Radio Propagation and Unexpected Loss of Signal? 48
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?"
batteries (Score:2, Funny)
But that would be too obvious (Score:1)
Radio technology (Score:2)
But from what I do know, radio is the most complicated, unpredictable phenomenenon around... my wireless LAN is a classic example of this!
Expert shrug their shoulders when asked how a particular antenna design works "hey, I don't know how it works, it just does".
Re:Radio technology (Score:1)
Re:Radio technology (Score:4, Interesting)
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.
Re:Radio technology (Score:3, Informative)
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....
Re:Radio technology (Score:1)
Standing waves? (Score:1)
This is the case. (Score:2)
Still, if you're in the same city as the transmitter, you should be able to hear it unless the antennas are aimed very oddly, and even then at 50 kilowatts you should hear something scattered from the sidelobes... Heck, when W2CXM (Cornell radio club) tunes into a *dummy load* at a kilowatt, you can hear it if you're tuned in a few miles away due to the watt or two that gets leaked out.
Re:This is the case. (Score:2, Informative)
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.
Reasons? (Score:5, Informative)
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 [qsl.net]
Lots of info on propagation effects [ac6v.com]
Re:Reasons? (Score:3, Informative)
Those rigs will pull damn near anything out of the air with a well-designed and erected antenna.
Re:Reasons? (Score:1)
It is true that it's hard to build tracking filters or other forms of selective front ends into affordable portables. However, at least some of the Sony portables are better on FE overload than competitors. Most portables need an attenuator or an exteranl tuned filter to avoid overload with external antenna, but Sony ICF-7600G has behaved well on a short randomwire for me.
Comments on FE Overload, local skip-zone both right on. HF isn't like MW where ground-wave is primary.
Re:Reasons? (Score:3, Informative)
I knew i'ld get to use that silly EE degree sometime.
Re:Reasons? (Score:1)
I've heard that referred to as the "umbrella effect" or simply as shadowing. Usually I would expect to see that happening quite near the tower, but it depends on the design of the antenna.
Depending on what kind of antenna (or array of antennae) WWV is using, the signal could have a narrow vertical beamwidth. No point in wasting power by directing it into the dirt, or at the moon.
Of course, if they are using a vertical 1/4 wave antenna, in the style of AM broadcasters, what I have been babbling about isn't relevant. (For those who aren't familliar with AM broadcast, the transmitting antenna IS the tower. The tower is built to a height of 1/4 wavelength of the signal, and is sitting on a big insulator)
Re:Reasons? (Score:3, Interesting)
Interesting, since most FM (and AM) stations have a small bootser antenna that broadcasts on a different frequency that is picked up by a really big antenna (pr antennas) elsewhere and rebroadcast really powerfully for the actual signal. Those big ones often have crappy reception right around them. (Okay, I just thought of one station that does sit under their broadcast antenna, so some do broadcast directly).
Amusingly, some small AM stations broadcast at a particular power level that leads me to believe that they used to be a big station, and sold their broadcast antenna and now only broadcast off their uplink antenna.
--
Evan
Re:Reasons? (Score:2, Informative)
Here you can see a dipole antenna pattern, and many more, and the cone you are speaking of.
http://www.rfcafe.com/references/electrical/
Here is an explanation on how antennas transmit which will other people understand how antenna patterns are effected a little, and how antennas transmit
http://www.arrl.org/tis/info/whyantradi
Re:Reasons? (Score:1)
Re:Reasons? (Score:1)
Looking on a map, Boulder is about 50 miles from Fort Collins. So it's certainly plausible that the ground wave is being screened off by local mountains and that due to the take off angle of their antenna, the first ionospheric bounce is coming down somewhere beyond a 50 mile radius. Having a few dead spots in central Colorado is a small price to pay if it gives them better domestic (and worldwide) coverage.
Re:Reasons? (Score:1)
Re:Reasons? (Score:1)
Also, imagine how warm and fuzzy you would feel if they did use ground waves and you were "just down the street" from a 50,000 watt transmitter (even with losses)! hehehe...pretty warm...
Ground waves (Score:4, Interesting)
Re:Ground waves (Score:2)
Re:Ground waves (Score:1)
This reminds me of a story I was once told about the BBC World Service. They broadcast from a very large and powerful shortwave transmitter. The BBC began to receive reports of poor reception in parts of the world. Upon further inspection, it turned out some guy had set up a large number of copper coils in his attic and was using the power induced in them to power his house!
At the time, they discovered there was no particular law against what he was doing... so they had to offer him a large sum of money to stop doing it.
This may or may not be an urban legend......
geography... (Score:1)
what I'm talking about is the Flatirons, which you've probably seen by now (or your blind). And they're just that, huge massive chunks of iron on the mountain faces, which I've heard work the same way mirrors work with light beams. So possibly the radio signal is being slightly reflected out of the valley, but thats just my guess
Re:geography... (Score:2, Interesting)
This is the first time I've heard of the Flatirons actually being made of "massive chunks of iron". I believe they are actually sandstone [gravmag.com]. They are named for their visual appearance.
I can attest to the awful reception of the WWV signals in Boulder, though they are occasionally intelligible. Still, this seems like a strange topic for /..
HF propigation can be like black magic... (Score:4, Interesting)
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!"
WWV Via Phone (Score:1)
Worst case, just call(303) 499-7111, and listen to WWV via phone.
A Question:
Do you know where to get other local phone numbers for the time like that? I'm especailly interrested in numbers in the 312, 708, 815, and 773 area codes or any where around Chicago.
I'm taking a wild guess here... (Score:2)
I'm not sure where the 303 area code is though, so I could be wrong.
Ask a local (physicist) (Score:3, Insightful)
Let's use the philosphy of "the right tool for the right job" in real life, too.
Re:Ask a local (physicist) (Score:1)
Couldn't get WWV (Score:2)
As far as dead spots in general go, AM radio is low enough frequency that it will bounce off of clouds and the lower atmosphere, which creates gaps where the signal is being reflected.
Receiver overload (Score:3, Informative)
From http://www.boulder.nist.gov/timefreq/stations/wwv. html [nist.gov]:
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.
Physical explanation? (Score:2)
Answer from NIST (Score:1)
Crimmony! Apparently no RF EEs or Hams here! (Score:3, Insightful)
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] [arrl.org]
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 [arrl.org]
JG
Re:Crimmony! Apparently no RF EEs or Hams here! (Score:2)
Just a simple case of ionospheric skip. Nothing more to see (or hear), folks, move along.
Suggestion (Score:2)
Take your radio out of the filing cabinet. Maybe the cabinet is shielded against those frequencies.
while in boulder (Score:1)