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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...
It's not that it's unpredictable, just ridiculously complex.
You can set up a very reliable 800 MHz trunked radio system, and with 50 watt radios in the cars and a 300 watt repeater on the top of a mountain you can be assured of 99.99 percent reliability. That's pretty much the average police department setup.
But if you're wanting to communicate over the horizon and don't have intermediate links to leapfrog off of, HF or satellite are pretty much the only ways to go. HF is basically relying on the ionosphere to bounce the signal, and you can predict to a certain extent what frequencies you'll be able to communicate best on, but other than that it's pretty much luck of the draw. Satellites require aimed antenna rigs (except for the Low Earth Orbiting AMSATs) and beefy stations (usually SSB, as FM modulation is a satellite battery power-sucker like no other); as well, satellite is an intermittent service - it's not geosynchronous in most cases, so you only get communications capability for a fraction of the day.
Hey Taco! Looks like you're using the "infinite monkeys and typewriters" scheme to generate Ask Slashdots again...
I don't know about broadcast radio, but in the early days of telegraphy a lot of effort was put into stopping the radio waves from propogating along the ground. The more signal they could beam up into the air the further they could transmit. Early antennas were so bad at this that farmers in Holland could put a wire around their cows horns with a lightbulb attatched - the current induced in the wire by the RF would light up the bulb so they could see where they were when it came time for milking. Another hoary old timers tale: when the Empire state building first went up it was considered prime real estate for radio transmission towers. There was so much RF going through the office that workers could keep warm by wrapping up in unplugged electric blankets, the radio waves would induce enough current in the blanket's wiring to keep them warm.
...but the possibiliity of front-end overload sounds plausible.My portable SW receiver has a Local/DX switch that throws an attenuator in. The radio in my ham station has all kinds of provisions for adjusting the gain in the first stage.
:-)
Are you receiving other HF stations? See if you can hear the Canadian time station CHU on 7.335 and 3.330 MHz. This will let you know your radio is OK. Then try various kinds of shielding or collapsing your antenna to check out the front-end overload theory.
You might be in a "skip zone" on one frequency, but I doubt it would apply to all of them. Our radio club here in Philly has Sunday morning networks on VHF, 10m (28MHz) and 75m (3MHz). This gives us an opportunity to observe ground-wave propigation (without an ionosphere bounce) at the two HF frequencies..the results always vary.
The VHF net meets on a repeater...that's pretty consistant.
Worst case, just call(303) 499-7111, and listen to WWV via phone.
73 de Maggie K3XS, who first was fascinated by shortwave as a kid by WWV.
"Radio station WWV, Ft.Collins Colorado. Give us twenty minutes and we'll give you....twenty minutes!"
-=Maggie Leber=-
Let's use the philosphy of "the right tool for the right job" in real life, too.
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....
The null spot is due to ground wave vs. sky wave differences and skywave propagation characteristics. Sky waves are completely propagated by refraction off of the ionosphere. Ground waves propagate along the ground in a fashion similar to propagation along a transmission line such as a wire or waveguide. The sky wave is received back on earth only because of Snell's Law of Refraction - remember "critical angles" from physics. Think of reflection in water that only happen at certain angles.
There are two factors in the case of the Boulder Null.
The first due to the fact that WWV transmitters and antennas are in Ft Collins while you are in Boulder with mountains in between. If you had a clear path to Ft. Collins (e.g. in nearby Denver) you could get a ground wave.
The second is due to antenna design. WWV is intended for world-wide time service so the antenna is designed to maximize sky wave and also minimize ground wave. The sky wave is optimized to have the lowest "wave angle" to maximize the propagation distance (to the other side of the earth, ideally).
However this also minimizes the local reception. You can't get a sky wave in Boulder because you are still too close to the transmitter to catch the first bounce off the ionosphere. At WWV's frequencies and likely antenna design, you'll usually catch the first sky wave reflection at distances not less than 250-500 miles from the antenna.
These combined help to create the null in Boulder.
The cover of this ARRL book "Your Guide to Propagation" has a diagram of exactly what's happening above: the antenna in the middle is Boulder; the antenna on the left is Ft. Collins WWV. The little arrow just to the right of Ft. Collins is the ground wave path. The arrows heading up into the sky are the sky wave paths. Then imagine a mountain between the left and middle antennas.
[Cover Image]
Some other clarifications for other comments:
1. The broadcast transmitters for WWV are in Ft. Collins, not Boulder. Front-end overload isn't a problem or cause.
2. Modulation doesn't affect propagation, per se - it doesn't matter if you use AM, FM or 64 QAM. Well, you could argue that sideband distortion is a propagation issue, but that's more of a 2nd order effect. You can subsume that as S/N ratio or Bit-Error-Rate (BER)spec. Only S/N ratio or BER is affected by modulation choice.
3. Atmospheric water doesn't have any effect on radio propagation until you get into VHF (30 MHz < f < 300MHz) and becomes a major component only at UHF and above (f > 300 MHz).
So FM radio (88-108 MHz) can be affected by the weather, but WWV (2.5, 5, 10, 15, 20 MHz) and broadcast AM (0.68 - 1.7 MHz) are not. The latter are affected by solar or geomagnetic weather i.e. sun spots and solar flares.
If you want to learn more pick up copies of the ARRL Antenna Book and the ARRL Propagation Handbook.
ARRL Books on Antennas & Propagation
JG