13 January 2013

Slant Alpha-Echo

Woo Hoo!  I passed the Extra exam!

I upgraded to General back in Oct 2011 and would have started studying immediately but the question pool changed part way through 2012.  So studying was put off until the new books with new questions were out.

Study started back in July.  It was a couple more months before issues with the test software were worked out.  Serious study started in November and I passed yesterday.

Most of what I know about electronics came from a second semester college physics class.  We experimented with capacitors and inductors and worked through the equations.  But that was back in Reagan's first term.  And I don't think we ever got into Smith Charts.

Some questions were easy, particularly the digital logic questions.  I learned a lot of that in HS and college classes and it hasn't changed much.

One question might have been difficult but instead was a "gimme":
E7C10 - Which of the following filters would be the best choice for use in a 2 meter repeater duplexer?
A.  A Crystal Filter
B.  A Cavity Filter
C.  A DSP filter
D.  An L-C filter
The reason this was easy question is because of first hand experience.  I have seen and touched a cavity filter, used for exactly the reasons asked in the question.  The SCSon's electronics group has one.  One meeting we talked about how and why it works.  A member brought in the RightToolForTheJob(tm).  We saw how it is tuned to the new frequencies.  Easy peasy.

I passed the test easily with a 90% score.  74% is passing and yes, that is also a test question [E1E05].  But there is still a lot I don't know.  I understand deciBels are units of power and that it's a logrithmic scale doubling every 3dB.  But I don't understand how they relate to the more usual Watts measurement and why deciBels are more useful in some cases.  Also I got the Smith Chart questions right, but I don't understand exactly how to get my measurements into it.

Some might say that means I'm not qualified to hold the license with such gaping holes.  I would argue I passed the test thus they say I'm good to go.  It's like so many government tests, the license is really a license to learn.  And I know some areas I need to study further.


Old NFO said...

Yep, it's a 'starting' point! And congrats!!!

Roberta X said...


DeciBels? Easy! Unlike Watts or Horsepower, they are a comparative measurement; if you don't have a reference, they tell you the ratio of two powers: 10 (there's the "deci") times the base-10 log of (P1 over P2). Everything else follows from that -- if all you have are the voltages, remember twice the log is the same as squaring the number, so dB = 20 * log(E1/E2). It is assumed the impedance is known or at at least constant for that trick.

dBm are common, deciBels referenced to a milliWatt; I see dBk at work, dB referred to a kW! You take that as one of the two powers and work the math.

And the kewl thing is, if you have power in dBm (say), coax loss in dB and antenna gain in dB, you can work out the effective radiated power by addition and subtraction. --Plus a little slight-of-calculator at the end to turn it back into milliWatts or Watts.

Wikipedia: http://en.wikipedia.org/wiki/Decibel