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Radio Propagation and Unexpected Loss of Signal?
Steven Wallace asks: "I'm currently attending an Internet2 related meeting (Joint Techs) in Boulder Co at the NIST building (same building as the most accurate atomic clock). WWV, the radio station that broadcasts the current time on 2.5, 5, 10, & 15Mhz is just down the road in Fort Collins Co. They transmit with about 50,000 watts. Surprisingly I can't receive any of the WWV broadcasts while here in Boulder (I carry a sony shortwave radio with me). The locals tell me that Boulder is a dead spot. I would think I'd hear the thing in my filings given the proximity and power. Anyone care to explain the radio propagation physics that prevent me hearing WWV while at NIST?"
There are two possibilities here.
Case A: Your receiver's front end is overloaded with the strong signal. Extremely strong transmitters will induce large voltages in receiver circuits that have automatic gain control circuits and 'fake them' into believing that the station they're listening to is extremely strong. That, or the front end amp simply can't handle the (relatively) large voltage coming in and it's just thrashing everything.
Case B: Skywave vs. ground-wave propagation. Radio signals from 1-50 MHz often bounce from earth to ionosphere and back, and often several times to get to your radio. That signal may be not propagating well via ground-wave to your current location, and the skywaves are effectively bouncing right over your head.
Here are a few links to get you started:
Realtime HF propagation news from qsl.net
Lots of info on propagation effects
Hey Taco! Looks like you're using the "infinite monkeys and typewriters" scheme to generate Ask Slashdots again...
From http://www.boulder.nist.gov/timefreq/stations/wwv. html:
WWV operates in the high frequency (HF) portion of the radio spectrum. The station radiates 10,000 W on 5, 10, and 15 MHz; and 2500 W on 2.5 and 20 MHz.
The signals broadcast by WWV use double sideband amplitude modulation. The modulation level is 50 percent for the steady tones, 50 percent for the BCD time code, 100 percent for the second pulses and the minute and hour markers, and 75 percent for the voice announcements.
Basically, the transmitter is AM modulated, which means that it is sending a continuous signal with varying strength (this is called modulation) on each frequency. The receiver is supposed to track the variation in power level (this is called demodulation), and send the variations to the speaker as sound (eg. if the signal strength varies 2000 times each second, create a 2kHz tone in the speaker). But guess what, the signal strength is so large because of the proximity to the high power transmitter, that the receiver can't see any variations in the signal strength at all. It detects only a VERY STRONG signal. This is called reciver overload.
Put the receiver inside a metal box (the trunk of a car ?), to attenuate the signal and remove the antenna (if it is possible), and it should works again.
RFC1925
He's right. Just go to http://www.arrl.org/ or http://www.fcc.gov/ for more information about propagation, which is *complex.*
ARRL has plenty of information about Amateur Radio and short wave radio, in particular. Look up information about shortwave, propagation, "skip," and "DX".
During the *day* HF/short wave also propagates differently than at *night,* since the ionosphere changes shape, due to the sun: UV, solar wind, solar flares, etc.
You'll notice on HF, AM, and CB that you can bring in many stations at night that you could not during the day -- for some frequencies, but not others.
I'd be willing to bet that you are either on the other side of a mountain (or mountains) from WWV, or their antenna array is not pointed your way (up, maybe?)
That reminds me, I need to update my Tech, No Code Amateur Radio licence....