Strong Medium Wave transmitters wreck VDSL performance

I've had VDSL for about 8 months, and it's never lived up to the expectations I had for its speed. I'm about 1000m from the cabinet, and achieve a solid 22.4Mb/s sync speed. The BT Wholesale Availability Checker reckons on 27.4 - 36.6 Mb/s for a clean line, with a Handback Threshold of 25 Mb/s. Recently I had a look at the bit loading figures on the combined modem/router, and there's a big hole corresponding to part of medium wave.

I live on the northern outskirts of London. I'm ~8km from Saffron Green (SG - where LBC and Gold are transmitted from), and ~10km from Brookmans Park (BP - a major BBC transmitter).

The really huge signals here on my communications receiver during daylight hours are:

Freq (kHz)StationStrengthTransmitter
909Radio 5 live endstop BP
1089Talksport endstop BP
1152LBC S9+50dB SG
1215Absolute endstop BP
1305Premier ChristianS9+50dB Chingford
1458Lyca S9+50dB BP
1548Gold S9+50dB SG
1584London Turkish S9+50dB?

My broadband feed comes overhead from a pole more or less opposite my house. I don't know about the overhead section of the line, but the dropwire is Drop Wire no.8 - not a twisted pair, just like glorified bell wire. This comes into the house to a GPO lightning protector (essentially 2 fuses), and then twisted pair to an early NTE5 equipped with an Openreach Mk3 VDSL filter plate.

The combined modem/router is a Zyxel VMG1312. This graph shows the bit loading (which is obtained with the command xdslctl info --Bits). It only goes up to bin 1863 because there's nothing in use on my line above that bin. Upstream bands are in blue, downstream in green. And there's a very obvious big hole.

From bin 205 to bin 306 there are 0 bits assigned (with the exception of bins 221 and 227-231 where there are 1 or 2 bits assigned). Then there are odd single bins with 0 bits assigned, which are 309, 338, 359, and 443. These bins translate to these frequencies:
The big hole is from 884kHz (bin 205) to 1320kHz (bin 306). That is the section of Medium Wave with the stations from Radio 5 Live to Premier Christian Radio.
Bin 309 is 1333kHz - ?
Bin 338 is 1458kHz - that's Lyca, from Brookmans Park
Bin 359 is 1548kHz - that's Gold, from Saffron Green
Bin 443 is 1910kHz - hmmm, something in Top Band, can't see what

The only really strong transmitter that doesn't wipe out (at least) one bin is London Turkish on 1584kHz (which is bin 367) - and when I look at the bin data the adjacent bins are allocated 8 bits while bin 367 is allocated 5 bits, so it's had some effect.

I suspect something is getting overloaded by the big Medium Wave signals.

I believe my early NTE5 has a gas-filled spark gap protector - rather than the 26A surge arrestor which is rumoured to affect DSL performance. And I don't think there's anything in the VDSL filter plate to overload. So is it the VDSL modem getting overloaded ?

Quiet Line Noise

In looking further into the problem, I found that the VDSL modem as part of the sync process has a look at the RF noise on the line, before the VDSL signals are present. It sort of turns the VDSL modem into a spectrum analyser for a brief test. The data is stored for the most recent VDSL sync, and can be obtained by the command xdslctl info --QLN.

So I've looked at my QLN data (actually on a different VDSL box, a Billion 8800NL, but it has the same problem as the VMG1312), and I think it shows a disaster area! Here's the graph, out to bin 1883:

And, zooming in to cover bins 0 to 400 (up to ~1.72 MHz):

The hole in the bit loading graph from bins 205 to 306 corresponds to the big signals with the wide skirts. They're pretty much destroying VDSL in that frequency range. The strong signals are:

BinFreq (kHz)Station
211909Radio 5 live
3031305Premium Christian
3671584London Turkish