40m VDSL filter for your NBN

Last Updated on November 3, 2021

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 of 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.

40m VDSL filter

The finished 40m VDSL filter.

TP link Archer VDSL modem

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


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.


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

Speed Test