RF Interference From HFC

I’ll start off by saying I am by no means an RF engineer, so I may be completely wrong about what I am about to write about. I am however, quite confident in my empirical observations over time.

I got into an interesting conversation about RF interference generated by HFC systems – (often touted as an “alternative” to the NBN) – with Gary Stark and Dan Warne on Twitter on Friday afternoon.

Gary raised the issue of signal leakage from HFC systems that interfere with terrestrial RF broadcasts – (such as radio, television, emergency communications, etc) – and the discussion led to the thoughts that any proliferation in the use of HFC for internet delivery – (as espoused by some) – would increase the problem, and possibly create others.

Gary highlighted the Bondi area of Sydney as being particularly RF congested. There are many competing frequencies in the area, that interference is a common occurrence.

This reminded me of my own observations around Geelong.

For example, have a listen to the interference caused to a terrestrial radio transmission as I drive past the TransACT headquarters – this was recorded on Friday afternoon:

Now – (and note I said earlier, I am no RF engineer, so I could quite easily have this ass-about, but stick with me for a bit) – that doesn’t seem good, does it?

Some people I am sure will say, “oh look, there is a radio tower and a mobile phone tower there too”. Well, those towers were in place before the HFC network was, and this interference did not occur in this location before the cable network went up.

I can also vouch for the fact that having worked in Geelong in a role that would see me drive all over the city at all times, similar interference occurs all over Geelong, particularly near splitters and boosters in this particular HFC network.

The Melbourne/Geelong, Ballarat, Bendigo, and West Gippsland broadcast areas – (for both radio and television) – have a considerable history of interfering with each other, and over the years there has been much juggling of spectrum.

So congested is the spectrum, that the advent of digital television created many headaches.

Metropolitan digital services for the Seven Network are broadcast on VHF channel 6, the same frequencies that were used for Nine Network-affiliated WIN Television from Ballarat since 1963 – which was moved to UHF channel 36.

Metropolitan Nine Network digital services are transmitted on VHF channel 8, conflicting with the Network Ten-affiliated signals originated by Southern Cross Ten from Bendigo and Gippsland. Similarly, metro Network Ten digital signals are broadcast on VHF channel 11, previously used by the ABC in Ballarat.

Melbourne’s 3AW and its sister station Magic1278 actually swapped frequencies some years ago to alleviate as much interference as possible from its 3AW signal – which moved from 1278AM to 693AM.

The ABC are trying to launch a local radio station in Geelong, but haven’t been able to find a suitable frequency so far, with the process now about five years old. A new ABC NewsRadio frequency in Ballarat and a community FM licence in Melbourne’s outer east regularly interfere with each other.

The ultimate point to make is that the RF spectrum is incredibly congested, in many areas. Not just Melbourne or Geelong or Bondi. It is a real headache for authorities in allocating frequencies for new services all over the country.

Locking the existing HFC networks into use for any “NBN-like” network does nothing to solve this problem. Putting more people on them may actually increase the problem.

HFC is also an analogue technology, not digital. It will also likely actually cost us more than switching to a primarily fibre-based network from the beginning.

HFC is still not the answer.