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

Ask a local (physicist)

[MasterLock]

You are in the same building as the folks who actually run the station and you come to /. for the answer? Walk down the hall and get the authorative answer.

Let's use the philosphy of "the right tool for the right job" in real life, too.

Crimmony! Apparently no RF EEs or Hams here!

[JGski]

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