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Noise Antenna to Reduce Nearby RFI

noise antenna

Get the noise antenna right, and the noise canceller will do its job. It’s simpler than you think. 

Setting up the external noise antenna for your ANC-4 or MFJ-1026 is the most important part of reducing RFI from noisy neighbors. There is lots of advice available, some good, some not. You should start by looking at the recommendations from the manufacturers.

According to MFJ, “If the primary problem is removal of local noise, it is preferable the noise (AUXILIARY) antenna hear the noise much louder than it hears desired signals. The noise antenna should be located as close to the noise source as possible, so the noise antenna picks up the least amount of desired signal and largest amount of noise possible. In this case the polarization is unimportant, and the spacing between antennas can be any convenient distance.”

According to Timewave, “For noises generated outside the home, we recommend that you mount a small noise dipole outside perhaps down in the shrubs or some other area a foot or two above ground, and broadside to the noise source, such as
parallel to power lines.  Any noise antenna that works, including combinations of horizontal and vertically polarized antennas, may be used.”

After conducting lots of experiments, I agree with these recommendations, particularly that from MFJ, and summarize my findings as follows:

  • Your noise antenna should do a good job of picking up the interference you want to cancel, and a lousy job of picking up the signals you want to hear. An easy way to find out what your noise antenna receives is to disconnect your main antenna, and just use the noise probe as an active antenna.
  • Polarization is not that critical, but RFI tends to be more vertically polarized. Start with a vertical noise probe.
  • You should not worry about impedance matching for the noise antenna. As long as it picks up lots of noise, it works. If the noise is too strong or not strong enough, you can easily attenuate or amplify to get the level right.

Unfortunately, users will find lots of “advice” on the Internet about using a “proper” antenna for the noise probe. Don’t, unless you plan to use the phasing devices for signal enhancement. For cancellation, users get best results from lousy antennas that just hear mostly noise.

Noise Antenna that Works for Me

At my location, the main noise sources are located in two neighboring houses. One is about 150 feet to the west, the other is about 75 feet east. I have put up a noise antenna for each source. The antenna is a 4 foot ground plane. The vertical element is a #14 wire inside a ½” plastic conduit. I 3D printed mounts for the wire that snap onto the top and bottom of the pipe. Each antenna is as close as I can get to the noise source, and vertically mounted on a fence post using a ½” clamp. See the picture above.

The noise probes are connected to runs of RG6 coaxial cable. I can switch between them in the shack, using “A” or “B” with my ANC-4. The antennas are largely hidden by surrounding foliage. Gray plastic conduit is not very noticeable, either.

These noise probes do nothing for RFI from within my house, but they work wonders on interference from either neighbor over the range of 6 – 25 MHz.

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