i'm so excited!

anyway, questions on test 2...


#2 Rm,dc = 1/sigmaAc, then *1000 (*500m, *2 wires, right?) PLUS 30 ohms, cause that's the load resistance, right? I got a total R of 117.81 ohms

then is it 6/that? i know it's not 36/ :-D


#3, 4, & 5 i just...botched, and don't know how to fix.

all i really got were #1 and #6. makes ya wonder how i still came out over the avg on this one...

any help would be greatly appreciated

12/4/2005 5:09:19 PM

yeah an explanation for #4 would really help!

12/4/2005 5:48:54 PM

so a1=-.318j, a-1=.318j; a0=1/2, or in a rational form:
s(t) = -1/jpi*exp(-j*2pi*f*t)+0.5+1/jpi*exp(j*2pi*f*t)

pull the sine out:

s(t)=2/pi*sin(2pi*f*t)+0.5
s(t)=2/pi*sin(2pi*250kHz*t)+0.5

for number 2, you can calculate Rdc to be 43.9ohms. then, show the entire system as a voltage source in series with a 43.9 ohm resistor (the send line), a 30 ohm load, and another 43.9 ohm load (the line back). you want the power across the 30ohm load, so do voltage division: 6(30/117.8) and then v^2/r = 1.53*1.53/30=0.78W.

number five is long. soooo the output, per the notes is y(t) = a0c0+2*sum( ak*|C(fk)| cos(2pi*k*fp*t+theta(fk) ) the input..that cosine just has one coefficient...well, one plus dc: a1=a-1=5, a0=5. find c0 = 500/(1500+j(0)) = 1/3. y(t) uses a0c0, which is 1/3 * 5=5/3.

spiffy. now, f1=200 --> C(fk) = C(f1) = C(2000) = 500/(1500+j2000) = .12-j*0.16. |C(2000)| = 0.2. theta(fk) = angle(C(2000)) = arctan(-0.16/0.12)=-0.93

put it all together according to the forumlae above... y(t) = 5/3 + 2 * (5) * 0.2 cos(2pi*2000*t-0.93) = 1.67+2cos(2pi*2000*t-0.93). 5/3 is a0c0, 5 is ak, and .2 is c(2000).

hope that isn't too hard to read =\

12/4/2005 6:41:18 PM

ok, in class he said he'd have 1 problem from ch12 or 13
and 2 from every other "testing period", if you will.

if ya'll had to choose 2 questions from every test, which ones do you think HE would ask? here are my guesses and reasons for those guesses :

test1: -#2 or something like #1 or #4 <--where it's just plugging in, but easy to mess up
and -#5 or #6 b/c he said in class those are the ones ppl missed the most

test2: - #6 b/c we did lots like that for hw/in class
- and something like combo of #1 and #2, finding total resistance and power.
~at the same time, i assume he'd give us a bessel eqn too; and 4 and 5 seem too hard, but if he feels like being mean, he'd throw 'em at us. *confused on this test*


test3: ~ i'm not sure yet..

agreements? disagreements/reasons?

12/5/2005 10:11:19 AM

Is this with Alexander? His final wasn't any harder than any of the previous tests. I think he put a question on array antennas but that was the only question that wasn't from previous material that I remember.

12/5/2005 2:27:43 PM

yeah alexander, thanks for the info man

12/5/2005 8:46:31 PM

not a specific problem but i have a very important question

what it is: i can only find test 3, the questions for test 1 and 2 i cannot find, but i have my answers, so if you have the exam sheet if you can scan, take a picture of or some variation i would be super happy

also if anyone did exceptionally well on test 1 or 2, or knows the correct answers pictures of that worked out would be wonderful... i can supply those for test 3 if anyone would like

[Edited on December 5, 2005 at 9:11 PM. Reason : .]

12/5/2005 9:06:23 PM

whered everyone go

12/5/2005 9:52:10 PM

On problem set 13, #4c: Where did the -0.5 come from? I remember figuring this out before but I have forgotten now.

^ and sorry, no scanner for me.

12/5/2005 9:54:01 PM

^ on number 4,
how did u get Tau0 and Tau1 (part b)?

i know u do at least this:
td = 1/2* 1/f * cos(60) , where 60 = phi,d
td = 250ns

12/5/2005 11:35:47 PM

the .5 is cosine of 60. you can make the more general case of if you want to steer the field's maximum towards a certain point:

G(phi)=4cos^2(pi/2 (cos(phi)-cos(phi_desired))

if you wanted to steer the minimum of the field to a certain point, just add 180 degrees to it -- i think.

12/5/2005 11:41:52 PM

set 12, #s 3 & 4...what am I missing? I've gotten very close to the answer but not THE answer yet. i keep getting .0171 instead of .0173 - is this rounding error?

my thinking right now is you use the (Io^2 * n) / (8 pi^2 * r^2) and Io = 3, n = 377, r = ? i've tried a lot of different things...and then THAT over the area of the circle, which I get to be 200pi = 628.3. solving backwards i get that my missing r should be 1.988 or something, but i don't know where i'd get that.

12/6/2005 1:52:09 AM

sorry, i woulda replied last night had it dawned on me what problem that was
you're right, .173 is a typo

12/6/2005 7:50:01 AM

good luck hiway!

12/6/2005 8:17:51 AM

no it is .173, i figured out my problem...the area of the circle doesn't mean anything and i was doing r funkily

12/6/2005 10:47:32 AM

DAMNIT

i just realized on the last question, i put something in my calculator wrong and got some REALLY WEIRD graph, didnt think to double check my equation

got something like infinity..

re-typed it in and graphed it at home, got a simple sinc functionish thing that computes 12 at theta = 0

i hope i get some sorta partial credit anyway

12/6/2005 3:52:52 PM

heh, i also put it in my calculator wrong. i forgot the squared in cosine squared. should be 12 @ theta=0, but i put like 8.something....i forgot the squared i am so embarrassed

that was a fun class, for the most part.

12/6/2005 4:16:11 PM

i accidently left off a parentheses making it sqrt(whole thing) instead of sqrt(1-cos^2(12theta))/(theta*cos(theta))


try it.... you get a CRAZY graph

i emailed him about it after i figured out my error to explain how I even got that graph and that my method of solving it was still correct

[Edited on December 6, 2005 at 4:18 PM. Reason : .]

12/6/2005 4:17:32 PM

only problem that "got" me was problem 6. other then that i thought the exam was fair.

12/6/2005 4:25:27 PM

fair as in pirates are fair





eh, it's all good. ONWARDS TO 420. really feels like 422 is the prereq for 420.

12/6/2005 5:58:59 PM

grades are up in "past semester grades" : Fall 2005

12/8/2005 6:25:26 PM

w00t

12/8/2005 6:29:39 PM

hells yea

12/8/2005 10:59:45 PM

Alexander is the best teacher. He curves at least a full letter grade.

12/8/2005 11:30:54 PM

at least he curved someone's grade then...

12/9/2005 1:09:16 AM

yeah i went from maybe passing to a C- :p

12/9/2005 11:38:21 AM