How to get back on 40m without the internet falling over.

My internet would drop within a few seconds of transmitting FT8 on 40m, as a result, I gave 40m away, but I’m back! This 40m VDSL filter stopped the annoying drops with a slight trade-off in download speed. It worked for me, but it may not work for you as modems and installations are different. It doesn’t cost much to make and everything to gain.

A simple 7 MHz filter in parallel across the phone line certainly fixed my problem of the internet dropping every time I keyed up on 40m regardless of TX power. It meant that when the family was streaming a movie or tv show, I couldn’t use 40m without incurring howls of abuse and protests. (In a nice way)

After many months, if not years of putting up with this, I decided to give this filter a go when it appeared in my inbox one day. There was nothing to lose… except 40m.

The calculations are straightforward. When the filter is placed across the phone line, no 7Mhz signals find their way into the modem. The trade off is a slight speed reduction. In my case 13 Mbs. The circuit consists of a 100pf 3KV ceramic capacitor in series with a 5uH inductor.

If you’d like to make your own filter for other bands, the formula used on this page was generated on toroids.info

The finished 40m VDSL filter.

The modem I have is a TP-Link Archer modem which is very common in Australia for the supply of NBN.

NBN Co release a paper called Mitigating Amateur Radio Interference to VDSL2 in March 2020. It’s available as a pdf from the NBN Co website and is worth a read as it goes into the technicalities of the VDSL technology. The preface of the document states…

This document is intended for technically-minded users or those providing technical support to users connected to the nbn™ network who are experiencing signal interference between their nbn™ service and amateur radio transmissions. It is assumed that the people referencing this document will have a reasonable understanding of electronics, wireless transmission and broadband transmission technology including digital subscriber line (DSL) and very high-speed DSL (VDSL). With that in mind, this document would be suitable for licensed cablers, telecommunications network technicians, operational support staff, network engineers and amateur radio operators.

Parts list

1 x 100pf 3KV ceramic capacitor from Altronics
1 x FT37-61 toroid ring from Minikits
a length of 0.4mm enamel wire (28 B&S)
1 x RJ12 Double adaptor from Jaycar
and an offcut of phone cable from an old handset. Jaycar

I have a basic kit of three parts required for the filter. Details here.

Installation

Plug the double adapter into the modem, then the filter and phone line into the two ports. Reboot the modem and let it resync.

To get a feeling for what this looks like, the following graphs show actual VDSL2 service transmit levels as a function of frequency. These particular graphed services have had notching applied in the 80 and 40 metre amateur radio bands. The dark blue line shows which frequencies and transmit levels that have been negotiated and are being used by the modem and node. The gaps in the line shows those other frequencies that have been disabled. The notched amateur radio bands are circled in red.

Before

After