Radio Shack PRO-71 - Manual (Page 37)

37
AVOIDING IMAGE FREQUENCIES
You might discover one of your regular stations on another frequency
that is not listed. It might be what is known as an image frequency. For
example, you might find a service that regularly uses a frequency of
431.875 also on 474.675.
To see if it is an image, do a little math.
Note the new frequency. 474.675
Double the intermediate frequency of 21.4 MHz
(42.8)
and subtract it from the new frequency. –42.8
If the answer is the regular frequency, 431.875
then you have tuned to an image.
Occasionally you might get interference on a weak or distant channel
from a strong broadcast 42.8 MHz below the tuned frequency. This is
rare, and the image signal is usually cleared whenever there is a
broadcast on the actual frequency.
FREQUENCY CONVERSION
The tuning location of a station can be expressed in frequency (kHz or
MHz) or in wavelength (meters). The following information can help
you make the necessary conversions.
1 MHz (million) = 1,000 kHz (thousand)
To convert MHz to kHz, multiply by 1,000:
9.62 MHz
×
1000 = 9620 kHz
To convert from kHz to MHz, divide by 1,000.
2780 kHz
÷
1000 = 2.780 MHz
To convert MHz to meters, divide 300 by the number of megahertz.
300
÷
7.1 MHz = 42.25 meters
20-311a.fm Page 37 Friday, February 12, 1999 4:51 PM