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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...
You're on the right track. The station radiates 10,000 W on 5, 10, and 15 MHz; and 2500 W on 2.5 and 20 MHz. 10Mhz is the most-reliably-heard frequency in most of the US, as it's one running at 10KW, at a good frequency compromise between minimizing number of hops to minimize absorption, and having short enough hops to leave very few dead spots ("skip zones"). Due to varying ground angles and indices of refraction along the way, it's well filled-in from about 200 miles on out until it's faded into the noise. since it's what, 40 miles from Ft.Collins to Boulder (down off Broadway, isn't it?), you get missed by skywave, even at the 2.5Mhz frequency. The 20Mhz signal might be detectable with a high-gain directional antenna, bouncing off the Flatirons.
I'm in Longmont (about 10 miles up highway 119 from you, and out of the valley). I just grabbed my rig and checked. I can her 2.5,5,10, and 15, but not a bit of 20Mhz. I would expect that 2.5Mhz would have enough ground-wave to maybe bend down into the valley. You might try that one. You may find it easier to find the signal if you can switch the receiver to U/LSB or CW mode. That way, as you tune through where the station should be, you may hear the carrier , dropping to 0hz near the indicated correct frequency. Set a bit of an offset where it's easy to hear the carrier, and experiment with antenna orientation and placement, looking for an increase in signal strength. Then, null the carrier, and switch back to AM mode. The WWV signal has components in its modulation that make it nearly impossible for me to understand, except in AM mode receiving. I think it's the 100hz subcarrier carrying the BCD time code. Whatever it is, it's much more easily ignored in AM.
If you still can't get a strong enough signal, grab some wire at the Radio Shack at Flatirons mall (i think it's on the north side of Pearl, around 31st street), and hook it to your rig. You might want to pick up alligator clips for the purpose. At receive, you can get away with crimping - no need to solder. Get as much wire as you can, going in one direction, as high as you can. If there's a grounding point on the rig, run a short straight wire from there to a good ground, or a long straight wire just laying along the ground as a counterpoise. Surely, you can pick up WWV on one of the 5 freqencies. Once you can do that, you can probably also get other SWB stations - BBC, VOA, etc..
General technical issues aside, yes, Boulder is a bit of a dead zone, at many wavelengths, to many locations. There's a lot of high ground between South Broadway in Boulder and Ft. Collins.
If you come across some time, come by for lunch, and we'll see if your rig is working, compared to my FT-817.
Alternately:(303) 499-7111 is a local call, back to the same building you're in.
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....