According to Wikipedia: "A Beverage consists of a horizontal wire one or two wavelengths long (hundreds of feet at HF to several kilometres for longwave) suspended above the ground, with the feedline to the receiver attached to one end and the other terminated through a resistor to ground."

http://en.wikipedia.org/wiki/Beverage_antenna

Memory Hint: Beverage antennas are receive antennas. When receiving a beverage, you want a receiver (glass) that's at least big enough to hold the the pour (beverage). When receiving RF, you want an antenna at least big enough to hold a wave.

Hint: long question - 'long' in answer

Hint 2: my favorite Beverage is a long drink.

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Atmospheric noise at the low HF frequencies referenced in this question (160 meter band = 1.8 to 2.0 MHz; 80 meter band = 3.5 to 4.0 MHz) is dominated by bursts of RF from impulsive events such as lightning strikes within thunderstorms. Since RF at these long wavelengths can travel quite far due to refraction (bending) provided by the ionosphere, lightning effects can be quite non-local and come from many directions, with especially large sources in rainy parts of the world (e.g. Caribbean, Far East, northern India).

Relevant for this question, the atmospheric noise below 4 MHz is very high because of these factors (up to 1,000,000 times or 60 dB that of normal background levels; see Kraus, "Radio Astronomy"; or Van Valkenburg, "Reference Data for Engineers"). Let's say your antenna has a reasonably high 20 dB of gain in its direction of maximum sensitivity. That means that the antenna will receive signals in other directions with at best 20 dB attenuation if you are between sidelobes. So if the atmospheric noise is 60 dB over background, the gain of your antenna only helped you attenuate it by 20 dB, and the noise comes crashing in at 60 - 20 = 40 dB (10,000 times) over background!

This means that for any practical antenna the amateur might construct, gain of the antenna is not going to solve your atmospheric noise problem, and therefore gain over a dipole is not important.

(To be complete, all these effects have a lot of variation depending on such factors as ionospheric conditions, time of day, season, weather fronts between the receiver and the source, and so on. But atmospheric noise is still a dominant factor in the low bands!)

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This question may actually be a little bit errant since RDF is also defined as Relative Directivity Factor.

In any case, the answer is Forward gain compared to average gain over the entire hemisphere. In open space this would actually be forward gain compared to average gain in all other directions.

The higher the RDF, the better an antenna generally is as a receiving antenna because it will receive mostly what it is pointed at while receiving less of signals (which may be noise) from other directions.

The other answers are wrong because: we are not comparing only to the reverse direction, we are comparing to average gain in other directions not isotropic, and we are not comparing to a dipole. Remember here that we are comparing the antenna to itself, not to another theoretical antenna, and we are comparing forward with all other directions not just back.

There is some more information available here.

Mnemonic: My relative is a dumb fool (RDF) who compares everything to a hemi.

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Loops are sensitive to magnetic fields, and the electric field interactions that may be present are an undesired effect. A grounded shield eliminates these interactions, which are most noticeable when the antenna is otherwise not receiving a signal, that is in the nulls.

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A wire loop antenna is symmetric, and therefore its pattern is the same along the planes of symmetry.

A resonant loop antenna has gain at right angles to the plane of the loop. That is, the "flat sides" of the antenna are sensitive and there are nulls near the edges. This provides ambiguity as to the direction the signal is coming from. Loop antennas are still great, though, because they're a very easy gain antenna to construct.

As the loop gets smaller than resonance, things change. Small loop antennas become sensitive on the edges and reject signals in the plane of the loop. In either case, the pattern is bidirectional, with a pair of peaks on opposite sides of the antenna and a pair of nulls on opposite sides of the antenna.

-icee

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A second dipole or vertical antenna known as a sense antenna can be electrically combined with a loop or a loopstick antenna. Switching the second antenna in obtains a net cardioid radiation pattern from which the general direction of the transmitter can be determined. Then switching the sense antenna out returns the sharp nulls in the loop antenna pattern, allowing a precise bearing to be determined.

-cfadams

Silly Memory Hint: Your Sensei will help One find Direction

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Hint: More voltage: More Length (either through loops or actual length of the antenna).

A receiving loop antenna (magnetic loop) works by sensing the oscillating magnetic field portion of a propagating radio wave, rather than the electric field portion that is much more familiar to typical antenna users.

The current induced in the loop by the radio wave obeys Ampere's law (see Wikipedia, "https://en.wikipedia.org/wiki/Maxwell's_equations"). This law states that electric currents and changes in electric fields are proportional to the magnetic fields circulating about the areas where they accumulate.

Note that part about "electric currents". So the amount of radio wave-induced current induced in your loop is a function of (a) the area of the loop and (b) the number of wire turns in the loop. This means that increasing one or both of these factors gives an increase in the amount of current flowing in the loop, and since the loop has an intrinsic resistance (all non-ideal conductors do), the induced voltage will also scale up as you increase one or both of these factors.

(Note that the same area and wire turns principle is used in other ways, e.g. transformers - as you increase the wire loop area or the number of turns, the induced current and hence the induced voltage goes up on the particular winding involved. The only difference here is that rather than an increase in core magnetic flux being the thing that induces current, the magnetic field of the radio wave itself induces current.)

Hint: 'increased' is in the question. 'Increasing' is in the correct answer.

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As the cardio part of the name implies, cardioid shape means heart shaped.

A transmitted cardioid radiation pattern has much of the radiated signal focused forward, in the direction that the antenna is pointing, with less and less signal energy as you go around the sides and up to almost behind antenna, then there is a distinct null (an area of no radiated energy) directly behind the antenna.

In the case of direction finding, if you rotate the cardioid patterned antenna connected to your receiver, until the incoming signal is lost, then it would result in the antenna pointing directly away from the signal source.

Note that it is the radiation pattern from the antenna that is heart shaped, not the antenna itself. There are a number of different antenna designs of various shapes that make a cardioid radiation pattern.

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