Wednesday, October 14, 2009

Extra Class Exam Grab Bag, Bob, No. 73

Tonight was supposed to be about current use of Spread Spectrum in Ham Radio but I couldn't find any information about it.  If anyone on frequency knows about it or knows where to point me to look please let me know.

Instead, tonight I will discuss a few test questions from the Extra Class exam. 

E9B08 (C)
How does the total amount of radiation emitted by a directional (gain) antenna compare with the total amount of radiation emitted from an isotropic antenna, assuming each is driven by the same amount of power?
A. The total amount of radiation from the directional antenna is increased by the gain of the antenna
B. The total amount of radiation from the directional antenna is stronger by its front to back ratio
C. There is no difference between the two antennas
D. The radiation from the isotropic antenna is 2.15 dB stronger than that from the directional antenna

The important point to take away from this is that the total radiation depends solely on the amount of power that is transferred to the antenna.  What the antenna design can do is shape or focus that radiation so that more goes in the direction you want and less in the directions you don't want.

E9D06 (C)
Why should an HF mobile antenna loading coil have a high ratio of reactance to resistance?
A. To swamp out harmonics
B. To maximize losses
C. To minimize losses
D. To minimize the Q

The reactance in the loading coil doesn't contribute to heat loss.  All heat loss comes from resistance.  Resistance acts directly on current, converting it to heat.  Reactance, being out of phase with current, doesn't produce heat and doesn't contribute to losses.
E9D08 (B)
What happens to the bandwidth of an antenna as it is shortened through the use of loading coils?
A. It is increased
B. It is decreased
C. No change occurs
D. It becomes flat

Which of the following types of conductor would be best for minimizing losses in a station's RF ground system?
A. A resistive wire, such as a spark-plug wire
B. A thin, flat copper strap several inches wide
C. A cable with 6 or 7 18-gauge conductors in parallel
D. A single 12 or 10 gauge stainless steel wire

The reason for the thin, flat copper strap is that it increases the Capacitive reactance vs. the Inductive reactance making the strap less likely to become resonant.  For similar reasons the RF ground strap should be as short as possible.
E9D15 (C)
Which of these choices would provide the best RF ground for your station?
A. A 50-ohm resistor connected to ground
B. A connection to a metal water pipe
C. A connection to 3 or 4 interconnected ground rods driven into the Earth
D. A connection to 3 or 4 interconnected ground rods via a series RF choke

We want a very low impedance path directly to earth ground.  We are not trying to match impedance here, just reduce it, so no resistors or chokes.  The metal water pipe might serve as a safety ground, although today, more and more pipes are PVC so don't count on that metal pipe you are connecting to, to be metal the entire length.  But even so, an all metal pipe system in your house makes a pretty active antenna system in itself.  This is not what you want for your RF ground.  The short-run ground strap to interconnected ground rods in the Earth is the way to go.

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