E9H

Receiving antennas and direction finding

Explain why Beverage antennas are terminated, why low-band receiving favors directivity over loss, and how loops and sense antennas find bearings.

11 pool questions1 on the exam

Syllabus: Receiving antennas: radio direction finding (RDF) techniques; Beverage antennas; single- and multiple-turn loops

A Beverage is a long wire with a resistor that eats the reverse signal

A Beverage antenna is a long wire close to the ground that receives a traveling wave, so it should be at least one wavelength long. A termination resistor at the far end absorbs signals from the reverse direction; set it for the least SWR variation across the band.

Wanted signals run toward the receiver. The termination resistor absorbs unwanted ones from the reverse direction instead of letting them bounce back.

RememberAt least 1 λ long. Termination resistor absorbs the reverse signal.

On 160 and 80 m, directivity beats efficiency

Atmospheric noise on the low bands is so strong that it sets the limit, not the receiver. A lossy antenna weakens signal and noise together, so signal-to-noise barely changes. Directivity helps because it favors one direction. Receiving directivity factor (RDF) is peak gain compared with the average over the hemisphere around and above the antenna.

Loss lowers signal and atmospheric noise together, so signal-to-noise barely changes. Directivity lifts the signal above the average noise.

Antenna loss10 dBBeam sharpness1

Receiving directivity factor4.3dB

Signal to noise14.2dB

RememberNoise-limited: directivity matters more than loss. RDF = peak ÷ average.

A small loop has two nulls; a sense antenna leaves one

Direction finders aim at a null, which is sharper than a peak. A small loop has a bidirectional pattern: two nulls, so the bearing is ambiguous by 180°. A sense antenna reshapes it into a cardioid with a single null. A terminated single-turn loop (pennant) is naturally a cardioid. An electrostatic shield removes unbalanced capacitive coupling, which deepens the nulls.

A small loop alone has two opposite nulls, so a bearing is ambiguous by 180°. A sense antenna adds an all-around signal and leaves just one.

Sense antenna strengthoff

Nulls2

RememberLoop alone: two nulls. Cardioid: one null.

More turns or more area, more voltage

A multiple-turn loop puts its turns in series, so its output voltage rises with the number of turns and the area enclosed.

Output voltage grows with the number of turns and with the area enclosed.

Number of turns2Loop area1× reference

Output voltage2×

RememberOutput voltage grows with turns and area.

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