Image received from the International Space Station

I received an image that was being transmitted from the International Space Station as it passed nearly overhead this evening. They broadcast at about 25W of power at 145.800 MHz; an amount that is easily heard and received since the communication is line-of-sight and only a few hundred miles away. They only use it on occasion so it was nice to hear they were planning on a few days of sending images. I used a generic SDR receiver and a simple dipole and piped the audio output to MMSSTV. Some of the noise in the center of the image was because I forgot to correct the receiver for the Doppler shift of the ISS as it passed by.

Re: Las Vegas Anomaly

A couple of images from that area.

Radio astronomy dish raising

Joe Cottral managed to get a spare 2.5m satellite dish from western Illinois to Chicago. The rest of the RAS had the responsibility of getting the dish from ground level up to our 6th floor aerie. We decided on raising the dish as reasonably quickly as possible, to reduce the annoyance factor for all non-involved. We also picked after much observation and discussion to raise the dish via the southeast corner of Ryerson which is an elevated turret. The reason being the turret allowed easy transfer over the parapets of Ryerson. The loading dock offered easy vertical lift, and the fire escape offered quick one-story lifts, but in the end, Lui's suggestion of the turret was the superior option.

Now, we have to 1. acquire a receiver capable of 1420MHz and 1.6GHz-ish, 2. Clean up the dish mounting steel, 3. Mount the dish on Ryerson with lots of Quikcrete, 4. Get microwave-capable cable, 5. get a low-noise-amplifier, and 6. put it all together. Minor bit there. Oh yes, and decide where to place it on the roof.

a very simple 2.4GHz meter

I built a very simple meter that reads the strength of radio energy in its vicinity, without amplification or anything fancy. It picks up radio waves roughly around 1 to 3 gigahertz. It does very well in detecting cell phones, microwave ovens, and wireless access points.

On a simple level, the antenna converts radio waves into an electrical AC voltage, which is then converted by the germanium diode into a pulsing DC voltage. A capacitor stores the pulses and smooths out the pulsing and leaves a very small DC voltage, which is measured directly at a tiny multimeter I picked up at American Science and Surplus. I set the meter to DC voltage, 200mV scale.

The biquad antenna is sensitive to vertically polarized waves and slightly directional as well. A lot of designs also stick a ground plane behind it to increase the directionality, but I was looking for more of a field strength probe, rather than having an antenna that got me the most gain. It's a trade-off since the detector is so simple and without amplification, but that's what you get.



What can we measure with this meter? My standard test suite for gigahertz-ish radio frequencies is delinquent*, so all I can think of is cell phones, microwave ovens, and wireless networking. I found that the office microwave oven puts much more energy out at the hinge side of the door and a fan vent on the side than it does the meshed window. It can saturate the meter at the 200mV scale (when right next to the fan vent). Cell phones periodically check in with their towers (you can also tell this with a set of computer speakers anywhere near a GSM phone). They also do put out a bit of gigahertz radiation; I can detect them sending text messages from about six feet away and more when talking--they can also reach 200mV. Wireless 802.11b and g networks are actually pretty low-power in the scheme of things--they hard to see except close-up until they are transmitting data; then I can detect them six or so feet away. During idle they emit a "beacon" 10 times a second. The wifi antennas are also a good source for checking the polarization of the biquad--I get nearly nothing from them if I rotate the receiving antenna 90 degrees.



Outside the signal level varies greatly. There is a pervasive field which is presumably cellular networks and the addition of all the 802.11b/g/n networks. On the University of Chicago campus near the Regenstein Library the average strength varies from 0.2mV to 0.6mV, with a couple of spikes to 1.0mV. There are also areas of much stronger than ambient. For instance, outside of the Medici on 57th street the average field strength is 2-3mV and peaks at moments at 8mV. There are cellular tower antennas on a school across the street; so it seems likely the area is getting a particular sector of the tower.

I used this Field Strength Meter for 2.4 Ghz Wireless LAN as the excellent template for the project. For my version I used a standard 1N34A germanium diode--this is a more sensitive diode, and turns on at 0.3V instead of 0.6V like a standard silicon diode. I painted the diode black, as the diode proved to be photo-sensitive (all PN junctions are light-sensitive, and ones in transparent glass tubes even more so). Also, I didn't tune the capacitance at all. I then ran two wires to the inside plugs and drilled a hole in the case and hot-glued the antenna to the front of the meter. It's nice and compact, although the GP23A 12V battery in it doesn't last very long.

I could see easy modifications of this system, putting a simple FET amplifier or such to increase the meter response. If I use one of the inexpensive multimeters from Harbor Freight I might have enough room for a prebuilt circuit and a more directional antenna. I'd also like to see if I can pick up both aviation and weather radars with such a simple system. Maybe I should make a Sardine Can antenna?

*delinquent is also a synonym for nonexistent.

The digital TV transition

In about a year and a half, analog broadcast TV will be ending and the transition to digital broadcast in the US will be complete. The New York Times carries an erroneous article about the transition, claiming "The V-shaped rabbit ears ... risk going the way of the eight-track tape player" because of the transition.

That's complete rubbish -- antenna use will increase, not go away (as some cable companies might claim), as people see huge quality improvements and the allure of free HDTV over-the-air. Instead of paying extra in order to get HDTV channels from Comcast, they'll re-discover the broadcast networks in HD, for free.

Some may argue the argument was for the VHF rabbit ears to go away, not the antenna in entirety, but they failed to note that at least here in Chicago, a major network (WBBM-DTV) broadcasts their HD signal on channel 3, in the low-VHF band, absolutely requiring the use of the so-called "obsolete" rabbit ears. See a long discussion about the HDTV situation here.