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Ionospheric propagation of BPL emissions

Concern has been expressed by some that there may be a significant increase in the HF noise floor due to ionospheric propagation of BPL emissions. Some go so far as to say it will "wipe out all of HF".

This article explores the risk on one amateur band that is subject to good ionospheric propagation through much of the day,

The model assumes a BPL link pair (upstream / downstream) that passes say, 16 homes uses about 5MHz of spectrum. About 80 homes can be passed by 5 such links pairs in the 2MHz to 30MHz region without frequency reuse.

If one of those links spanning the 7Mhz band leaks energy to the limit allowed by US FCC Part 15.209 (30uV/m at 30m from source), the radiated power in 2KHz of bandwidth (typical equivalent noise bandwidth of SSB Rx) is of the order of -52dBm.

The path loss for an ionospheric path varies with conditions, but a middle value is somewhere around 130dB. (As a check, consider a 100W tx (50dBm), less 130dB path loss give a rx power of -80dBm, that is approximately equivalent to S8 (S9=-73dBm, 6dB/ S-point).)

The signal strength at the remote 2KHz wide receiver from the single link will be about (-52dBm - 130dB)dBm or -182dBm. That level is about 74dB below the ambient noise level at a very quiet location (ITU-R P.372-8 suggests -108dBm).

If there were 25 million such sets of links passing 80 homes (2,000,000,000 homes passed), the combined emissions propagated by an ionospheric path would lift the background noise at a very quiet location by just 3dB (half an S-point).

This additional interference by ionospheric propagation from BPL passing 2,000,000,000 homes would be barely discernible at a quiet location, it isn't strong enough to "wipe out all of HF". To "wipe out all of HF", you might need another 40dB, or 20,000,000,000,000 homes, or more.

Though BPL system have frequently been documented to radiate emissions at up to 20dB above the limit set in FCC Part 15.209, the risk of significant increase in the noise floor from a deployment scenario as described above seems quite low.

The potential for BPL emissions to interfere with radiocommunications by direct propagation paths is well known and very high, see the article Are you ready for BPL enablement of your home and neighbourhood? .

 


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