 PolarBear03The bear formerly known as aaron8301Premium
join:2005-01-03 | Why 30Mhz? This is probably going to sound really stupid, but why can't BPL just use a higher, unused frequency? Why must BPL hover around the same bands that Ham and others already use?
Also, in reference to a post above, I think a different kind of backbone(fiber, wireless, etc.) as close to the last mile as possible is a very good idea, to avoid interfering as much as possible. That way the BPL only uses as short a powerline run as possible, interfering as little as possible, regardless of what frequency it is using. |
| Not a stupid question at all. The answer, unfortunately, is not a simple one.
One factor is the ratio of the conductor diameter to the wavelength of the signal. HF signals are (relatively) long wavelengths (10 to 100m), and so "travel well" on power conductors whose diameter is a few centimeters. As the wavelength gets shorter (higher frequencies), those few centimeters become a significant fraction of the wavelength. For example, at 300MHz, the wavelength is just 1 meter.
At VHF, those signals start to travel only on the outer surface of the conductor (skin effect), the resistance becomes higher, leading to higher I^2R losses, and the need for higher power levels (or more frequent repeaters). (Ultimately, at microwave frequencies, no current flows through the interior of the conductor, and waveguides become the preferred transmission medium.)
Other factors leading to the use of HF, rather than VHF/UHF, systems, might include the generally lower cost of HF components.
By the way, the division of the RF spectrum into categories such as HF (High Frequency, 3-30MHz), VHF (Very High Frequency, 30-300MHz), etc. is essentially arbitrary, based (probably) on the fact that a 1 meter wavelength is 300MHz, 10m = 30MHz, 100m = 3MHz, etc. In fact, circuit, transmission, and propogation characteristics all change continuously with frequency: 29.9MHz has essentially the same properties as 30.1Mhz.
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