Contributed by Al, KN1O
As every Ham quickly learns there are all kinds of interference. Some interference is simply caused by other stations that are too close in frequency to the desired signal. The interference could also be noise caused by an arc somewhere in the electrical grid, or a buzz caused by a poorly designed LED light fixture, appliance, or one of the many small power adaptors feeding your computer, cell phone, or tablet. There are several good books on how to track down and eliminate sources of interference (check out the ARRL book store) and while their advice may offer solutions to your noise problem the sad reality is that if you live in an urban environment you will likely never eliminate all of them. The good news is that modern radios provide some very useful tools for reducing interference if you know how and when to use them. Think of them as your radio’s hearing aids.
Have you wondered what some of those buttons and knobs on your new, or not perhaps not so new, radio actually do? Perhaps they have mysterious labels such as NB, NF, PBF, ANF,ATT, P.AMP etc. You may have tried some of them and they did not appear to do anything or they made signals sound very strange indeed. Without getting too technical the intention of this article is to demystify those buttons and knobs and provide some guidance as to when and how to use them to assist with interference reduction.
A feature that has been offered on receiving equipment for many years is the ability to control the radio’s bandwidth. If your radio is tuned to 14.200 MHz, for example, and the bandwidth setting is 2 KHz then you are listening to any “signal”, including noise, that lies between 14. 199MHz and 14.201MHz. The wider the bandwidth setting on the receiver the more noise and interference you will hear in your speaker. At the same time if the bandwidth is too narrow the audio quality of the received station will suffer. In addition to the tuned circuits used to control bandwidth in older receivers, manufacturers started offering plug-in crystal filters and mechanical filters specifically designed for SSB (Single Side Band voice) or CW (Code). SSB filters provide “communications quality” audio, not as pleasant to listen to as a high quality AM audio. However, since most SSB transmitters limit the audio bandwidth of their signals there is no improvement in audio quality if you select a filter with a wider bandwidth setting. CW signals have very narrow bandwidth so a CW filter can also have very narrow bandwidth without impacting the desired signal. If you have a CW filter installed, or if you have a more modern radio with digital filters, you may notice a significant reduction in noise from your speaker when switching from SSB to CW or from AM to SSB. Of course if you use the CW filter and try to listen to SSB or AM signals the received signal will be unintelligible because most of the audio falls outside the bandwidth of the CW filter. For CW you may ask yourself which filter do I buy? 500Hz? 250Hz? While an ideal 250 Hz filter would be great for CW the reality is that when a filter is too narrow it starts to produce artifacts of its own often referred to as “ringing”. In my experience the 500Hz filters are the best choice for most radios. Fortunately newer radios have built in adjustable filters that allow you to select almost any bandwidth so you won’t have to make that decision. In summary, for reduction of noise and interference use the narrowest bandwidth setting that still provides acceptable audio quality.
Before continuing the discussion on bandwidth it is important to mention the most overlooked and perhaps most useful knob on your radio. It is the RF gain control. It doesn’t control bandwidth but it may help the filters in your radio do a better job. Noise is generally broadband relative to the desired signal. Thus there may be a lot more energy in that noise than is present in the desired signal. While reducing the RF gain will attenuate the signal you want to hear it also reduces the huge amount of broadband energy entering your radio’s bandwidth filters allowing them to do a better job for you. The result is that you may be able to hear the weak signal better with the RF gain reduced than you did with the RF gain all the way up. You will probably need to increase the volume control (typically labeled AF) to compensate for the loss of volume. If you have an attenuator button on your radio it does the same thing as reducing the RF gain but it may insert too much attenuation thus eliminating the desired signal. Experiment with the RF gain control at the same time you are trying different bandwidth settings to determine what combination of gives the best results . If you hear a lot of “ringing” and “rumbling” when you activate your filters that is an indication that you may want to reduce the RF gain a bit. Note also that if your radio has a preamp it should be left off in most situations.
Pass band filters (PBF). The SSB and CW filters mentioned above are Passband filters, they reduce the IF (intermediate frequency) bandwidth of the receiver. With older radios you were forced to select a specific bandpass when purchasing a plug in SSB or CW filter however new radios allow you to select just about any passband you wish. Typically newer radios have preselected factory settings for AM, SSB and CW but those can be changed if desired. On more compact radios you may need to dig down in the menus a bit to get to the manual adjustments. You may also have a “sharp” vs. “soft” option on the filter settings. A sharp setting is intended to maximize the effectiveness of the filter for elimination of interference but a sharp setting also tends to introduce more filter “ringing”. I prefer the soft setting on my radio.
In the AM setting the IF bandpass is typically centered on your dial frequency. If your bandwidth setting is 3KHz, for example, then you are hearing everything from 1.5KHz below the tuned frequency to 1.5KHz above the tuned frequency. When you switch to SSB you will notice that your radio selects either LSB (lower side band) or USB (upper side band) The convention is to use LSB for frequencies below 20M and USB for 20M and higher (with the exception of 60M). To avoid unwanted interference the receiver is now only listening to frequencies below the dial setting for LSB or above the dial setting for USB. And since only one sideband is being used the IF bandwidth is reduced to roughly half that of the AM setting. For that reason alone you may notice a big drop in background noise just by switching from AM to SSB. You can listen to an AM station in SSB mode. Since you are only hearing one of the two identical sidebands, and only letting in half the noise as a result, you may hear a weak AM station better in SSB mode than you do in the AM setting. Audio quality is likely to be reduced but intelligibility is improved in high noise situations. If there is very little noise and interference you may chose a wide filter setting to enjoy higher quality audio.
It is worth noting that the pass band filtering described above is done in the IF (Intermediate Frequency) section of the radio. Another approach that can be used to upgrade older radios is to use an external audio passband filter. These audio filters are very effective for CW since they can be adjusted to just pass the desired tone (600Hz) from the receiver and block everything else from making it to the speaker.
IF shift is another option on modern receivers. As mentioned above the SSB option controls whether your receiver is just listening above or below the frequency indicated on your dial. Let’s say you are in LSB mode and another station just above your frequency, also operating in LSB mode, is overlapping your received bandwidth a little causing interference often referred to as “splatter”. By shifting your IF down a little bit you may be able to cut off that interference while not having too much negative impact on the audio of the station that you want to hear.
Dual passband filters. Let’s take the above situation where you have a station above yours that is overlapping your passband but there is also a station below the desired station you want to hear that is causing interference. The dual passband filter option provides some ability to reduce the interference from both. You only hear signals that lie in the overlap of the two passband filters. In effect dual passband filters are doing the same thing that you could achieve by adjusting the IF shift and IF bandwidth controls but they make the process easier because the controls are completely independent. If you reduce bandpass but then shift the IF you will likely need to go back and forth between the two controls to try to eliminate the two sources of interference. With Dual Bandpass filters you can work on the upper and lower interference problems independently. The displays on modern radios provide a very nice graphic showing the passband of the two filters and their overlap.
Noise blankers (NB) have been available in radios for over 30 years. Their original intent was for the elimination of automotive ignition noise and they do that incredibly well. Since ignition noise consists of regularly spaced pulses of short duration, the noise blanker can determine when those pulses are present and blank the receiver for the short period of time those pulses are happening. Since the receiver is blanked out for such a brief period of time for each pulse it has no noticeable effect on the audio of the desired signal but in most cases completely eliminates the interference. The first time I had that option in a receiver it was like magic! The drawback of course is that these filters only work with very regularly pulsed interference and have absolutely no impact on other kinds of interference. Some other sources of noise may create regularly spaced pulses and the Noise Blanker can help reduce or eliminate them. Noise blankers can distort some signals (AM stations in particular) so you probably do not want to leave yours on all the time.
Noise reduction (NR) is an option on most of the newer radios with Digital Signal Processing (DSP) capability. These filters employ digital algorithms that try to differentiate between noise and desired signals. Their effectiveness continues to improve as signal processing gets better with each new generation of radios. In my 15 year old radio I find it somewhat helpful but only at a relatively low setting (about a 3 on a scale of 15). Above that setting it creates a lot of distortion in the audio.
Automatic notch filter (ANF). This is another filter that works like magic but has a very limited application. If you are using SSB and have an interfering carrier resulting in a loud squeal from your speaker, pressing this button should eliminate that annoying noise. What it is doing is detecting the frequency of the interference and it inserts a “notch” in the radio’s audio output frequency (or in the IF bandpass) to eliminate the undesired carrier. You will probably notice that this feature is not available when your radio is in CW mode because it would try to notch out the desired CW signal!
Manual notch filter (MNF). This filter also inserts a “notch” in the bandpass but allows you to manually select where that notch is located within the receiver’s passband. Since two can be better than one using the ANF and MNF together may help to eliminate multiple interfering carriers. Some modern radios even offer multiple manual notch filters. Here is a use for the MNF that wasn’t documented in my manual. As mentioned earlier, some filters generate interference of their own often described as “ringing”. I have found my MNF very helpful at reducing ringing and hissing sounds in the audio.
Noise cancelling devices. These are noise elimination devices that are not included in your radio. They can work magic or they can be completely ineffective depending on your particular situation. So how do you determine if you should spend $100 plus on one of these items and just what do they do? Their principle of operation is that they take a sample of the interference (and only the interference), shift the phase of the interfering signal 180 degrees, and inject that out of phase interference signal at exactly the same level as the interference coming in from the main antenna. That cancels the interference but leaves the desired signals. The trick is to obtain a sample of the interference and only the interference. If you collect a sample of the interference that also includes the desired signal then both will be cancelled!
I have an underground high voltage power line that runs down the street in front of my house. It generates noise that registers +20dB on my S meter on several ham bands making them completely unusable. I can pick up the noise with just a few inches of wire placed close to any AC outlet in my home. Since all I needed for a sample of the noise is that short wire, it is too small to pick up any of the ham signals I want to hear. Since I have a good sample of just the interference the noise canceler is able to eliminate the same interference that comes in on my main antenna without harming the desired signals. It reduces the +20dB interference to nothing! It requires readjusting the phasing control and the level control on the device whenever I make significant frequency changes but it allows me to use several bands that were not usable before.
Another ham in town is experiencing interference from a neighbor’s plasma TV. If he could place a sample antenna next to the neighbor’s TV and run a coax to the appropriate input on his noise canceller then it is likely that he could eliminate the interference completely. However, that is not practical, especially since the neighbor is across the street, so as a compromise he placed a small antenna in the front window of his house to pick up as much of the noise as he could while not receiving too much of the desired signal. The noise canceler provides a little improvement in his situation.
Another option for severe interference situations
Some combination of the above “hearing aids” should allow you to hear signals under most situations. However if you find yourself in a hopeless interference situation on one or more bands here is another thought: obviously the noise only impacts your reception, not your ability to transmit. Many hams are making use of free internet SDR (software defined radio) receive sites and listen via their computer and transmit from their local station. Keep in mind that in the event of an emergency the power is likely to have failed and your interference sources will be gone. In the meantime you will have maintained an effective Ham station, honed your operating skills, and will be prepared if your station is needed to assist with communications in an emergency. But equally important you will be able to continue to enjoy hamming thanks to the generosity of the hams operating those SDR sites (check out WEBSDR.ORG)