Kill FM Interference With Two Antennas

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FM listeners often wish to tune in a distant station which is adjacent-channel or co-channel to a stronger local signal. Often no amount of tuner selectivity or capture ratio will enable reception of the weaker signal.

Adjacent-channel reception is particularly difficult, and tuner designs 30 years ago did not permit adjacent channel reception. Indeed, when the FCC set up broadcast standards for FM transmission, adjacent-channel reception was considered impossible. Today, FM tuners can be built with 100 dB adjacent-channel selectivity.

The improvement in tuner selectivity has unfortunately not been followed by a reduction in the spurious emissions of FM transmitters in their adjacent channels. This interference cannot be tuned out by a selective receiver, since it is already spilling into the desired channel at the transmitter end.

Co-channel interference may also be a problem. You may live between two stations, or in the middle of a triangle of three stations, and wish to receive just one of them. Instead, you get all at once, even when using a directional antenna and rotor.

Hope still exists for the listener facing these reception problems; the solution involves erecting a second antenna.

Method

We call this method a "phase-cancelling" antenna system, and it is often used in military and commercial point-to-point communications systems where interference must be minimized. Two separate receiving antennas are connected such that the desired signals add in phase, and the undesired signals add out of phase. Thus, the wanted signal is reinforced and the interfering one eliminated, or nearly so.

In order for the system to work, the antennas must usually be physically separated in a horizontal direction.

Mounting the two antennas on the same mast will not work. Consider first the case where the two stations are at different compass directions from the receiving location (Fig. 1); they may be co-channel or adjacent-channel. Point both antennas at the desired station and adjust their horizontal separation until the undesired signal cancels out.

It is easy to obtain as much as 60-dB rejection of the undesired signal with a phase-cancelling antenna of this type. As long as the two stations are at different compass headings, the scheme will work easily. If the stations are in the same direction from the receiving location, the problem is more difficult. This special case will be discussed later.

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Fig. 1 Phase-cancelling antenna system. Desired and undesired stations at different compass headings.


Fig. 2 Use of an existing TV antenna, or existing TV-FM cable signal, combined with the FM antenna in a phase-cancelling system. Place the attenuator in the cable to the stronger of the two undesired FM signals.

Fig. 3 Easy, "quickie" phase cancelling scheme that will work in many cases. Hook the rabbit ears to the splitter with any convenient length of cable, and then walk around the room with antenna while watching signal strength of undesired signal. You may need to place an attenuator in line to the outside FM antenna. Juggle attenuator settings and placement of the rabbit ears for minimum interference on the desired station or minimum pickup of the undesired station. This scheme fails when the attenuator setting becomes large enough to kill the desired signal.


Fig. 4 Phase-cancelling system using two identical indoor antennas. A pair of B1C Beam Boxes should work well, but the author hasn't tried them.

Move and adjust the two antennas for best results. Physical spacing of the antennas should be large, at least 10 feet, or on opposite sides of the room.

Fig. 5 Scheme to try if the desired and undesired stations are in the same direction (zero degrees) or opposite directions (180 degrees). The idea is to obtain vertical spacing of the antennas so both pick up the strong undesired station, but only the big antennas on the tower picks up the weak undesired station. Connect antennas as in Fig. 3 and then adjust the position of the indoor antenna and the setting of the attenuator of the outside antenna for best results.

(a) Layout for phase-cancelling antenna system using (b) Electrical hookup for the phase-cancelling system two identical antennas, shown in (a).

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Stations on Different Headings

Best performance is achieved using two identical antennas on separate masts. All connections are made with coaxial cable of the same length and type. Arrange the antennas as in Fig. 1.

Calculate the distance between the antennas, as

D (meters) = 300 / 2f (MHz),

where the frequency, f, is of the undesired station.

For example, if the undesired station is on 100.1 MHz, the distance D will come out to:

D=300 / (2 x 100.1) or about 1.498 meters or 4.92 feet.

It may be easier to measure the distance d between the two antenna masts; this is given by: d=D= sin Θ

where Θ is the angle between the desired and undesired stations (Fig. 1). What happens if you do not want to crawl all over your roof with protractors and slide rules, putting up masts, or digging up your back yard and putting in expensive tower foundations? Is there an easier way? There are some simpler tricks, which won't work as well, but often give adequate results. An existing TV antenna can be combined with the FM antenna, if they are on different masts. You may even use your existing FM antenna and a piped-in cable FM signal to cancel an unwanted local FM station.

Both of these schemes are outlined in Fig. 2. You must be prepared to do some tedious cutting and adjusting of the lengths of the cables from the signal sources to the tuner. Adjust the cable length from one antenna for minimum undesired signal, and then set the attenuator to further minimize the undesired station.

Sometimes simple rabbit ears can be used as a phase-cancelling antenna, as in Fig. 3. This scheme will work well only if both signals are fairly strong. A "quickie" setup that actually worked at the author's home used the method of Fig. 3 to cancel a local station three miles away and allowed the reception of an adjacent-channel distant station 140 miles away.

If you are apartment-bound and cannot put up outside antennas, a scheme using two indoor antennas in a phase-cancelling hookup may work.

Set up the two antennas as in Fig. 4, and move them around until the undesired signal is minimized. Cable FM combined with an indoor antenna may also work, as in Fig. 2 earlier.

Stations on the Same Compass Heading

If the stations are on the same compass heading, you are in trouble because the distance d (Fig. 1) becomes infinite as sin 9 = zero. Even my back yard on top of a country mountain isn't that large! Here is a trick that may work when the angle between the stations is near zero degrees, i.e. they are in the same direction. Try a large vertical spacing between the antennas (Fig. 5). This scheme works especially well if the desired station is distant and the undesired station is local. The big outside antenna picks up both stations, and the indoor antenna picks up just the local, allowing cancellation to work. If the desired station is very weak, it may not be possible to effect cancellation with the attenuator; the cancellation may be just right when the desired station disappears into the noise! Stations located at an angle e = 180° (opposite directions from the receiving location) are a little easier to handle. If your antenna had an infinite front-to back ratio, a phase-cancelling system would not be needed, so the first thing to try is a better antenna. If your antenna has the best possible front-to back ratio and still you have interference, the phase-cancelling scheme of Fig. 5 will work. Results are best if the undesired station is nearby.

Conclusion

While at McIntosh in 1970-1972, I received requests from FM listeners to solve reception problems just discussed. Today letters still come; phase cancelling antenna hookups do work for those willing to make the effort to try them out.

I purposely avoided too much technical detail here. If you want to try an elaborate phase-cancelling antenna, it is best to work through a local TV-FM dealer experienced in exotic antenna systems. Realize that you must really want that distant station, so write that station's management and ask if they intend to keep the program format you like so much. Imagine the frustration after putting up two $1,000.00 towers in your back yard to pick up that elusive classical station 150 miles away, and just as you finish they switch to the same taped disco format that's on the station you were trying to cancel! Listeners with specific reception problems or questions may write to me for more information, and I will be glad to help. (Note: If I'm flooded with letters, the replies may be slow.) Write to Rich Modafferi, c/o Electroacoustical Labs, Inc., 16 E. 42nd Street, Suite 918, New York, N.Y. 10017.


Article by Richard Modafferi (adapted from Audio magazine, Jan. 1980)

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Updated: Sunday, 2016-10-09 11:13 PST