# MHz Scanner Users By Bill Kelsey, W6QC

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3 3 AUDIO AMP An electronic circuit that amplifies the low level audio frequencies to a level sufficient to drive a loud speaker. SPEAKER An electromechanical device that converts electronic audio frequencies into sound vibrations that the human ear can hear. Mathematically, we can express the superheterodyne principal for the above FM receiver as follows: B-A = C or MHz MHz = 10.7 MHz Unfortunately, the superheterodyne technique produces another solution besides the one we have just shown. Exploring this second solution we find: A-B = C or MHz MHz = MHz In this particular case this is a nonsense solution as the solution is a negative number. If we continue this process by substituting another frequency for A such as MHz and B remains at MHz we get the following: A-B = C or MHz MHz = 10.7 MHz This solution is one that could work, and in fact in the real world it will work under many conditions. This second solution is referred to as the image frequency. This is simply the frequency on the other side of the mixer conversion process that mathematically can be present. The receiver cannot tell the difference between these frequencies and will receive them both simultaneously. You will observe that mathematically this image occurs at two times the I.F. frequency (frequency C ) higher than our desired receiver frequency A. A + 2 C = Image Frequency or MHz + 2(10.7 MHz) = MHz This image frequency presents a solution that we do not want for our FM broadcast receiver and requires special efforts in the design of the actual receiver to minimize this effect.

6 6 The image frequency problems described above have always been present. They are most commonly found in the single conversion scanners or any dual or triple conversion scanner with a first IF frequency of 10.85, 10.7, or 10.8 MHz. You rarely noticed them before as these images of the Amateur service repeater transmitters fell on frequencies that were offset by 12.5 khz from the 470 channels you have been monitoring. You would have needed to tune the scanner to a channel that is half way between the 25 khz spaced channels that are currently in use on the 470 MHz band. The MHz band is currently on 25 khz channel spacing (0.025 MHz) with the band starting with the first channel at MHz. An example of this spacing is: An example of three adjacent channels: MHz MHz = MHz MHz MHz MHz A 12.5 khz ( MHz) offset channel would then occur at: MHz MHz = MHz (which is halfway in-between & MHz) Therefore the image frequency for MHz with a MHz I.F. is: A - 2 C = image frequency or MHz - 2 (10.85 MHz) = MHz The previous Amateur service channel plan was on 25 khz spacing starting at MHz. An example: MHz, MHz, MHz, MHz Before May 1, 1999 tuning your scanner to MHz would cause you to hear the image channel of MHz, as well as any traffic on the selected MHz channel. There typically is no activity on MHz, or similar offset channel. The new Amateur service channel plan is now on 20 khz channel spacing again starting with MHz. An example: MHz, MHz, MHz, MHz, MHz

8 8 Before spending money on a triple conversion scanner, ask to see the technical specifications on the scanner, and then make some calculations for the frequencies that you are interested in and make sure the image frequencies fall into areas of minimal concern. Generally, the better triple conversions scanners will convert to a much higher first I.F. frequency than 10 MHz. To eliminate the problem with MHz images for MHz you do not want a first I.F. anywhere near 10 MHz. If the current scanner you have will allow operation up to at least 495 MHz, then you can try using the image frequency phenomena to your advantage. To do this tune (input a frequency or channel) the scanner to the frequency that causes the image frequency to fall on the desired channel. In the greater Los Angeles area TV channel 17 ( MHz) is not used. A scanner tuned to a 492 MHz channel will hear the image frequency with little interference because there are no signals on the 492 MHz frequency. Due to the poor performance of most TV receivers the lower adjacent TV channel is not assigned. Los Angeles has TV channel 18 ( MHz) so TV channel 17 is not used. An example of this is as follows: You wish to listen to MHz but are bothered by the ARS activity that occurs near the image frequency of MHz. This example assumes a first I.F. frequency of MHz (one of the most common). To find the correct frequency to use in the MHz area, make the following calculation: Desired channel to monitor + 2 (I.F.) = Actual channel to tune ( MHz) or MHz + 2 (10.85 MHz) = MHz. If you now tune the scanner to MHz, you will be listening to this frequency (part of the spectrum that belongs to TV channel 17) and the image frequency of MHz. Because there is no activity on MHz, you will now hear the LACO Fire Dispatch and no amateur activity. Please see the MHz Scanner Channels as an Image Frequency for 10.85, 10.7, and 10.8 MHz 1st I.F. chart at the end of this article. This chart depicts MHz channels and the three most commonly used I.F.s and gives the appropriate TV channel 17 frequency to use.

10 MHz Scanner Channels as an Image Frequency for 10.85, 10.7, and 10.8 MHz 1st I.F. 470 MHz MHz 10.7 MHz MHz 471 MHz MHz 10.7 MHz MHz Channel I.F. I.F. I.F. Channel I.F. I.F. I.F. Frequency Frequency Frequency Frequency Frequency Frequency MHz MHz MHz MHz MHz MHz MHz MHz NOTES: Frequency MHz = Frequency to set "Scanner" to for selected 1st I.F. ScannerImage1.xls WFK 11/26/99

11 MHz Scanner Image Frequncies For 10.85, 10.7, and 10.8 MHz 1st I.F. 470 MHz MHz ARS ARS 10.7 MHz ARS ARS MHz ARS ARS Channel I.F. Image Channel Freq. I.F. Image Channel Freq. I.F. Image Channel Freq. Frequency Offset Frequency Offset Frequency Offset MHz MHz MHz khz MHz MHz khz MHz MHz khz NOTES: I.F. Image Frequency = Frequency of image for selected 1st Intermediate Frequency of Scanner ARS Channel = Closest Amateur Radio Service channel to selected image frequency ARS Freq. Offset = Offset in khz of ARS Channel from selected image frequency ScannerImage.xls WFK 11/26/99

12 MHz Scanner Image Frequncies For 10.85, 10.7, and 10.8 MHz 1st I.F. 471 MHz MHz ARS ARS 10.7 MHz ARS ARS MHz ARS ARS Channel I.F. Image Channel Freq. I.F. Image Channel Freq. I.F. Image Channel Freq. Frequency Offset Frequency Offset Frequency Offset MHz MHz MHz khz MHz MHz khz MHz MHz khz NOTES: I.F. Image Frequency = Frequency of image for selected 1st Intermediate Frequency of Scanner ARS Channel = Closest Amateur Radio Service channel to selected image frequency ARS Freq. Offset = Offset in khz of ARS Channel from selected image frequency ScannerImage.xls WFK 11/26/99

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