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?"

Comments

Reasons?

[Jonny 290]

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

Re:Radio technology

[Jonny 290]

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.

Ground waves

[pshanks]

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.

HF propigation can be like black magic...

[MaggieL]

...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!"