Mar 072013

The US Air Force runs the “Space Fence” system which is used to track objects in orbit. It’s a bi-static radar and you can read all about it on Wikipedia. The basic idea is to transmit a high-power (I’m not kidding) continuous carrier upwards in a big fan-shaped pattern from a few different locations. The radio waves are scattered by objects in space and picked up at multiple receiving stations.

In a nutshell, correlating the received radio signals from each transmitter at multiple receivers allows the system to locate objects as the pass through the fence.

The biggest transmitter in the system is located at Lake Kickapoo, Tx and it transmits 768 kW (Wow!) of RF straight up on 216.983 MHz.

In the past, I’ve picked up the scatter from big satellites and meteors using a simple three-element Yagi antenna feeding my Yaesu VR-5000 receiver. Most of the time I hear static, but once in a while I hear a ping/whistle due to the Doppler shifted signal reflecting off of the object.

I just built a better antenna by scaling the WA5VJB six-element 222 MHz Yagi to 217 MHz. Scaling Yagis is easy: you just multiply every dimension in the Yagi by the ratio 222/216. That’s about a 2% increase in the spacing and length of each antenna element.

I also used my RepRap to print nice brackets to hold the antenna elements onto a piece of furring strip. The driven element is 10-gauge copper and the other elements are just aluminum clothesline wire. Here’s the result.

217 MHz Yagi under construction.

Here’s the final antenna leaning against the bench.

 Posted by at 11:39 PM
Mar 062013

Analog computing in the dark!

I got interested in analog computers over the summer and ended up buying a Comdyna GP-6 off of eBay. They still make the GP-10 version and the Comdyna finds application in teaching labs and in control systems engineering. You can read about the history of the Comdyna GP-6 here.

The unit works and as a first computation I patched it up to simulate the Lorenz system.

The Lorenz system of ODEs is given by:

\frac{dx}{dt} = s (y-x)

\frac{dy}{dt} = r x-y-x z

\frac{dz}{dt} = x y - b z

The classic Lorenz butterfly results when s=10, r=28, and b=8/3.
Here’s the resulting program:

The “program” on the Comdyna GP-6.

And here are a few time-exposures of the program output.

The x-z plane of the phase space.

Another view of the x-z plane.

The x-y plane.


 Posted by at 6:01 PM
Dec 192012

Since curve tracers can go to large voltages, you can look at the I-V characteristics of things like neon bulbs. After playing with a bulb for a few minutes, I wanted to see if the photoelectric effect was observable on the curve tracer. Sure enough, you can see the effects on quantum mechanics in a cheap Neon bulb!

Here’s the video:

 Posted by at 8:59 PM