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Registered Member #10
Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424
I remember talking to a tech from the US base in Exmouth Australia that runs a ULF station. Can't remember the frequency but it was real low.
"They are over 1,000 feet tall, the tallest being 1,271 feet tall, and spread out over 2 1/2 Km in diameter. It's the world's largest VLF transmitter, and can transmit over 2Mw of power." "Quite a few tons of buried copper too to improve ground resistance."
So, if you have a big back yard and want to talk to your nuclear sub under the north pole .....
... not Russel! Registered Member #1
Joined: Thu Jan 26 2006, 12:18AM
Location: Tempe, Arizona
Posts: 1052
Yep, that is quite true. Anything under 9khz is pretty much fair game, so long as you ensure that your transmitter isn't radiating any significant harmonics. I can't imagine designing a low pass filter for 9khz would be a very pleasant task, though.
I've seriously considered building a transmitter of significant power for this frequency, but the math is always pretty appalling to behold. A halfwave dipole at 8.9khz is over 10 miles long. You could probably shorten it to a mile or two with some pretty severe loading coils, but at quite a performance loss. Also, unless it's very high off the ground (several thousand feet), you're going to send pretty much all your energy straight up, which isn't exactly the most ideal radiation pattern.
If you have the resources of an entire nation, the most ideal solution is probably a network antenna, like the one pictured below at Exmouth. It's actually a VLF station, transmitting at 19.8khz. There is another one near here, at Cutler, Maine, transmitting at 24khz. Its signals are easily receivable at any time of the day or night, simply by sticking a length of wire into the line-in of my soundcard. With a loop antenna, I was actually able to demodulate the MSK signals into data. Unfortunately, the data is encrypted and contains nothing interesting. The station in Cutler also has a significant amount of buried copper, and is built on a salt marsh, to take advantage of increased ground conductivity.
For the amateur experimenter, the only reasonable antenna to consider is a magnetic loop, but it's still quite a daunting situation. Assume you've built a reasonably efficient transmitter that can deliver 3.2kW of power to the antenna. Assume the antenna is constructed from 3/4 inch copper pipe, which is available in 20 foot sections here in the US. Let's say you've bought four of them, constructing a square-shaped loop that's pretty massive in size, but not totally unbelievable. Let's say the bottom of the loop is 10 feet off the ground, meaning the top is about 30 feet in the air. Tricky, but could be done if hung in some trees. Given all that information, a loop design calculator suggests that you'll have to construct an 11.5uF capacitor, capable of standing off 1200 volts, and that radiated power will be right around ten microwatts.
The attractiveness of the unlicensed lowfer band (160-190khz) in this case is obvious. Yes, you have to limit yourself to one watt and 15 meters of wire, but an externally located transmitter connected to a square loop made of 15 meters of 3/4 inch copper pipe gives 50 microwatts of radiated power, in a band that is far, far, far less noisy. You still don't need a license, and the initial cost of materials is greatly reduced.
... not Russel! Registered Member #1
Joined: Thu Jan 26 2006, 12:18AM
Location: Tempe, Arizona
Posts: 1052
atomicthumbs wrote ...
I was actually able to demodulate the MSK signals into data. Unfortunately, the data is encrypted
Can you say "decryption"?
I am relatively certain that the US Navy's encryption codes are, for all intents and purposes, unbreakable. After all, if I can hear them clearly with just a few inches of wire jammed into my mic jack, I'm sure anyone in the northern hemisphere with a good receiver and a big back yard can tune in.
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Under 9 Khz Wide Open
