Sample questions from the Amateur Extra exam pool

Amateur Extra: Variable-Length Coding

Which of the following HF digital modes uses variable-length coding for bandwidth efficiency?

A. RTTY
B. PACTOR
C. MT63
D. PSK31

Now, I guess this answers my question about last weeks Extra question. Mainly why is it important.  It’s important because it does matter in transmission efficiency.  So which mode would be a good and efficient method of sending digital transmissions.  Well, once more lets examine the possibilities.

RTTY?  RTTY or Radio Teletype is more or less and ancient method of sending text via radio.  It actually dates back into the 1930’s.  RTTY uses a fixed-bit encoding.  It’s NOT variable.  So that basically rules it out as an answer.  (See: https://en.wikipedia.org/wiki/Radioteletype)

PACTOR?  PACTOR is a much more recent development, but its primary use is to send large messages, essentially being packet over HF.  It has built-in error correction, and other features.  Unfortunately, PACTOR I uses anywhere from 400 to 2600 Hz bandwidth, so not all that efficient.  (see: https://en.wikipedia.org/wiki/PACTOR)

MT63?  Now we’re getting somewhere in terms of bandwidth efficiency.  But again, has a bandwidth of anywhere from 500 to 2000 Hz. (https://en.wikipedia.org/wiki/MT63)

PSK31?  Hello!  Here’s what you need to know.  A proper PSK31 signal has a bandwidth of just 100 Hz.   That’s all.  Take a look at the image above, there are several PSK signals being seen here.  Yes, its a variable length coding.  See https://en.wikipedia.org/wiki/PSK31

So there’s your answer: D. PSK31

[E2E09]

Amateur Extra: Using ASCII Code

What is one advantage of using ASCII code for data communications?

A. It includes built in error correction features
B. It contains fewer information bits per character than any other code
C. It is possible to transmit both upper and lower case text
D. It uses one character as a shift code to send numeric and special characters

This is yet another one of those questions that I’m not sure why it’s on an amateur radio exam, really.  But, there it is.  Let’s examine the options and figure out which is the “one” advantage.

A. It includes built in error correction features? FALSE.  ASCII is just a numeric representation of letters/numbers/symbols and doesn’t have any such thing.

B. It contains fewer information bits per character than any other code.  FALSE again.  There are much more efficient methods for this.  ASCII has its roots in computers, not radio.  It’s designed for programming, not efficiency in transmission.

C.  It is possible to transmit both upper and lower case text.  TRUE.  Different case of letters have different ASCII codes.  i.e. “A” is 65 where “a” is 97.

D.  It uses one character as a shift code to send numeric and special characters.  FALSE.  Look up any ASCII table and its plain to see that every symbol, whether its punctuation, numbers, letters, etc. has its own distinct code.  There is no “shift” modifier.

The answer then, would be C.  It’s possible to transmit both upper and lower case text.

I know, I know.  Why is this really important to amateur radio?  I guess its important working digital modes and needing to know the details behind how a data stream is coded.  Beyond that, its superficial knowledge.

But its on the test, and you should know the answer!

[E8D11]

(Image: https://commons.wikimedia.org/wiki/File:ASCII_full.svg)

Amateur Extra: Distance From Monitoring Facilities

Within what distance must an amateur station protect an FCC monitoring facility from harmful interference? [E1B03, 97.13]

A. 1 mile
B. 3 miles
C. 10 miles
D. 30 miles

The answer is: A. 1 mile.

Why? Because the FCC has said so, basically.  In section 13 of Part 97.

(b) A station within 1600 m (1 mile) of an FCC monitoring facility must protect that facility from harmful interference. Failure to do so could result in imposition of operating restrictions upon the amateur station by a District Director pursuant to §97.121 of this part. Geographical coordinates of the facilities that require protection are listed in §0.121(c) of this chapter.

 

Amateur Extra: Toxic Materials

Which insulating material commonly used as a thermal conductor for some types of electronic devices is extremely toxic if broken or crushed and the particles are accidentally inhaled? [E0A09]

A. Mica
B. Zinc oxide
C. Beryllium Oxide
D. Uranium Hexaflouride

The answer is C. Beryllium Oxide.  One of the hazards we face as radio enthusiasts is dealing with technology, and quite often the very materials we gain enjoyment from are also toxic if not handled with respect.  Beryllium Oxide is one of those.  So long as you don’t crush or cut the material, its pretty harmless.  The dust is what gets you.

I have run into a similar problem in antique radios I restore.  Occasionally I will run into a sheet of material acting as a vacuum tube heat shield.  You guessed it, its asbestos.  I simply remove the piece of material, very carefully, place it into a plastic zipper bag, and put it in the trash, taking great pains not to tear or crumple it.  It, too, is harmless if intact, but broken or torn it can release particles that could be inhaled and cause problems.

 

Amateur Extra: Estimating RF Fields

Which of the following would be a practical way to estimate whether the RF fields produced by an amateur radio station are within permissible MPE limits? [E0A03]

A. Use a calibrated antenna analyzer
B. Use a hand calculator plus Smith-chart equations to calculate the fields
C. Use an antenna modeling program to calculate field strength at accessible locations
D. All of the choices are correct

Lets examine all the possible answers and see if we can get the correct one by elimination….

A. Use a calibrated antenna analyzer.  This might tell you if you’ll be able to transmit through your antenna with a low SWR on a given frequency, but it won’t tell you anything about the radiation pattern and field strength.

(We just eliminated D. All of the choices are correct!)

B. Use a hand calculator plus Smith-chart equations to calculate the fields.  Much like A. Using a Smith chart won’t help you determine the RF fields coming off of the antenna.

The only way to estimate the RF pattern and field strength coming from an antenna or antenna system is to use software (or do it the very hard way!) to model the system and provide theoretical field values at given locations.

C. Use an antenna modeling program to calculate field strength at accessible locations is the correct answer.

(photo: https://commons.wikimedia.org/wiki/File:NEC_pattern_Helix_vgain.png)

Amateur Extra: ASCII Parity Bit

What is the advantage of including a parity bit with an ASCII character stream? [E8C12]

A. Faster transmission rate
B. The signal can overpower interfering signals
C. Foreign language characters can be sent
D. Some types of errors can be detected

The definition of a “parity bit” according to Wikipedia is: a bit added to the end of a string of binary code that indicates whether the number of bits in the string with the value one is even or odd. Parity bits are used as the simplest form of error detecting code.

Given the other answers don’t deal with error detection at all, its a safe bet that the answer is D. Some types of errors can be detected.

Amateur Extra: EME Communications

Which of the following digital modes is especially useful for EME communications? [E2D03]

A. FSK441
B. PACTOR III
C. Olivia
D. JT65

Due to the time it takes for signals to get to the moon and back, (EME = Earth-Moon-Earth) we need a mode that is highly dependent on time synchronization.  This way everyone knows when to transmit, and when to listen. D. JT65 is just that mode.

JT65 is a highly structured digital mode that requires the operator’s station computer to be synchronized to the world time servers.  Communications are then alternated at 60 second intervals, 47 seconds transmit, with 13 seconds for decoding.  Transmissions are limited to 12 characters, and the QSO follows a strict format of exchanges.

All of the other modes might be capable of making the EME trip, but all are independent of any external reference that makes them a reliable option.

 

Amateur Extra: Rectangular Coordinates

What do the two numbers represent that are used to define a point on a graph using rectangular coordinates? [E5C11]

A. The magnitude and phase of the point
B. The sine and cosine values
C. The coordinate values along the horizontal and vertical axes
D. The tangent and cotangent values

This might seem like a strange and really lame question, but I think the reason for it is to make sure you understand the difference between rectangular and polar coordinates.

In a rectangular coordinate system, the two numbers represent C. The coordinate values along the horizontal and vertical axes.  That might seem rather obvious.  For example, if you think of a checkerboard, for example, and measure over three squares, and up four, then those coordinates would be 3,4.

Polar coordinates on the other hand, are represented by a radius and an angle.  For example if you were to take a string and pin it to the center of the checkerboard, then rotate that string around the center, at a given length.  So if your string was 6″ long and it was at 90º from the horizontal, your polar coordinates would be 6∠90º.

See the image above for what this looks like.

Amateur Extra: Reactive Power in an Ideal Circuit

What happens to reactive power in an AC circuit that has both ideal inductors and ideal capacitors? [E5D09]

A. It is dissipated as heat in the circuit
B. It is repeatedly exchanged between the associated magnetic and electric fields, but is not dissipated
C. It is dissipated as kinetic energy in the circuit
D. It is dissipated in the formation of inductive and capacitive fields

First, recall the definition of reactive power.  Reactive power is mathematically there, but doesn’t really “exist.”  It arises from the complex or “imaginary” components of reactance that arise in a circuit with inductors and/or capacitors exposed to alternating current.

In an ideal circuit, that is one where the capacitors and inductors don’t have any resistive components, the power B. is repeatedly exchanged between the associated magnetic and electric fields, but is not dissipated.  Of course there are always resistive physical elements at play, and this power gets bled off in the form of heat, since a portion manifests as real or non-reactive power.

Since it does exist in mathematical terms, reactive power from one circuit can affect and interact with the reactive power in another circuit, such as how power grids interact.

Here’s a more in depth explanation of True, Reactive, and apparent power: http://www.allaboutcircuits.com/textbook/alternating-current/chpt-11/true-reactive-and-apparent-power/

Amateur Extra: Op Amp Output Impedance

What is the typical output impedance of an integrated circuit op-amp? [E7G15]

A. Very low
B. Very high
C. 100 ohms
D. 1000 ohms

A A. Very low output impedance is a property of an IC op-amp.   In an ideal case, it would be zero.  But of course that’s never possible.

In reality, the behavior of an op-amp depends on the circuit that its in, and it can be used in many different types of circuits (see image above.)

The reason for the low impedance, is so that the voltage gain that is output by the device acts on the rest of the circuit, and not the op amp itself.  If the voltage output had to contend with a high impedance across the output terminals, The power would go there, instead of actually being output.

More information: http://www.learningaboutelectronics.com/Articles/Why-does-an-op-amp-need-a-high-input-impedance-and-a-low-output-impedance

(image: https://commons.wikimedia.org/wiki/File:Op-amp_circuits.jpg)