An isotropic radiator is a theoretical point source of electromagnetic waves which radiates the same intensity of radiation in all directions. It has no preferred direction of radiation. It radiates uniformly in all directions over a sphere centred on the source.

Isotropic radiators are used as reference radiators with which other sources are compared.

Source: Wikipedia - Isotropic Radiator

One Word Key "theoretical"

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A reference dipole antenna is defined to have 2.15dBi gain.

A useful conversion between dBd and dBi is as follows:

dBi = dBd + 2.15

dBd = dBi - 2.15

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Isotropic antennas are ideal (theoretical) antennas that have equal power in all directions. They are used as references for antenna gain.

The word "isotropic" means "uniform in all orientations/directions".

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The feed-point impedance of an antenna can be influenced by near-by conductive objects, including proximity to the ground.

Antenna height is the only answer that has anything to do with the antenna and its surrounding environment. The other answers do not affect the antenna.

Another point of view - the question is about what effects the feed point of the antenna. The transmission line length, settings of an antenna tuner, and input power are not part of the impedance at (and after) the feed point of the antenna.

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Radiation resistance is that part of an antenna's feedpoint resistance that is caused by the radiation of electromagnetic waves from the antenna, as opposed to loss resistance (also called ohmic resistance) which is caused by ordinary electrical resistance in the antenna, or energy lost to nearby objects, such as the earth, which dissipate RF energy as heat.

The radiation resistance is determined by the geometry of the antenna, whereas the ohmic resistance is primarily determined by the materials of which it is made and its distance from and alignment with other nearby conductors or semi-conductors, and what those are made of.

Both radiation and ohmic resistance depend on the distribution of current in the antenna.

Eliminate the answer referring to "transmission line" because that is not what they are referring to as an "antenna system".

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The key is to remember that this antenna is one wavelength long "folded" in to a thin loop one-half wavelength long.

For more information see wikipedia: Folded Dipole Antenna

Silly memory trick: It's folded in half.

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Bandwidth is a measurement of frequencies, as in the width of a range of frequencies.

An antenna's bandwidth is the range of frequencies it works best on, though it's impossible to give an exact formula without the exact performance requirement.

For example the performance requirement might be "SWR less than 1.7" in which case the antenna could be modeled or tested and the exact range of frequencies for that requirement specified.

Hint: A Band "performs". The answer contains the word "performance".

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Hint: GROUND losses is in the SOIL. The only answer talking about the ground. m

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A 1/2-wave dipole antenna has approximately \(2.15 \text{ dB}\) of gain over an isotropic antenna. So \[6 \text{ dB} - 2.15 \text{ dB} = 3.85 \text{ dB}\]

(The directive gain of a half-wave dipole is 1.64. It has a 2.15 gain over an isotropic antenna or \(10\log_{10}(1.64)\approx2.15\text{ dBi}\))

The distractor of 8.15dB relies on you remembering the 2.15dB difference between isotopic and dipole antennas, so pay careful attention to whether you're adding or subtracting 2.15dB!

Silly way to remember: The 1/2 way number is in the answer, which is a sum of the ends: 3.85... 3+5=8

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Given: It's well-known that a half-wave dipole has \(2.15 \text{ dB}\) gain over an ideal isotropic radiator.

Let \(H =\) the gain of an ideal isotropic radiator

Let \(W =\) the gain of a half-wave dipole

Therefore \(W = H + 2.15 \text{ dB}\), or solving for \(H\), \(H = W - 2.15 \text{ dB}\)

Now, let \(X =\) the gain of the antenna in question

If that antenna in question exhibits \(12\text{ dB}\) over an isotropic antenna, then

\(X = H + 12 \text{ dB}\)

Substituting, we get

\(X = W - 2.15\text{ dB} + 12\text{ dB}\) \(= W + 9.85 \text{ dB}\)

Therefore, the antenna in question (\(X\)) exhibits a gain of \(9.85\text{ dB}\) over that (\(W\)) of a half-wave dipole.

Remember that a half-wavength dipole has 2.15 dB gain over an ideal isotropic radiator. Then take the difference between 12 dB and 2.15 dB to arrive at the answer of 9.85 dB.

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The electrical resistance of an antenna is composed of its ohmic resistance plus its radiation resistance. The energy lost due to radiation resistance is the energy that is converted to electromagnetic radiation.

It isn't the combined losses of antenna elements and feed line because radiation resistance is not related to feed line losses.

It isn't the specific impedance of the antenna because the radiation resistance is only a portion of the antenna's impedance.

It isn't the resistance in the atmosphere that an antenna must overcome because the radiation resistance is not a property of the atmosphere. It is determined by the geometry of the antenna.

The correct answer is therefore the value of a resistance that would dissipate the same amount of power as that radiated from an antenna

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