplifier versions were produced:
- Nuvistor vacuum-tube version
- Push-pull 13CW4 Nuvistor triodes.
- Mechanical (magnetostrictive) filter with approximately 30 Hz bandwidth.
- Solid-state version
- Dual JFET front end.
- Crystal-filter or transformer-coupled output stages.
Relevance to Radio Astronomy
Although designed for frequency-standard reception rather than astronomy, the HP 10509A is interesting because:
- It is an early example of an active magnetic loop.
- Similar loops are used for Sudden Ionospheric Disturbance (SID) monitoring and VLF propagation studies.
- VLF observations can be used to study solar flares and ionospheric changes, topics that overlap with amateur radio astronomy.
It would not be suitable for 1420 MHz hydrogen-line work, but it could be useful for monitoring VLF transmitters and ionospheric effects associated with solar activity.