Audioclinic (Oct. 1979)

Outdoor Antenna Noise Revisited

Mr. Giovanelli made two mistakes in his reply to "Outdoor Antenna Noise," in the June, 1979, "Audioclinic" column. The first one results merely in some misinformation; the other is dangerous.

According to John Young of Belden's engineering department, the shielding effectiveness of coaxial cable is not greater than that of shielded twin-lead. Shielded twin-lead provides superior electrostatic shielding (99 per cent) to that of common braid shield, co-ax 90 percent or less. The electro static shielding effectiveness of shielded twin-lead is exceeded (by a fraction of a percent) by foil-shield coax with overbraid, but shielded twin-lead also offers immunity to electromagnetic pickup (by virtue of its balanced configuration). Coax has no immunity to electromagnetic signal pickup. See page 109 of M.J. Salvati's book "TV Antenna and Signal Distribution Systems" for full details. Also, see page 58 of the February, 1978, Audio for his brief statement on this matter.

Mr. Giovanelli's recommendation to raise an antenna to clear power lines is potentially very dangerous if the installer does this in close proximity to the power lines. An antenna raised above ("to clear") the power lines would likely contact them if it tipped over. Over 200 people were electrocuted last year by doing just this.

-Richard Reeves, Flushing, N.Y.

Comments: Because I have been an amateur radio operator for over 30 years, it would never occur to me to put any antenna close to a power line. But an antenna can be some distance from a power line and still pick up noise, especially in damp weather where there are slight breakdowns in the insulators. A. person certainly can be killed during the process of raising the antenna if he does this when close to a power line. He would not even have to wait for the antenna to come down. As long as we are on the subject of antennas, think what could happen if the antenna fell on adjoining property, perhaps seriously damaging a roof or striking a child.

In order for an antenna to stay up, good guying is essential, and I recommend the use of aviation control cable. This will hold most installations of simple TV masts. The section of masting above the rotator may have to be guyed separately by means of what are known as floating guy rings. The bottom of the mast can be anchored to the roof with roof mounts. Chimney straps can sometimes be used, but only where the mast height is not great and where the antenna system does not provide a serious wind load.

Never attach an antenna to an attic vent. I have seen instances where toppling of the antenna tore the vent right out, not to mention the damage created by the falling antenna. You should not use the vents even for guying. Rust-proof screw-eyes put right into the roof beams is the surest means of securing the guy wires.

If the antenna tower is free-standing, it should be mounted solidly to a slab of thick concrete and guyed with elevator cable. If the tower is high enough, you should not have to use much masting above the rotator. If, for any reason, you do use a lot of masting above the top of the tower again use a floating guy ring and aviation control cable to keep the masting from giving way in a high wind. If the tower is mounted against a building, secure it to the side of the building by appropriate plugs and bolts. In addition, if you are not exactly certain how to do all of this, find a qualified person.

Stereo Interference with Cable Reception

Q. My year-old tuner is connected to cable TV. I get a high-pitched tone when my tuner is in the stereo mode.

The filter helps a little, but the sound is still bad. Reception was fine before I got cable TV. What is wrong? What can I do?

-Charles David, Ft. Wayne, Indiana

A. If your FM reception deteriorated immediately upon installing the cable, I would have to think that your stereo interference is caused by the reception of "direct" signals at the same time that you are receiving signals from the "cable." I suggest that your cable company should improve the shielding of any transformers, adaptors, etc. which may be involved with your particular installation. Unfortunately, there are some tuners which are, themselves, not well shielded. In the presence of only moderately strong signals, they might pick up some signal, even when there is no antenna connected to their input terminals. If your tuner is one of these, then there is probably little that can be done to eliminate the problem, assuming that "direct" pickup is the root of the problem.

If you made some modification to the original installation, such as running an extension line from the cable installation point into another room, perhaps this is giving rise to the problem. Often such extensions are made with 300-ohm twin-lead. It is best to have your cable company make the installation properly. (This assumes that the company does put the FM signals on the line.)

If you wish to check to see if this direct pickup is really what is taking place, disconnect the tuner from the cable and connect it to a regular antenna, which may be an indoor di pole for this application. Determine whether the stereo is still plagued by the high-pitched whistle. If it is, chances are that there is something amiss within the tuner.

Rise Time

Q. What is meant by "rise time"? How does rise time pertain to the sound of an amplifier?

-Rawn Stafford, Gainesville, Fla.

A. "Rise time" refers to the amount of time required for a pulse fed into an amplifier or other device to produce full amplitude at its output. Hopefully, this will happen as soon as the pulse enters the amplifier. In fact, however, there is a certain amount of time required for the various circuits to pro duce their outputs. The faster the rise time, the better the equipment will reproduce transient sounds such as percussion instruments.

(adapted from Audio magazine, Oct. 1979; Joseph Giovanelli )

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