AUDIOCLINIC (Aug. 1990)

FM Antenna Length

Q. I am cutting wire to construct an indoor FM antenna. Is there a formula by which to calculate the optimum length of an antenna used for FM reception for the carrier frequency of a given station? I am mainly interested in receiving stations between 88.5 and 91.1 MHz.

-Name withheld

A. I don't think that getting too serious with math will help you very much.

The Q of a typical FM dipole antenna is rather low. This will mean that the length can be off quite a bit and the strength of the received signals will not drop off very much. If the antenna is to be used indoors, it is not in the clear, as it should be if it is to operate according to theory.

To give you a flavor for wavelength calculations, the speed at which radio waves at any frequency travel in space is about 186,000 miles per second--or 300,000,000 meters per second.

This speed is constant regardless of the wave's frequency. If the frequency is low, the number of such waves passing you each second is small, and the length of each wave will be large.

As the frequency goes up, the wavelength goes down--you can calculate it by dividing the wave's speed by its frequency. A frequency of 100 MHz, roughly in the middle of the FM band, will have a wavelength of about 3 meters (300 million meters per second divided by 100 million Hz), or about 9 feet, 10 inches. Most antennas are half the wavelength (some are a quarter wavelength), so you can divide the wavelength by two and be close.

Now that you have the idea, here's a shortcut as long as you keep your frequency in MHz. To obtain a half wavelength for an antenna in feet, divide 468 by the frequency in MHz. Thus, the length of the antenna for 100 MHz is 4.68 feet. The length of an antenna for 88.5 MHz is 5.29 feet. The length needed for proper resonance at a frequency of 91.1 MHz is 5.14 feet.

If your antenna is a form of "rabbit ears," you'll end up adjusting the length of the two elements for maximum signal strength and make no calculations at all!

MM Cartridge Considerations

Q. I own two moving-magnet cartridges. The cheaper cartridge has more output than the more expensive one, 4.6 mV as opposed to 3 mV; I like the sound of its high end better than that of the more costly cartridge. Which unit is really better, and what specs can I use to determine the best cartridge, whether the "best" is what I already own or not?

-Gusztav Marokity; Van Nuys, Cal.

A. The cartridge which sounds best to you is the one you should use, regardless of its price or other considerations. Wanting to be sure we have the best is fine up to a point. Yet because of the subjective nature of the way we perceive what we hear, there is no absolute best cartridge or other audio product. In fact, even my own thoughts on this matter are subjective.

You might also find that the more pleasing high end you hear from your less expensive cartridge is not just a function of the cartridge's own performance but of other factors in your system which affect its response, such as the load resistance and load capacitance into which the cartridge works.

For flattest frequency response, the proper capacitance must be provided by a combination of the phono cable's capacitance and that of the preamplifier's input. The load resistance should also be correct; for the most part, this is standardized at 47 kilohms for moving-magnet cartridges, and most phono inputs have this resistance value.

It is best if your equipment's phono input sensitivity specification is lower than your cartridge's output voltage spec. This insures sufficient output from your system and also results in the best possible signal-to-noise ratio for your system. If your system produces more than sufficient output and its signal-to-noise ratio is greater than the background noise of the records themselves, you will probably be able to use a cartridge having less output than is expected by your phono inputs.

The mass of your tonearm may also be important when attempting to find the "best" cartridge. The maker of the tonearm will usually offer guidelines as to cartridge requirements.

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(Source: Audio magazine, Aug. 1990, JOSEPH GIOVANELLI)

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