E9D-2012: Directional antennas: gain; satellite antennas; antenna beamwidth; losses; SWR bandwidth; antenna efficiency; shortened and mobile antennas; grounding
How does the gain of an ideal parabolic dish antenna change when the operating frequency is doubled?
Gain increases by 6 dB
Gain does not change
Gain is multiplied by 0.707
Gain increases by 3 dB
How can linearly polarized Yagi antennas be used to produce circular polarization?
Arrange two Yagis perpendicular to each other with the driven elements at the same point on the boom and fed 90 degrees out of phase
Stack two Yagis, fed 90 degrees out of phase, to form an array with the respective elements in parallel planes
Stack two Yagis, fed in phase, to form an array with the respective elements in parallel planes
Arrange two Yagis collinear to each other, with the driven elements fed 180 degrees out of phase
How does the beamwidth of an antenna vary as the gain is increased?
It increases geometrically
It increases arithmetically
It is essentially unaffected
Why is it desirable for a ground-mounted satellite communications antenna system to be able to move in both azimuth and elevation?
In order to track the satellite as it orbits the Earth
So the antenna can be pointed away from interfering signals
So the antenna can be positioned to cancel the effects of Faraday rotation
To rotate antenna polarization to match that of the satellite
Where should a high-Q loading coil be placed to minimize losses in a shortened vertical antenna?
Near the center of the vertical radiator
As low as possible on the vertical radiator
As close to the transmitter as possible
At a voltage node
Why should an HF mobile antenna loading coil have a high ratio of reactance to resistance?
To minimize losses
To swamp out harmonics
To maximize losses
To minimize the Q
What is a disadvantage of using a multiband trapped antenna?
It might radiate harmonics
It radiates the harmonics and fundamental equally well
It is too sharply directional at lower frequencies
It must be neutralized
What happens to the bandwidth of an antenna as it is shortened through the use of loading coils?
It is decreased
It is increased
No change occurs
It becomes flat
What is an advantage of using top loading in a shortened HF vertical antenna?
Improved radiation efficiency
Greater structural strength
What is the approximate feed point impedance at the center of a two-wire folded dipole antenna?
What is the function of a loading coil as used with an HF mobile antenna?
To cancel capacitive reactance
To increase the SWR bandwidth
To lower the losses
To lower the Q
What is one advantage of using a trapped antenna?
It may be used for multiband operation
It has high directivity in the higher-frequency bands
It has high gain
It minimizes harmonic radiation
What happens to feed point impedance at the base of a fixed-length HF mobile antenna as the frequency of operation is lowered?
The radiation resistance decreases and the capacitive reactance increases
The radiation resistance decreases and the capacitive reactance decreases
The radiation resistance increases and the capacitive reactance decreases
The radiation resistance increases and the capacitive reactance increases
Which of the following types of conductor would be best for minimizing losses in a station's RF ground system?
A wide flat copper strap
A resistive wire, such as a spark plug wire
A cable with 6 or 7 18-gauge conductors in parallel
A single 12 or 10-gauge stainless steel wire
Which of the following would provide the best RF ground for your station?
An electrically-short connection to 3 or 4 interconnected ground rods driven into the Earth
A 50-ohm resistor connected to ground
An electrically-short connection to a metal water pipe
An electrically-short connection to 3 or 4 interconnected ground rods via a series RF choke
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