40m VDSL filter for your NBN

Last Updated on September 26, 2021

This 40m VDSL filter may stop the annoying internet drops.

40m and Australia’s NBN delivered over FTTN just doesn’t mix. It’s like oil and water. Transmit on 40 metres and kill the internet. It’s quite a common problem here and probably anywhere the internet is delivered over VDSL.

Having suffered the 40m line drop for a couple of years, I took interest in an email I recently received detailing a simple 7 MHz notch filter that sits in parallel to the phone line. I decided to give it a shot and build the filter. It’s a really simple tuned circuit that’s resonant at 7 MHz.

You will need:


The formula calculated can be found on toroids.info.

40m VDSL filter calculation

Soldering up the filter is straightforward. All components in series.

40m VDSL filter for your NBN series components

The inductor consists of 9 turns on a FT37-61 toroid ring.

40m VDSL filter for your NBN circuit diagram


My internet used to drop within a minute of transmitting on 40m. When the modem resynced, it would drop again a few minutes after. Having installed the filter, 40m FT8 has been running for a few hours without the internet dropping once. I’m calling that a great success. Everyone’s situation and equipment are different, so with a bit of luck, there’s a good chance that this simple 40m VDSL filter for your NBN phone line will work for you and you can rediscover the 7 MHz amateur band.

Installing the filter is straightforward. You’ll need an RJ12 double adapter. Simply plug the double adapter into the modem, the filter into one socket and the phone line into the other.

The modem I have is an ISP supplied TP-Link Archer VDSL modem. Probably not the best modem on the market which probably explains the line drops with 40m. I have not tried bands lower than 40m.

TP link modem

You will notice a download speed drop.

The trade-off for using 40m with the notch filter is about 10 MBs. Before the installation the download speed was 70 MBs, and after it was 57 MBs. In my case 13 MBs. A small price to pay when the family wants to stream a movie while you work some DX or FT8 on 40 meters.

40m VDSL filter for your NBN speed test BEFORE filter fitted
40m VDSL filter for your NBN speed test AFTER filter fitted