Q Signals

Technician: Q Signal for interference?

This week’s Tech question comes from sub-element 2 (Operating Procedures,) group B (Q signals) [T2B10]

Which Q signal indicates that you are receiving interference from other stations?

A. QRM
B. QRN
C. QTH
D. QSB

There’s really no good way to understand what the Q signals mean except to memorize them.  I know, I know, it goes against what we teach here at HRA but I don’t think there’s much logic behind them.

The answer is A) QRM.  It might help to remember the “RM” part as “Receiving a Mess” or something.  QRN means “The atmospherics are strong.”  QTH is your station’s location.  (You should remeber this one, as its used quite often.)  QSB is used slightly less, and means “your signal is fading.”

Of course any Q code can be turned into a question by adding the ? after it.  So, for example, QSB? becomes “is my signal fading?”

A complete list of Q-Codes can be found here, in the Ham Radio Academy Resources section.

Radio Receiver

Amateur Extra: Minimum Discernable Signal

The Amateur Extra torture question of the week is taken from sub-element 4 (amateur practices) section C (receiver performance) [E4C07]

What does the MDS of a receiver represent?

A. The meter display sensitivity
B. The minimum discernible signal
C. The multiplex distortion stability
D. The maximum detectable spectrum

If you stop to think about the possible answers, as they relate to radio receivers, options C and D don’t look very good.  They actually look sort of like gibberish.  Let’s get rid of those.

Option A, Meter Display Sensitivity, sounds like a plausible answer.  But its wrong.  The correct answer is B. The minimum discernible signal, or minimum detectable signal

But what is MDS, anyway?

It’s the lowest power signal your receiver can be received by the input, and then turned into any sort of useful information.  Basically the smallest signal that would be audible (or visible) over background noise.

The actual value of MDS is dependent on a few factors, including bandwidth and temperature.  Theoretically, the lower you can get the temperature of your receiver, the weaker the signal you should be able to detect.

NiCd Battery Disassembled

General: NiCd Battery Resistance

This week’s General Class exam question is from sub-element 6 (Resistors; capacitors; inductors) section B (batteries)… [G6B13]

What is an advantage of the low internal resistance of nickel-cadmium batteries?

A. Long life
B. High discharge current
C. High voltage
D. Rapid recharge

Nickel Cadmium, or NiCd, rechargeable batteries aren’t as popular as they once were, due to the current popularity of Lithium Ion (LI) and Nickel Metal Hydride (NiMH) batteries.  They do still have their place, though.

Let’s examine the incorrect answers first.  A) Long life?  A low resistance would seem to indicate that there’s nothing to keep the charge in the battery, right? So that one is probably not correct.  C) High voltage? NiCd batteries usually only have a 1.2V capacity, instead of the typical 1.5V.   D) Rapid recharge? This is not a normal quality of most rechargeable cells, as the potential for overcharging (and explosion) exists.  Slow or “trickle” charging is best.

The correct answer is B) High discharge current.  “High” is of course a relative term, but in order for a battery to do any good, it has to have a high enough current to generate suitable power, whether its in a flashlight, radio, or electric motor.  A typical 1.5V AA NiCd cell can supply up to a maximum 1.8A! That’s quite a bit for such a small package.  This answer also negates answer A) Long life, as a high discharge rate will unfortunately drain a battery quickly.

Technician: Automatic Gain Control

This week’s Tech Class question is pulled from sub-element 4 (Amateur radio practices and station set up), section B (AGC) [T4B12]

What is the function of automatic gain control or AGC?

A. To keep received audio relatively constant
B. To protect an antenna from lightning
C. To eliminate RF on the station cabling
D. An asymmetric goniometer control used for antenna matching

Let’s examine these answers one by one logically, starting from the bottom up.

“An asysmmetric goniometer?”  Considering a goniometer (yep, its a real thing) is a device used by physical therapists to measure the range of motion of, say, your elbow or knee…  I think we can eliminate that one.

RF on the station cabling? That’s done by making sure your station is all properly grounded, connected properly, and if its still a problem, placing a few ferrites on your cables to eliminate the rest.

Protect from lightning?  Nothing will completely protect your antenna from lightning, but proper grounding might help a bit.

Also consider that none of these are related to “gain” in any way, shape, or form.  The only choice that could even be related has to be related to audio.  Your volume control is labelled “AF (Audio Frequency) Gain” for a reason.  AGC circuits have been in radio receivers since the early days, basically it tries to eliminate any hard spikes from the signals, originally to protect your speaker from being overdriven, but now just to keep you sane.  Ideally you won’t even notice it working.

The answer, then, is A) To keep received audio relatively constant.

Charge Transfer

Amateur Extra: N-Type Charge Carriers

Extra question of the day is from sub-element 6 (circuit components) section A (n-type semiconductors) [E6A16]

What are the majority charge carriers in N-type semiconductor material?

A. Holes
B. Free electrons
C. Free protons
D. Free neutrons

First, remember what the N-type designator means.  It indicates that the device uses Negatively charged carriers.   Protons and “holes” are positively charged carriers, and neutrons can’t carry any charge at all.

Electrons are the only negatively charged particles in the list of answers, so the correct choice is B. Free electrons.

Electrical Components

General: Resistance and Temperature

The General class exam question of the week is from sub-element 6 (circuit components) section A (resistors) [G6A06]

What will happen to the resistance if the temperature of a resistor is increased?

A. It will change depending on the resistor’s reactance coefficient
B. It will stay the same
C. It will change depending on the resistor’s temperature coefficient
D. It will become time dependent

The answer is C. It will change depending on the resistor’s temperature coefficient.  That number basically defines how the material in the resistor reacts to temperature.

A resistor doesn’t have a “reactance coefficient”, as reactance is a property of circuits and elements that respond to AC (Alternating Current.)  It might be measured in Ohms, but the resistor doesn’t particularly care either way.

Staying the same would be ideal, but that is never the case.  And becoming “time dependent” really doesn’t make sense here, unless you’re applying heat, in which case the temperature would continue to increase…. but the resistor’s response to that temperature would still be defined by the temperature coefficient.

 

http://www.fcc.gov/

Technician: Station Inspection

The Technician class question for this week is from sub-element 1 section F, essentially “the rules.” [T1F13]  [97.103(c)]

When must the station licensee make the station and its records available for FCC inspection?

A. At any time ten days after notification by the FCC of such an inspection
B. At any time upon request by an FCC representative
C. Only after failing to comply with an FCC notice of violation
D. Only when presented with a valid warrant by an FCC official or government agent

I could run down an explanation of each incorrect answer and say why they’re incorrect, but what it all boils down to is this:

At the end of the day, like it or not, the FCC (Federal Communications Commission) runs the show with respect to amateur radio, and especially your license.  That license basically says that you’ll play by the rules, and agree to certain things in return for the privilege of using the amateur bands.

So, if a random FCC representative shows up at your door and asks to inspect your station….  Well, you’ve agreed to that.  Granted the odds of that actually happening are pretty slim, (unless you’ve been misbehaving, intentionally or not!) but it could still happen.

The answer, therefore, is B) At any time upon request by an FCC representative.  You’ll notice that the rest of the answers have conditions and qualifiers, none of which are relevant.

Transmitter tank inductor

Amateur Extra: LC Current at Resonance

This week’s Amateur Extra question is from sub-element 5 (Electrical Principles) group A (characteristics of resonant circuits) [E5A06]

What is the magnitude of the circulating current within the components of a parallel LC circuit at resonance?

A. It is at a minimum
B. It is at a maximum
C. It equals 1 divided by the quantity 2 times Pi, multiplied by the square root of inductance L multiplied by capacitance C
D. It equals 2 multiplied by Pi, multiplied by frequency “F”, multiplied by inductance “L”

OK, buckle down, kids, because here we go.  Time to roll up those sleeves and dive into some electrical math junk.  I know, I know, but when we’re done, it will make sense.  I hope.

The answer is B) It is at a maximum.  We’ll explain here.

The current in a parallel RLC circuit is the sum of the current in each section, or branch.  We’ll call them IL and IC.  Remember those equations for reactance, because we’ll use them here.  (as XL and XC.)

IL = V/XL = V/2{\pi}fL and IC = VXC = 2{\pi}VfC

if we sum these “vectors” together (remember the complex aspect of these!) we get

I = \sqrt{(IL + IC)}

at resonance, the currents in the capacitor and inductor are equal, but 180º out of phase, so they cancel each other out….

…. BUT ….  What we’ve just described it that the current leaving the circuit is zero.   How can the answer be “at a maximum?”

Another term for a parallel LC circuit is a “tank circuit.”  If you take this figuratively, consider this circuit as a tank of water, and the current the flow of water.  If the water going in doesn’t also come out, it has to go somewhere.  Where? Into the “tank.”

Remember above that the two vectors canceled each other out?  The effect on the AC current is the same.  The C current is going one way, and the L is going the other, so it cancels out.  You can even think of this as two halves of a loop, hence the term “circulating” current.  All the current goes in and just keeps turning around in circles, never leaving.

I know, I know, its a little weird.  But that’s how it is.  All the current gets stuck in the LC circuit and never leaves.  So the “circulating current within the components” is at a maximum.

Any questions? Good, because I’m not even sure I understood that completely now!!

In practical use, an example of a “tank circuit” might be the notch filter on your radio transceiver, it will block signals of a particular frequency from passing through the circuit.

Ionospheric absorption

General: Ionosphere’s Maximum Height

The General question of the week is from sub-element 3 (propagation) section C (ionospheric layers) [G3C02]

 

Where on the Earth do ionospheric layers reach their maximum height?

A. Where the Sun is overhead
B. Where the Sun is on the opposite side of the Earth
C. Where the Sun is rising
D. Where the Sun has just set

The answer to this question makes the most sense if you look at the following image:

Ionospheric absorption

http://commons.wikimedia.org/wiki/File:Ionospheric_absorption_%28en%29.svg

As you can see, the ions in the ionosphere get there because of the sun.  The sun emits charged particles, aka the solar wind.  The part of the earth that catches most of these particles, is of course, the part directly facing the sun.  The charged radiation collects here, and excites the atmosphere, so the top layers increase in height.  Where the sun is setting or rising, the angle these particles hit the earth is so low, they reflect right off.  And of course, the part facing away from the sun, doesn’t get any exposure at all.

Therefore, the answer is A. Where the Sun is overhead. 

SWR Meter

Technician: SWR and Output Power

The Technician question of the week comes from sub-element 7 (antenna measurements) section C (SWR measurement) [T7C05]

What is the approximate SWR value above which the protection circuits in most solid-state transmitters begin to reduce transmitter power?

A. 2 to 1
B. 1 to 2
C. 6 to 1
D. 10 to 1

Once you understand what SWR (Standing Wave Ratio) is, and how it affects your transmitter, this question becomes common sense, really.

SWR is a measure of how much power is being output by your transmitter, compared to how much power is actually being transmitted by your antenna.  In an ideal situation, you have a 1:1 SWR, where all the power from your transmitter is being emitted as RF by your antenna.

Practically speaking, though, its almost never that good.  The energy that doesn’t actually get transmitted reflects back to the transmitter, and it turned into heat.

We can automatically eliminate answer B) 1 to 2, since that isn’t a valid SWR measurement.  It’s not possible.  Your SWR will always be equal to or greater than 1:1.

C) 6 to 1, and D) 10 to 1 are way, way, way too high.  If you transmit at any appreciable power with these SWR ratios, you will damage your radio.  It will be too late.

The only answer left is A) 2 to 1.  This means that a full 50% of the power leaving the transmitter is getting reflected back into it.  Most modern transmitters will start to dial back the power to protect the internals above this point.  While C and D might also be correct, approximately 2:1 is where the dial-down starts.