Sunday, 9 October 2016

Interference (3) VDSL continued

Just before I got the notches on my VDSL line I experimented a lot with ferrite and my ground system and hoped for a good outcome. I already wrote a concept post on my blog including this video. And actually wanted to make a video as soon as I had a solution for the problem. After all that was not as easy as I thought.


This video shows what the interference does. First time short burst PSK on 60m it only gives a few hickups, second time the VDSL signal was completely gone. Modem had to be restarted and had to wait 5 minutes before the signal was back again. If it was only the TV it would be less a problem but the same modem does also internet and our telephone.

Still, I would like to fix things myself as a real radioamateur and experimenter. I decided to call for help, first tried to get a password for the official remoterig forum to ask about their RF-protection filter. Till this day I'm still waiting for access. But wait, there is also a remoterig yahoo group. I asked the about the RF-filter there. Luckely got some helpfull answers from some friendly fellow radioamateurs. Kostas SV1DPI mentioned that the RF-protection filter must be a kind of low pass filter not suitable for the high frequency of VDSL these days and normal ferrite clamps are not efficient enough to reduce RFI at low frequencies like 1,8-10 MHz. I should use #31 or #43 mix ferrite rings or clamps. Alle wires should be twisted (twisted pair) and use as much shielded CAT cabel as possible. He also directed me to a very usefull document regarding RFI. In which the RFI on DSL is described. However the solution is not a simple one and most important you should have chokes of >5K Ohm at the feedpoint of all your antennas. Difficult if you use a open feeder and a horizontal loop but I can manage it for the vertical. W8JI Tom (the master of RFI ;-)) came with a explanation and told a differential filter (with caps to ground) in your VDSL line will cut off the frequencies the line is using. He doesn't think the internet modem is the problem the internals are too small to pick up any of the HF transmissions. He suggested to stop common mode RF flowing from the VDSL line to the power source. According to his e-mail: A differential filter in the DC line or low resistance high impedance chokes would help. DC wires could be bypassed to ground (if available) or each other (with very small caps). All signal wires (and you could include power and ground after the filter) should go through a high impedance RF core as a parallel bundle with the exact same turns. This will effectively bypass all of the leads to each other for common mode. It will not bother the signal. This will probably require a stack of large diameter ferrite cores with very high permeability and low Q, much like used in HF high power tuner baluns.

Well enough tips and suggestions to go on with solving this problem. However the interference has already solved at my side by adding HAMradio frequency notches at the telecom side of the line. So there is nothing left to test for me. It probabely will help others that have the same problem. However I decided to buy some 3E25 ferrite with a very high permeability for solving RFI from low frequencies, they succesfully used them to solve RFI at other telephone/internet modems. I removed the ferrite clamps and added them in all the lines including power supply winding with as much wire on it as possible. I also ordered some FT240-31 cores to build a choke blocking especially the low frequencies and to insert at the feedpoint of my vertical. According to G3TXQ's common mode choke impedance chart it has a high resistance especially on low frequencies although a FT240-43 core might be better.

A few weeks ago PE1BVQ Hans was visiting me as he was on vacation nearby. He brought a line-isolator (FT240-43 inside) with him to test. As he had very good results with it at home. Using it between the tuner and endfed antenna. He didn't cause any interference anymore when transmitting and before there were same problems as here. Now, I got another antenna system with a autotuner at the feedpoint of the vertical. But though, it was worth trying the line-isolator behind the palstar tuner in the shack. Watching the touch-screen monitor, the only device I have some interference on in my shack, I noticed the interference was almost gone with the line-isolator connected. Tried other places as well, behind the radio, behind and before the connectorpanel (with grounding connection) but that made no difference at all. So, I left it connected behind the palstar. Hopefully it will help, time will tell.

If you have problems with interference on the VDSL line it will be a difficult task to solve the problem. Hopefully this post and some post before will help anyone that has the same problem.


2 comments:

  1. Hi Bas

    Thank you for sharing these. I have also had problems here with VDSL. There has been some repairs to the infrastructure on the street but the issue is not fully resolved. No offer to notch out the amateur radio freqs yet.
    73 Angel

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    Replies
    1. Hello Angel, I had the luck to find someone that could place the notches. If it is possible it certainly is possible at your side as well. Although just calling the helpdesk doesn't help I think. They probabely don't know what you're talking about. I found someone using a hamradio forum. You might try the same. 73, Bas

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Thanks for your comment. Bedankt voor je reactie. 73, Bas