Yep, it’s Christmas again, and I’m such a geek, I can imagine all sorts of things that link it back to radio stuff. If you follow the link below, you’ll find that one of the first (if not the first) audio broadcasts was made on Christmas Eve, 1906, by Canadian scientist Reginald Fessenden. These first radio transmissions were made down at the very bottom of the radio spectrum, at frequencies of just a few kilohertz, and were produced by an alternator, which served as a mechanical oscillator to produce continuous waves. Very cool. You can read more about Fessenden on his Wikipedia page:

Reginald Fessenden – Wikipedia, the free encyclopedia.

Oh, and Merry Christmas!

G3XBM pointed out silent key KL7R’s modification of a Heathkit HW-8 to do double sideband voice. It looks pretty straightforward, and would be an interesting modification. My own HW-8 isn’t exactly pristine, but I am torn between keeping it in its native state, and performing a modification to make it more versatile. Bookmarked for later.

DSB with a Heathkit HW-8

I’ve never attended Dayton or FDIM, but I am thinking that perhaps I will soon (hopefully this year). An affiliated event is the Four Days in May: the QRP event for the QRP Amateur Radio Club International. They are hosting a building contest this year:

QRP Amateur Radio Club International – The FDIM 2010 QRP Challenge.

The long and the short of it is to build a complete QRP transceiver that has only 72 parts. This doesn’t seem too hard except the receiver is specified as being single signal, and you are only allowed one IC in the radio. Seems like an interesting project. It’ll give me something to think about while I’m on the airplane this evening.

Addendum: An example of a transceiver that would not qualify would be the Pixie II.

By my count, the Pixie II has 24 parts, and implements a complete transceiver, but it’s a simple direct conversion receiver, and therefore isn’t single signal. Still, an interesting launching point.

Addendum: Steve Weber’s AP-80 looks like a much better radio, but again, I think it’s a simple direct conversion receiver.

As I might have mentioned, I am trying to teach myself a bit about electronics and radio design. I find the problem with being self taught is that often you read something, and it doesn’t seem clear to you why it should be so, and you uncover the basic lack of understanding that you have to go back and fill in before the later material makes sense. This happened to me while I was trying to work through some more amplifier designs, and I realized that I had a basic misconception about how impedance matching actually worked, and this was perhaps even more basic, and could be illustrated with a simple circuit, consisting of a voltage source, and then in series a source resistance R_S and a load resistance R_L. The question is given an R_S, what value of R_L maximizes the power dissapated in the load R_L?

Well, it’s not really too hard to figure out. First, we can determine the total current going around the loop. For a voltage V, the current is simply:

       V
I = -------
    R  + R
     S    L

From this we can easily determine the voltage drop across R_L:

      R  V
       L
V  = -------
 L   R  + R
      S    L

And, since we know the current through and the voltage across the resistor, we can determine the power as their product:

            2
        R  V
         L
P  = ----------
 L            2
     /R  + R \
     \ S    L/

When is power maximized? Well, we can differentiate the power equation with respect to R_L, and we get:

                           2
dP         2         2 R  V
  L       V             L
--- = ---------- - ----------
dR             2            3
  L   /R  + R \    /R  + R \
      \ S    L/    \ S    L/

Setting this equal to zero, and solving for R_L, we find that a maximum occurs where R_L equals R_S, in other words, power is maximized when the source and load resistances are matched. The total disappated by R_L is then:

      2
     V
P  = ---
 L   4 R

Thus, if we had a load of 50 ohms and a voltage source of 12 volts, we’d end up with a maximum power of 720 mw.

Addendum: As a double check, the power passing through both resistors is V^2 / 2R, which would have been 1.44 watts, and obviously since both resistors are the same, the power is evenly split between the two.

Addendum²: I forgot to mention what I was confused about. It’s not really this (which occurs in DC circuits) but the corresponding circuit which occur in AC circuits with complex impedances. I’ll work through this later.

Addendum³: Hmm. Subscripts and superscripts seem to not work right with this theme. I’ll fix it.

OZ9AEC shows how you can use free software and web-based radio receivers to decode amateur digital communications. Very nifty. I tried this myself using WebSDR, and it worked great.

So, for fun, I entered the circuit for the bidirectional amplifier used in the BitX20, and did some basic simulation. You can see it in the image linked below. I was trying to figure out how to instrument the schematic so I could determine the input and output impedance (and eventually things like return loss as well). Still puzzling over stuff like this. But the circuit is pretty straightfoward. I may have to build this amplifier, just for amusement.

Addendum: If you don’t know what a BitX-20 is, you can find out lots by googling around, perhaps beginning with Farhan’s original page. If you check out his schematic, you’ll find three copies of this bidirectional amplifier.

Got some interesting spots overnight. VK6BMW is located in Perth, Australia, which is pretty close to the maximum distance I’ve heard before. PA0LSK is in the Netherlands (any European spots are pretty rare for me). CO7WT is my first Cuban station spotted I think. Not a bad night.

Well, one of the advantages of getting a new Windows 7 laptop is that I now have a much nicer environment for running LTSpice. Does anyone have any ideas of a good, simple project that I could do to help me learn the ropes of LTSpice, and which I could also build for real to test my understanding? Perhaps something like a VFO + buffer amplifier + power amplifier chain, that could be done in steps? The simple transmitter in EMRFD would seem to be a reasonable choice, but the oscillator is crystal controlled, and I’ve heard that LTSpice can have some difficulty simulating these kind of “high Q” circuits.

What would you all suggest?

Left my FT-817 beaconing for the last 48 hours or so, mostly on 40m, and got some interesting DX, including someone I think I never reached before, KG6DX in Guam.

Well, I finally dug up all the cables I needed and hooked up my little HP netbook to my radio again, and got it going with the new WSPR 2.0 software. As of this moment, I’m beaconing out at 5w on the traditional 30m watering hole, and getting pretty good domestic spots. I might try shifting to a different band a bit later in the day (maybe 15m, I’ve been curious about how good 15 has been lately).

17 spots: