HF External Noise Reduction
Noise, QRM, QR-Mary, it's everywhere these days. RFI from TVs, computers, monitors, alarm systems, etc just wreak havoc on the HF bands. Where I live, it's S9+ most of the time and even higher once everyone starts getting home from work. Outside of moving, how can the average ham in a suburban area combat this problem?
The first thing that needs to be done is to make sure your own QTH is quiet. Start by powering your HF rig from a battery that will last a couple of hours, then flip the master breaker on your home. Once that's done, turn off all the breakers in the panel. I won't be going into further details as the BCB and RFI Resolution page has all the details on limiting the amount of RFI from your own QTH.
Still, the noise level was too high, especially on 160/80/60/40 meter bands. I've tried one of those noise cancelling devices in the past - the Timewave ANC-4 and honestly I couldn't get it to work all that well. Why was that? Had to do with the noise antenna. The internal whip didn't work at all and the horizontal wire I tried wasn't much better. Fast forward a decade and the external noise is even worse. This time I spent more research on the two major noise cancelling units I could afford - the venerable ANC-4 and the MFJ-1026. After much deliberation, I decided on the MFJ-1206 because it also had the ability to adjust main antenna gain as well as the noise antenna gain. The ANC-4 lacks this ability.
The most important aspect of reception is not the overall signal strength, but the signal to noise ratio. What that simply means if you have a high noise level, desired signals that are less than the high noise level won't be heard and signals that can be heard must be way above the high noise level to be understood. This is one of the ways the S-meter on your rig lies to you! It will indicate the level of both desired and undesired signals!
It's easy enough to install a noise cancelling unit but the most crucial part of the installation is the noise antenna. What kind should be used? How do I know I have the correct type? Where's the best place to mount it? How big?
First a quick explanation on how a noise cancelling devices work. Simply put, your main antenna hears the noise at a certain level plus the desired signal. The noise antenna must hear the same noise at the same level as the main antenna but WITHOUT hearing the desired signal. When the noise received from the noise antenna, that signal is inverted 180 degrees, adjusted to the same amplitude as the noise received on the main antenna then mixed in the noise canceler. This process effectively cancels out the noise but because the main antenna only hears the desired signal, the desired signal can't be canceled out and gets passed on to your receiver untouched. So, the key to a successful noise cancelling system is with the noise antenna. What kind though?
It's important to know that all man-made noise is vertically polarized. No one seems to know exactly why but it is. In order to receive the noise it requires a vertical antenna, even if you're using a horizontally polarized antenna. A horizontal noise antenna will work but not as well as a vertical noise antenna. This took some thinking on my part because my antenna is neither vertical or horizontally polarized. I'd say it's polarized at 45 degrees! Knowing I needed a vertical antenna, I installed a 9' whip attached to the aluminum frame of my patio cover.
Indeed, it worked very well! Here's a sample:
Problem with this noise antenna is that it didn't collect enough noise an 160/80/60/40 meter bands. Obviously, I needed a much larger noise antenna, but by how much? After much cogitation I came up with a simple procedure to determine the correct size of noise antenna. It really bent my brain around thinking about how to increase received noise level! I took a spool of wire and horizontally extended about 70' along the top of my fence. I tuned to WWV on 5 MHz and flipping the noise canceler on/off I could instantly compare signals. I could hear WWV at about the same strength on both antennas so I started shortening the noise antenna until I could barley copy WWV. Flipping between the main antenna and the noise antenna, the noise level was within about 2 S-units. The length of the noise antenna was about 47'. Remember I wrote all man-made noise is vertically polarized so the noise antenna also needs to be vertically polarized. I have no simple way to stand-up a 47' vertical. More cogitation was required! It hit me that I don't need a 50 ohm match to the feedline as this antenna is not going to be used for transmitting so I helically wound all 47' on a 9' crappy fiberglass fishing pole and replaced my first noise antenna with this:
Looking carefully you can see the spiral wire that's compacted down from 47' to 9'. Interestingly enough, the noise level actually increased some and WWV decreased! A win, win right there!! So, here's the end results:
So, there you have it! It's like using a totally different station. I don't suffer from "receive fatigue" and this setup makes operating more enjoyable. The settings on the noise canceler are rather broad-banded. Sometimes I need to touch up the noise antenna gain or the phase but usually not by much. If your location suffers from QRM, give this a try. Determining the length of the noise antenna is easily repeatable and will be different than my location.