Myth or truth???

[zachtheterrible]

I heard somewhere that lower frequency radio signals travel farther than higher frequency because of their long wavelengths. It seems reasonable 2 me, but is this true? I'm not talking about how the lower frequency waves can bounce off the earth's atmosphere or any natural phenomena like that.

 

[Klaus]

True, the longer wavelengths can easier bend around the earth's curvature while the shorter ones can not.
Just look at your radio, you can hear medium wave AM stations a lot further from the transmitter than ultra high frequency FM stations.

Low frequency signals require a lot more power to travel far since it is difficult to build directional antenna's for that size. Once you get to shortwave frequencies, directional antennas become practical, even for home amateur stations.

Generally, the higher the frequency the more line of sight the reception, However, you can receive very high frequency signals from outer space simply because it IS line of sight :wink:

 

[Dean Huster]

Actually, there are limits on both ends of the spectrum. Above around, oh, call it 70MHz or so, and transmitted signals are considered pretty much to be line-of-sight. Below that, especially with the HF shortwave spectrum, you get into ionspheric skip, which is really prevalent when we're on the peak of the 11-year sunspot cycle.

At night, on the standard AM broadcast band, you also get skip, which is why you can hear the clear channel stations like WOAI in San Antonio over a big chunk of the nation.

Below 500 KHz, skip disappears and the signal is primarily ground wave reception. But that ground wave can be pretty strong, even for lower powered stations. WWVB, the NIST time and frequency station, broadcasts on 60KHz and the signal covers the lower 48. Until the advent of more accurate methods of delivering time and frequency out of Colorado, users like the VLF signals vs. the HF signals of WWV. The WWV signals would skip and/or the ionsphere would change in antitude and cause all sorts of shifts in phase and really mess up trying to calibrate a decent frequency standard. The VLF signal is a heck of a lot more stable, but still, a cal lab would average out over a few days of reception. Now days, cal labs can tap into the GPS system and get cesium-accurate frequency and time for around $5000. Sounds expensive, but it gives nearly the same accuracy as having a primary cesium standard in-house at a cost of several tens of thousands of dollars. In that regard, it's cheap accuracy.

The U.S. Navy ELF system transmits near the audio range and the signals cover the earth, penetrating even into the ocean so that they can whisper to the subs to come to periscope depth so they can receive a satellite signal. However, the lower frequencies suffer in that they have little bandwidth, so you can't transmit a lot of intelligence over them. The Navy ELF basically can do a really slow Morse code -- don't expect them to transmit full-motion video over that low of a frequency.

Dean