Understanding the Null of a Shielded Active H-Field Loop Antenna

Shielded active H-field loop antennas, like the  RF.Guru OctaLoop, are popular among ham radio operators due to their excellent noise rejection, directionality, and deep nulls in their reception pattern. These characteristics make them highly effective for reducing man-made interference, locating signal sources, and enhancing weak-signal reception. However, many hams wonder if they need to mount their loop antenna on a rotor for HF reception or if simple manual orientation adjustments are sufficient. This article addresses these concerns and explains when the null of such an antenna applies.

The Null in a Vertically Mounted OctaLoop

The gray line represents the loop antenna seen from above.

In 3D it looks like this.

When looking at a loop antenna from the broadside (perpendicular to its plane), you are directly aligned with its deep null. The loop’s null is a region where the received or transmitted signal strength is at its weakest.

Physically, this means that signals propagating from the deep null outward to the exterior are barely picked up by the antenna. Conversely, maximum reception or radiation occurs along the plane of the loop, perpendicular to the null direction.

If you imagine standing inside the null and looking through the loop, it’s as if you’re peering into a quiet zone where signals are almost absent. This effect is crucial for direction-finding applications, as the deep null can be used to pinpoint the direction of a signal source by rotating the loop until the received signal reaches a minimum.

What Determines the Null in a Loop Antenna?

The null in a loop antenna occurs when the received signal is at an angle where destructive interference cancels it out. For a perfectly balanced loop, the null is perpendicular to the plane of the loop. However, real-world conditions such as ground effects, environmental noise, and antenna construction can slightly alter this behavior.

Several key factors determine when the null applies:

Signal Polarization

• H-field loops primarily respond to magnetic field components, which means they naturally reject vertically polarized signals that have weak magnetic field components in the plane of the loop.
• Horizontally polarized signals can still be received effectively but are subject to the loop's directional properties.
• Circularly polarized signals may not be completely nulled, as they have both vertical and horizontal components.

Frequency Dependence

• Lower frequencies (below 1 MHz) tend to exhibit stronger ground-wave propagation, which can interact with nulling effects.
• Mid-range HF frequencies (1.8–30 MHz) show significant skywave propagation, where reflections from the ionosphere can alter signal angles, affecting how deep or broad the null appears.
• Higher frequencies (>30 MHz) tend to behave more predictably in direct line-of-sight paths, where nulling is more consistent with theoretical expectations.

Signal Source and Path

• Groundwave Signals: If the signal source is primarily groundwave, nulls are most effective at expected perpendicular angles.
• Skywave Signals: Ionospheric reflections can modify the incident angle of the received signal, which might cause the null to shift or reduce in effectiveness.
• Local Interference: If the interfering signal originates from a nearby source, multipath effects can alter nulling behavior, sometimes causing the antenna to exhibit multiple null zones instead of a single deep null.

Do You Need a Rotor for HF Reception?

For most HF ham radio applications, a rotor is not necessary. Since skywave signals arrive from multiple angles due to ionospheric reflection, precise nulling with a loop antenna may not always be as predictable as with groundwave signals. Instead, manual rotation of the antenna during operation can often achieve sufficient noise reduction and directionality.

However, in certain cases, such as direction-finding (DF) applications or in environments with significant local noise, a rotor can be beneficial. It allows for fine adjustments to null out unwanted interference or to focus reception on a desired station. A simple manual positioning setup may be adequate for casual listening, while a rotor system can be useful for serious DXing and interference mitigation.

Using Two Loops for Optimal Noise Reduction

One effective strategy for further minimizing interference is using two loop antennas positioned at approximately 90 degrees relative to each other. This setup provides multiple benefits:

• Deeper Nulls: By orienting two loops at right angles, you can combine their nulling effects for greater interference rejection.
• Optimized Positioning: The best placement is to position both loops so that their nulls are directed towards the most significant noise source (e.g., your house or nearby power lines). Alternatively, placing both nulls at a 45-degree angle relative to your house can further minimize noise pickup from multiple sources.
• Improved Signal Reception: This configuration allows more flexibility in adjusting the null pattern without requiring constant repositioning.

A phasing network or switching system can be used to combine the outputs of both loops for optimal noise rejection and directional selectivity, giving superior reception performance compared to a single loop.

Active Magloop Reception of NVIS Signals on 80m and 40m

An active magnetic loop (magloop) on 80m and 40m can receive NVIS (Near Vertical Incidence Skywave) signals, but its effectiveness depends on several factors.

How an Active Magloop Receives NVIS

• Omnidirectional in the Horizontal Plane: A magloop’s radiation pattern is mostly circular in the horizontal plane, meaning it picks up signals from all azimuth directions equally.
• High Angle Sensitivity: A horizontally positioned magloop will have strong response to signals arriving at steep angles, which is ideal for NVIS reception. A vertically positioned magloop favors lower-angle DX signals but still picks up NVIS to some extent.
• Magnetic Field Reception: Magloops primarily respond to the magnetic component of electromagnetic waves, reducing local electric noise (QRM) and making them quiet receivers for weak NVIS signals.

Best Orientation for NVIS Reception

• Horizontal Magloop (Best for NVIS): Placing the magloop horizontally at a height of 0.1λ to 0.2λ (e.g., 4–8m for 80m, 2–4m for 40m) enhances NVIS reception.
• Vertical Magloop: In a typical vertical mounting, the loop has deep nulls broadside to the plane, which makes it less optimal for NVIS but still usable.

Antenna Type	NVIS Reception	Notes
Horizontal Dipole (λ/2 at low height)	Excellent	Classic NVIS antenna at 0.1λ–0.2λ height
Horizontal Magloop (Low Height)	Good	Better if elevated slightly
Vertical Magloop	Fair to Good	Picks up NVIS but favors lower angles

Conclusion

For most HF ham radio operators, mounting a shielded H-field loop like tthe RF.Guru OctaLoop on a rotor is not strictly necessary. Manual orientation adjustments can effectively null out interference and optimize reception. However, for hams who are serious about direction finding or live in high-noise environments, a rotor system can be a valuable addition. Additionally, using two loop antennas at a 90-degree angle can provide superior interference rejection and flexibility. Understanding the interaction of polarization, frequency, and signal path with the loop’s nulls can help operators make the most of their antenna for improved noise rejection and better HF reception.

Written by Joeri Van Dooren, ON6URE – RF, electronics and software engineer, complex platform and antenna designer. Founder of RF.Guru. An expert in active and passive antennas, high-power RF transformers, and custom RF solutions, he has also engineered telecom and broadcast hardware, including set-top boxes, transcoders, and E1/T1 switchboards. His expertise spans high-power RF, embedded systems, digital signal processing, and complex software platforms, driving innovation in both amateur and professional communications industries.