Thursday, March 31, 2011

Lesson 35: General Class Exam Course G0B

Here is lesson 35, the last lesson in the General Class Exam Course!  This final lesson focuses on electrical safety rules.  There are a few numbers you will need to memorize:

-The minimum wire size for a 20 ampere current is AWG #12.
-The fuse you would want to install along AWG #14 wire is a 15 ampere fuse.
-60 Hz alternating current is more dangerous to humans than direct current or RF energy.
-The maximum amount of current that can run through a human body without causing damage is 50 microamperes.

That's it!  I hope these videos helped you in your studies for the General Class exam!  Until the next series of lessons...


Lesson 34: General Class Exam Course G0A

Here is lesson 34!  This lesson primarily covers RF safety and Maximum Permissible Exposure (MPE) limits.  RF energy can heat and damage bodily tissue.  None of the material in this section is much of a stretch and most of the answers fall within "common sense."  One thing I will recommend is to do a quick scan of the FCC OET Bulletin 65 which covers RF exposure limits and RF safety.

There is one question that is a little out of the ordinary.  When multiple transmitters are operating at a single site, the transmitters which are contributing more that 5% of the MPE limit are the ones responsible for ensuring RF safety compliance.  The actual way this is determined is a little more complex than what the question seems to be asking.  It is probably best just to memorize this one.

One more lesson to go!

As always, please feel free to leave any suggestions, comments, or questions in the comments box.


Wednesday, March 30, 2011

Lesson 33: General Class Exam Course G9D

Here is lesson 33 and the G9D section.  This lesson goes over some aspects of specialized antennas.  There are two antenna types you will need a basic understanding of: Log Periodic Antennas and Beverage Antennas.

A log periodic antenna looks a lot like a type of Yagi antenna, but the log periodic antenna is capable of handling a much larger bandwidth than a Yagi.  The elements of a log periodic antenna are basically a series of dipoles connected to each other along a boom.  The longest element is cut to 1/2 wavelength of the lowest desired operating frequency.  From there the length and spacing of the elements is determined by a logarithmic formula.  Thus the "log" in log periodic antenna.

A Beverage antenna is basically a very long wire.  Beverage antennas are great for low HF directional receiving but are very bad for transmitting.  Beverage antennas are characterized by their long length, low height above the Earth, and their ability to be very directional receiving antennas.  They can be extremely long, miles in some cases.

NVIS, or Near Vertical Incidence Skywave, is a term you will need to be familiar with as well.  An NVIS antenna is a horizontally polarized antenna that is placed between 1/10 and 1/4 wavelength above the Earth.  The result this has is that the majority of the signal strength is radiated more upward than off to the sides which allows for shorter skip distances during the day.

The one and only number you need to memorize for this section is the gain of two 3 element vertically stacked Yagis 1/2 wavelength apart.  By stacking Yagis in this fashion, the two Yagis will produce a 3 dB gain over just a single 3 element Yagi.

No sweat!

As always, please feel free to leave any suggestions, comments, or questions in the comments box.


Saturday, March 26, 2011

Lesson 32: General Class Exam Course G9C

Here is lesson 32 which deals with directional antennas.  This lesson primarily works with three type of directional antennas: Yagis, Cubicle-Quads, and Delta-loops.  The video for this lesson is long, but the information is relatively easy to absorb.

The basic Yagi antenna consists of three elements: The driven element, reflector element, and the director element.  The driven element is the element that is connected to the feedline and thus the transceiver.  The other elements are what are called "parasitic" elements and their function is to focus the signal to and from the driven element.  By increasing the length of the Yagi and adding director elements, the gain of the Yagi is increased in the Yagi's focal direction.  The length of a Yagi's driven element is generally 1/2 wavelength of the desired frequency.

Cubical Quad antennas are the second directional antenna this section deals with.  The basic quad antenna consists of a driven element and a reflector.  Directive elements can be added to increase gain.  The driven element of a quad antenna equals a full wavelength of the desired transmit frequency.

Delta-loops are similar to quads except that they are triangular vice square.

A few points to memorize: The gain of a 3 element Yagi, 2 element quad, and 2 element delta-loop are all about the same.

The relative gain of a Yagi antenna in its focal direction is 9.7 dBi.

And that's about it.  As always, please leave any comments, suggestions, or questions in the comments box.  Thanks!


Monday, March 21, 2011

Lesson 31: General Class Exam Course G9B

Here is lesson 31!  Just four more to go!  This lesson covers basic antennas.  There are a few things you will need to remember for this section.

There are two formulas you will need to know for determining the length of a 1/2 wavelength and 1/4 wavelength antennas.  For the 1/2 wave antenna:

Length (in feet) = 468/frequency in MHz

For the 1/4 wave antenna:

Length (in feet) = 234/frequency in MHz

Keep in mind that a 1/4 wave antenna is half the length of a 1/2 wave antenna and 234 is half of 468.  That should help in memorizing the formula.

There are also some feed-point impedance relationships to antenna construction that are not very intuitive and you will need to memorize.

-For ground plane antennas (think vertical antennas), downward sloping radials increase feed-point impedance.
-A dipole's feed-point impedance steadily decreases as the antenna is lowered below 1/4 wavelength above the Earth.
-A dipole's feed-point impedance steadily increases as the feed-point (where the feedline attaches to the dipole) moves from the center to the ends of the antenna.

As always, please feel free to leave any comments, suggestions, or questions in the comments box.


Lesson 30: General Class Exam Course G9A

Here is lesson 30!  This lesson deals primarily with antenna feedlines, impedance, and SWR.  This lesson is fairly straight forward with no complicated formulas or concepts, but there are a few items you will need to memorize.  Those things are:

-The characteristic impedance of feedline most often used in amateur radio is 50 and 75 ohms.

-Flat ribbon TV type twin lead is 300 ohms.

-Attenuation is the reduction in signal strength expressed in dB.  When you see attenuation, think signal loss.

-RF feedline loss is most often expressed in dB per 100 feet.

-To find Standing Wave Ratio (SWR) when all the impedances are known, simply divide the larger impedance by the small impedance.  SWR is expressed with the larger number first with the second number always reduced to 1.  For instance: 4:1, 3:1, and 2:1.  It is not expressed 1:4, 1:3, 2:1.5, etc.

That's about it for this lesson.  As always, feel free to leave any comments, suggestions, or questions in the comments box!


Thursday, March 17, 2011

Lesson 29: General Class Exam Course G8B

Hello again!  The majority of lesson 29 deals with things we've covered in previous lessons.  The majority of this section deals with mixer and multiplier stages, deviation, and bandwidth.  There is also a little on data modes as well.

Mixers are stages in transmitters and receivers which combine two frequencies to produce a third.  This process is called heterodyning (remember the super heterodyne receivers we talked about in previous lessons).  When two signals are combined in a mixer, the new frequency will either be the sum or the difference of the two combined frequencies.  For instance, if a 10 MHz signal is received by an antenna is sent to a mixer and combined with an 8 MHz signal produced by an oscillator, the result will be an intermediate frequency (IF) of either 18 MHz or 2 MHz (10 MHz + 8 MHz or 10 MHz - 8 MHz).  The process to determine whether the IF is the sum or difference is fairly complicated and not important for the purposes of passing the exam.  The important things to know about mixers are they combine, or mix, two different frequency to produce a new frequency for the purposes of producing an IF or a desired transmitting frequency.

Multipliers perform a similar function as mixers in that they take an input signal and produce a new signal at a different frequency.  The difference is that a multiplier produces the new frequency by generating the harmonic of the input frequency.  This new frequency is usually double the input frequency.  For instance, if a 10 MHz signal is sent to the multiplier, the output frequency will be 20 MHz.  Multipliers are often used in VHF FM transmitters to change a modulated HF signal to a frequency in the VHF range.  These signals are often sent through several multiplier stages, doubling the frequency each time, until the desired transmitting frequency is reached.  The key word to remember when dealing with multipliers is "harmonic".

Another concept which may require some explanation is deviation.  Deviation primarily deals with FM signals.  A very simple explanation of deviation is that it is the difference between a carrier signal and frequency modulated signal, or the frequency differences between the modulated signal and the original carrier wave.  Deviation is expressed in terms of frequency (kHz, MHz, etc.).  For this section of questions, the thing to remember is that when a FM signal is sent through a multiplier, there is more or less a direct relationship between the signal frequency and the deviation.  If the frequency doubles, so does the deviation.

There is also a bandwidth formula you will need to memorize.  The formula is very simple.  If the deviation and modulated frequency are known, the formula is:

Bandwidth = 2(deviation + modulated frequency)

Easy enough.

Please leave any comments, suggestions, or questions in the comment box.

Until the next lesson...