brainwagon "There is much pleasure in useless knowledge." — Bertrand Russell

3Aug/090

Decoding packet….

It's late, so I am not figuring this out, but I seem to be getting quite a bit of ripple in the output of this demodulator. I bet that it is because I don't know what I am doing. Oh well. More for later, I guess.

Picture 3

Filed under: Amateur Radio No Comments
3Aug/094

No luck on ISS SSTV, but NOAA 17 looks nice

I tried to record the pass of the ISS as it came over, but I heard nary a peep (or beep) from it as it went over. Annoyed, I did a quick search, and found that the NOAA 17 satellite was just coming up. I retuned and hand tracked that, yielding the following rather nice image:

n17

3Aug/091

FET Transistor Homemade From Cadmium Sulfide Photocell.

I've blogged about the experiments of Nyle Steiner before, but just recently got back to his Spark Bang Buzz website again, and found that he's added a couple of interesting bits. In particular, he shows how to modify a common Cadmium Sulfide Photocell to act like a field effect transistor. The voltage gain provided is quite small (about 1/10) but the power gain is better and the principle is quite interesting. Check it out.

FET Transistor Homemade From Cadmium Sulfide Photocell.

Filed under: Amateur Radio 1 Comment
3Aug/090

Passing the Amateur Extra test by guessing…

The Amateur Extra test is 50 questions, multiple choice, with 4 answers per question. A passing grade is 35 or more. A few minutes of programming this morning, even before I had any coffee yielded that the exact probability of passing was:

      4677523340461106447
------------------------------
158456325028528675187087900672

or about 1 in 33.9 billion.

This wasn't that interesting of a question, but to solve it, I hacked up a quick but limited implementation of rational arithmetic in Python. I was wondering if there was a better way to implement this in Python so overloading would "just work". I didn't know how, and the problem was simple enough, so I didn't try. Here's my solution.

#!/usr/bin/env python

def gcd(a, b):
    if (a < b):
        a, b = b, a
    while b != 0:
        a, b = b, a%b
    return a 

class Rational:
    def __init__(self, a, b):
        self.a = a 
        self.b = b 
    def __str__(self):
        return "[%d / %d]" % (self.a, self.b)
    def pow(self, p):
        return Rational(pow(self.a, p), pow(self.b, p))
    def mult(self, r):
        tmpa = self.a * r.a ;
        tmpb = self.b * r.b ;
        d = gcd(tmpa, tmpb)
        return Rational(tmpa//d, tmpb//d)
    def imult(self, i):
        tmpa = self.a * i ;
        tmpb = self.b ;
        d = gcd(tmpa, tmpb)
        return Rational(tmpa//d, tmpb//d)
    def add(self, r):
        tmpa = self.a * r.b + r.a * self.b
        tmpb = self.b * r.b 
        d = gcd(tmpa, tmpb)
        return Rational(tmpa//d, tmpb//d)

p = Rational(1, 4)
q = Rational(3, 4)

def fact(n):
        c = 1 
        while n > 1:
                c = c * n 
                n = n - 1 
        return c 

def comb(a, b):
        return fact(a)/(fact(b)*fact(a-b))

total = Rational(0, 1)

for t in range(35, 51):
        x = p.pow(t).mult(q.pow(50-t)).imult(comb(50, t))
        total = total.add(x)

print "Exact probability is",  total
print "Only about 1 in", total.b // total.a, "will pass"