Audioclinic (Q and A) (Mar. 1973)

by Joseph Giovanelli

Are Signals Too Strong?

Q. Can an FM signal be strong enough to cause distortion?

I have a Finco turnstile antenna on the terrace of my apartment. I get perfectly good reception of FM stations transmitting from New Jersey, Connecticut, Pennsylvania and surrounding regions. I get distortion and static only from those FM stations which transmit from the Empire State Building.

Can it be possible that the FM signals from the Empire State Building are so strong in my area (I live about eight miles from the building) that this causes the trouble? Is there anything I can do to reduce the distortion?

-Salvatore A. Mele, Brooklyn, N.Y.

A. If an FM signal is too strong, this fact is USUALLY not manifested by distortion. (I have seen that happen, but so seldom that I have to think in other terms, as will be shown.) What usually happens is that the strong signal appears at more than one place on the dial or is heard as background to other, weaker stations.

I do not think, therefore, that your problem with FM stations broadcasting from the Empire State Building has anything to do with their signals being too strong. I think that you are either well shielded from such stations by other buildings or else you are getting multipath distortion. The fact that you hear "static" in addition to the distortion would lead me to believe that you may be a victim of both situations.

Your FM antenna is not suited to the elimination of these problems since it is omnidirectional. Therefore, it will pickup reflected signals as well as direct ones, with resulting distortion.

What is required is a directional and rotable antenna. If it is impractical to rotate the antenna, simply point it at the Empire State Building. If distortion is still present, reorient the antenna so that it is pointed in the direction of the reflected signals. It sometimes happens these reflected signals are stronger than the direct signals and produce significantly better audio quality than when the antenna is oriented to favor the direct signal. Because of the directional characteristics of the antenna, some of the weaker signals to which you have listened in the past may no longer be usable. This is because the antenna is not aimed at them. At such times as you wish to hear these stations, the antenna should be aimed toward them. They will once again be usable. In fact, they will be considerably stronger than they were at the time they were received by your turnstile antenna. If your receiver has a signal strength meter, and if this meter is at nearly full scale--or perhaps pinning--you well may be the victim of severe overload.

The directional antenna will add to this situation, but will reduce multipath distortion effects. Place an attenuator between the FM set and the lead-in cable; this should provide less signal and reduce or eliminate the overload.

FM Reception Problem

Q. Last year I bought a Kenwood TK-140 receiver. I had been using a little wire antenna that came with it until last week when I put up an outside antenna. Everything works fine except one thing. With the outside antenna I get a very annoying “frying" sound. Further, when listening to one particular station I get a `motorboating." When I disconnect the antenna, the noise stops. Therefore, the problem is not the receiver. What is my problem? What can I do about it?

-Lloyd Luper, Inglewood, Calif.

A. I am not sure I know what is taking place in your receiver to bring about the "frying" and "motorboating." My best guess is that the increased signal strength provided by your new antenna is causing overload problems in your receiver.

The sole purpose your antenna serves is to increase signal strength; it does not produce noise of its own (at least not of the kind you have described; antenna noise could only be a factor with the most advanced possible circuitry when receiving very weak signals.) The only conclusion I can come to is that the receiver is to blame-in some way lacking the ability to handle strong signals. This condition should not exist. The receiver should be checked.

"Motorboating" is generally associated with power or r.f. bypass capacitor problems. It may be that yours is a marginal case, manifesting itself only under strong signal conditions.

"Frying" is often an indication of some stage breaking into oscillation--another indication of defective bypassing.

If you have background in servicing electronic equipment, I suggest that you shunt various r.f. and i.f. decoupling capacitors with new ones known to be good. Shunt them one at a time while the set is in operation WITH THE OUTSIDE ANTENNA CONNECTED.

(Keep the leads as short as possible so the inductance in the leads won't minimize the by-passing action of the capacitor. This is most likely to be a factor when an r.f. stage is involved.) One capacitor probably will be found which will eliminate the "frying."

Intermittent FM Antenna Problem

Q. I am experiencing an intermittent problem in my FM antenna system. I am using a Finco FM-4G antenna, mounted on a rotator and supported on a 40 ft. telescoping pole. I use two antenna-mounted boosters; one is a Jerrold Powermate transistor booster of fairly recent manufacture which is designed for TV and/or FM use. The other is an older model Blonder-Tongue tube-type unit for FM only. The problem I have manifests itself when the wind blows. It is an intermittent popping sound in the speakers. At the same time the signal-strength meter jumps.

Incidentally, the receiver is a brand new Marantz Model 22.

I first changed the lead-in, thinking this was the obvious answer to the problem. No change. I then substituted the older booster for the transistor one. Still the same problem. I have double checked all connections. I am certain there are no loose ones. I am now completely puzzled. I do not know what to try next.

-John F. Wieland, Liberal, Kansas

A. Because your problem with FM reception occurs only when the wind blows, the receiver cannot be the cause of the trouble. The receiver, after all, is not subject to wind. You are on the right track, therefore, when you think in terms of the antenna system.

You said that you changed the lead-in. Have you changed the cables which inter-connect the antenna mounters and the antenna? The cables must have slack in order for the rotator to turn.This slack loop may blow in the wind. If the conductors break under the insulation, you will not see the break.

Nevertheless, the cables will become intermittent as the contact is made and broken with the cable's motion. The antenna itself can cause trouble. Check all rivets to see that they are tight.

Loose rivets mean loose elements, which will result in poor contacts as the antenna is blown in the wind. It may well be that you have made the connections secure at the antenna terminals as far as the cables are concerned. Have you checked to see that the screws are snugged up against the driven elements? If the lock nuts for these screws are loose, this looseness will cause intermittent operation of the antenna.

I suggest that the mast be grounded. If this has already been done, check that the ground is intact, with no broken insulation and no wires broken under the insulation. Check that the guy lines are tight and thus unlikely to make intermittent contacts with surrounding objects.

(Audio magazine, Mar. 1973)

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