E9H: Receiving Antennas

On the low HF bands — 160 and 80 meters — atmospheric noise dominates, and the criteria for a good receiving antenna differ fundamentally from those for transmitting. Specialized antennas designed for directivity and noise rejection are essential for serious low-band work and radio direction finding (RDF).

This lesson covers the Beverage antenna design and termination, receiving directivity factor (RDF), loop antennas for direction finding, electrostatic shielding, the sense antenna, cardioid patterns, and multiple-turn loops.

Low-Band Receiving: Why Directivity Matters

On 160 and 80 meters, the dominant interference is atmospheric noise — static crashes and the aggregate noise from distant lightning strikes. This noise arrives from all directions, but it is so intense that antenna losses become a secondary concern. What matters most is the antenna's ability to reject noise arriving from undesired directions while accepting signals from the desired direction. Therefore, for these bands, directivity is much more important than losses in the receiving antenna.

Receiving Directivity Factor (RDF)

The Receiving Directivity Factor (RDF) is a figure of merit specific to receiving antennas. It is defined as the ratio of the antenna's peak gain to its average gain over the hemisphere around and above the antenna. An antenna with high RDF concentrates its sensitivity in a narrow range of directions, providing better discrimination between the desired signal and diffuse atmospheric noise arriving from all directions.

Beverage Antenna

The Beverage antenna is a long horizontal wire antenna specifically designed for receiving on the low bands. Key design requirements and characteristics:

• Length: Should be at least one wavelength long at the desired frequency. Longer Beverages are more directional and have better RDF, but even one wavelength provides useful directivity.
• Terminating resistor: A resistor at the far end of the Beverage absorbs signals from the reverse direction, converting the antenna from bidirectional to unidirectional. The correct terminating resistance value is found by minimizing variation in SWR over the desired frequency range.

Loop Antennas for Direction Finding

A small wire loop antenna produces a bidirectional null pattern (figure-eight) — it has sharp nulls at 90 degrees to the plane of the loop and broad response in the plane of the loop. This bidirectional null is a challenge for direction finding because the null points in two opposite directions, making it impossible to determine which of two possible headings is correct without additional information.

Electrostatic Shielding

Placing an electrostatic shield (an incomplete conducting loop around the antenna loop) eliminates unbalanced capacitive coupling between the antenna and its surroundings. Without the shield, nearby objects — ground, structures, the operator — couple electrostatically to the loop, distorting the pattern and degrading the sharpness of the nulls. The shield allows only the magnetic field component to couple to the loop, improving the depth of its nulls and making the pattern more symmetric.

Sense Antenna

A sense antenna resolves the 180-degree ambiguity of a loop's bidirectional null. By combining the loop signal with a signal from a small omnidirectional antenna (the sense antenna), the loop's figure-eight pattern is combined with a circular pattern. When the two signals are added in the correct phase, the result is a cardioid pattern with a null in only one direction. The operator can then determine the unique bearing to the signal source.

Cardioid Pattern and Its DF Utility

A cardioid pattern — produced by a single-turn terminated loop such as a pennant antenna — has a single deep null in one direction and a broad forward lobe. The feature that makes a cardioid pattern antenna useful for direction finding is precisely this single null: unlike the bidirectional null of an unterminated loop, a single null uniquely identifies one bearing. By rotating the antenna for minimum signal (the null), the operator can determine the direction to the transmitter without ambiguity.

Multiple-Turn Loop Antennas

The output voltage of a receiving loop antenna can be increased by increasing the number of turns and/or the area enclosed by the loop. Both the number of turns and the loop area directly multiply the induced voltage. Larger loops intercept more of the magnetic flux from the arriving wave; more turns add the induced voltages in series. This is why ferrite rod antennas (many turns wound on a high-permeability core) can provide useful signal levels even at very small physical size.

E9H Practice Questions