How I eliminate low band HF interference

[Nashua Area Radio Society]

I am Paul Dumont K1NOB & have been a member since late 2010.  I am using a 160m 1/2-wave-sized-dipole fed with 450-ohm twinlead at ~35 feet from the ground for the 160 through 6 meter bands with good results.  In the past six months, I have been plagued by RF hash-sounding interference at S9 level.  I took the time to troubleshoot and identify the noise source and was successful.  Here’s my story.

First, the initial conditions.  We use Comcast-Xfinity for a triple-bundled communication package of cable TV, internet and Ma Bell-style land line telephone.  The land line phone is a VoIP system, using the RF cable from the street.  We “rent” a Comcast-Xfinity modem-switch-router-WiFi-VoIP box that provides internet and a modular phone jack.  When I installed it years ago (before getting my license) I plugged the phone jack into the built-in house phone lines wand disconnected the line outside which was fed by the (Fairpoint?) line from the street.  So, the land line phones work as they always did.  The multifunction box has served us well for at least 10 years, and up until last summer didn’t interfere with my HF radio.

Friday (2/23/18) I had the day off from work and with the wife out working I took the opportunity to diagnose the problem without having to explain myself.  I hooked up my IC-706mkIIG to a 12v battery and verified that the noise was still at S9.  With the whole house shut off the background noise on 75m was a much better S5 or 6.  So the noisemaker was inside the house and therefore under my control.  After shutting more circuits off one by one the noisemaker was found to be the cable modem-switch-router-WiFi-VoIP box mentioned above.  When the box was shut off the Rx noise was S5 or 6, and no change when I put back the dedicated 12V power supply on the ‘706.  The noise path was puzzling because the modem etc. box couldn’t be further from my radio shack.  The only conductors radiating anything would be the phone lines in the walls, one of which is 3 feet from my radio gear.  When I disconnected the phone line at the modem box I observed the desirable S5-6.  How do I filter that noise out and still have the land line?  At that point I had to leave the house for a bit and while out I thought of installing an old DSL lowpass in-line phone jack filter I had stashed away.  It solved the problem.  I you have a DSL filter, don’t throw it away.

If you have a similar box from your cable company, you may be getting noise into your radio as well.

[Nashua Area Radio Society]

Paul – good problem and the solution you used is good information to share with our members.  The approach you used to isolate the source of the noise is a good one – especially when you consider that we will all experience this type of problem at sometime in our life.

Broadband noise sources can be caused by a wide range of sources both inside and outside a home. Poorly designed switching power supplies (i.e. low voltage DC wallwart supplies), arcing AC wiring switches and contacts, HV AC power line leakage or low level plasma discharge, and/or poor NETWORK wiring can all be a potential source for this type of problem.

Most of the noise sources we’ll encounter will usually be broadband due to the rich harmonic content associated with transient and/or non-sinusoidal signal (digital) sources. The use of an old AM portable radio tuned to an unused AM channel near the low end of the AM dial can often be helpful in locating the signal source.  Note: As an aside, monitoring around 500 KHZ at the low end of the AM broadcast band is a good warning of approaching electrical storms.

Your addition of an old DSL filter to your cable network phone line was an effective solution and I would like to add that clamp on ferrite RFI filters might offer a good working solution as well.  (It is usually helpful to have a couple of clamp on ferrites your shack and many major electronic part suppliers stock them).

I searched the web to learn more details about the internals of a typical DSL filter and stumbled across a couple of interesting articles.  W8JI documents a slightly different problem where his 160M operation would crash his DSL network.  The details of his problem and a low pass filter that he reportedly added (to prevent his RF leaking into his DSL network) offers another interesting RFI prospective.  You might enjoy reading W8JI’s “My ADSL Modem Filter” for future reference.

Again, good information and thanks for sharing your experience with the forum readers.

[Author]

Posts