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Receive loops and active antennas
Small receiving antennas for noisy locations.
Small receiving antennas for noisy locations.
On the lower HF bands a receiving antenna does not have to be big or efficient. It only has to hear the signal as far above the noise as possible (see Beverage antennas for why). That makes two small antennas useful: the small receiving loop and the active antenna, a very short probe with an amplifier right at its base. Their strength is not gain; it is that they can be small, placed away from noise, or aimed so a noise source falls in a null.
A loop smaller than a tenth of a wavelength responds to the magnetic part of the wave. It hears best in the plane of the loop, and a signal arriving broadside to it, straight through the loop, is nulled. The null is sharp, so you can turn the loop to put it on a local noise source or a strong interfering station. Output voltage rises with the number of turns and the area enclosed, which is why a multi-turn loop of modest size can still deliver a usable signal.
Close to a noise source such as a switching supply or a power line, much of the noise is in the electric field, and a loop mostly ignores that. Loops are therefore often reported to hear less local noise than a whip beside them, although the result depends on the noise source and location. A resonant loop, tuned by a capacitor, is also narrow-band, which helps reject strong signals on other frequencies.
A single-turn terminated loop, such as a pennant or flag antenna, has a cardioid pattern with one null, and so a definite direction.
A whip much shorter than a quarter wavelength behaves like a tiny capacitor with a very high reactance and almost no radiation resistance. Connect it directly to coax and nearly everything is lost to the mismatch. An active antenna puts a low-noise amplifier with a very high input impedance right at the whip and delivers 50 Ω to the coax. The same coax usually carries DC power up through a bias tee in the shack.
On the low bands where external noise is far above the receiver's, the small signal loss does not hurt the signal-to-noise ratio, and the antenna is tiny and works across a wide frequency range without tuning.