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Subelement E9
ANTENNAS AND TRANSMISSION LINES
Section E9H
Receiving Antennas: radio direction finding antennas; Beverage Antennas; specialized receiving antennas; longwire receiving antennas
When constructing a Beverage antenna, which of the following factors should be included in the design to achieve good performance at the desired frequency?
  • Its overall length must not exceed 1/4 wavelength
  • It must be mounted more than 1 wavelength above ground
  • It should be configured as a four-sided loop
  • It should be one or more wavelengths long

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

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Which is generally true for low band (160 meter and 80 meter) receiving antennas?
  • Atmospheric noise is so high that gain over a dipole is not important
  • They must be erected at least 1/2 wavelength above the ground to attain good directivity
  • Low loss coax transmission line is essential for good performance
  • All of these choices are correct

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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What is an advantage of using a shielded loop antenna for direction finding?
  • It automatically cancels ignition noise in mobile installations
  • It is electro statically balanced against ground, giving better nulls
  • It eliminates tracking errors caused by strong out-of-band signals
  • It allows stations to communicate without giving away their position

Direction finding is an activity that’s both fun and useful. One of the ways that it’s useful is to hunt down noise sources. It can also be used to hunt down stations causing harmful interference.

A variety of directional antennas are used in direction finding, including the shielded loop antenna.

An advantage of using a shielded loop antenna for direction finding is that it is electro-statically balanced against ground, giving better nulls. (E9H12) The main drawback of a wire-loop antenna for direction finding is that it has a bidirectional pattern. (E9H05)

Source: KB6NU.com - Extra Class question of the day: Direction finding

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What is the main drawback of a wire-loop antenna for direction finding?
  • It has a bidirectional pattern
  • It is non-rotatable
  • It receives equally well in all directions
  • It is practical for use only on VHF bands

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

A resonant loop antenna has gain normal 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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What is the triangulation method of direction finding?
  • The geometric angles of sky waves from the source are used to determine its position
  • A fixed receiving station plots three headings to the signal source
  • Antenna headings from several different receiving locations are used to locate the signal source
  • A fixed receiving station uses three different antennas to plot the location of the signal source

You need three or more receiving sites distributed at some distance from each other and in different locations around the approximate search area. Each directional receiving antenna is rotated for maximum RSL and the direction azimuths are then plotted on a map. Where the lines cross is the location of the transmitter.

One receiver could drive around the search area, repeating the direction-finding process. However, this approach takes more time.

dbrejda

Hint: The question has the word Triangulation in it. Tri= 3. Thus it would usually require 3 or more separate antenna/stations to determine the location.

-KE0IPR

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Why is it advisable to use an RF attenuator on a receiver being used for direction finding?
  • It narrows the bandwidth of the received signal to improve signal to noise ratio
  • It compensates for the effects of an isotropic antenna, thereby improving directivity
  • It reduces loss of received signals caused by antenna pattern nulls, thereby increasing sensitivity
  • It prevents receiver overload which could make it difficult to determine peaks or nulls

Radio signals generally fall off with the inverse square of distance. When you are very close to a radio transmitter, the received signal can be very, very strong-- to the point that it is difficult to determine whether or not your directional antenna is pointed at the receiver.

Even in a "null" where your directional antenna provides, for example, 25 dB of attenuation, there may be no discernible static and the signal strength meter may still read its maximum value. Adding additional, constant attenuation lowers the signal and makes it easier to find.

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What is the function of a sense antenna?
  • It modifies the pattern of a DF antenna array to provide a null in one direction
  • It increases the sensitivity of a DF antenna array
  • It allows DF antennas to receive signals at different vertical angles
  • It provides diversity reception that cancels multipath signals

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

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Which of the following describes the construction of a receiving loop antenna?
  • A large circularly polarized antenna
  • A small coil of wire tightly wound around a toroidal ferrite core
  • One or more turns of wire wound in the shape of a large open coil
  • A vertical antenna coupled to a feed line through an inductive loop of wire

Loop antenna or Radio antenna is made of a loop of wire or some electrical conductor with a circumference equal to the wavelength. There are two basic sizes, small (magnetic) and large (resonant) type. The ends of the loop are connected to a balanced transmission line.

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How can the output voltage of a multiple turn receiving loop antenna be increased?
  • By reducing the permeability of the loop shield
  • By increasing the number of wire turns in the loop and reducing the area of the loop structure
  • By winding adjacent turns in opposing directions
  • By increasing either the number of wire turns in the loop or the area of the loop structure or both

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.)

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What characteristic of a cardioid pattern antenna is useful for direction finding?
  • A very sharp peak
  • A very sharp single null
  • Broad band response
  • High-radiation angle

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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