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June 2018

DISH DOCTOR
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WHY DAS ABOVE 300 MHZ?

Q: Why do all DAS Headends, operate above 300 MHz? Is there is any reason? Is it more LCO friendly? Or easy for maintenance?

Prashant Bale, By E-Mail.

ANS: There are actually 2 reasons for this:

1. Historical

2. Technical

HISTORICAL

When cable TV digitisation began, cable networks carried both Analog and Digital signals for several years until the Analog signals were completely switched off only recently.

The prime channels for Analog signals were Channels 2 to Channel 12, as those could be received even on very old TV sets with mechanical tuners.

Hence digital signals were simultaneously introduced at frequencies beyond Channel 12, usually in the UHF band that began at 300 MHz.

TECHNICAL

The frequency spectrum from 30 MHz to 299 MHz is referred to as the Very High Frequency (VHF) Band. This corresponds to wavelengths of 10 meters (30MHz) to 1 meter (300 MHz).

There are many services offered simultaneously in the VHF band, viz:

Amateur Radio: 50 MHz to 54 MHz

TV: VHF Band I (48 MHz to 70 MHz)

FM Radio: VHF Band II (88 MHz to 109 MHz)

Aeroplane Navigation Beacons: 108 MHz to 118 MHz

Air Traffic Control: 118 MHz to 137 MHz

Amateur Radio (2 meter): 144 MHz to 148 MHz.

TV: VHF Band III (110 MHz to 300 MHz)

TV signals are transmitted in VHF Band I and VHF Band III. As you will observe the frequency bands are not continuous because FM radio is transmitted on VHF Band II ie. 88 MHz to 108 MHz for FM radio. It is not efficient to club Band I and Band III by leaving out Band II.

Further, there are a lot of other services that can cause interference with the TV signal. Hence it is better to avoid frequencies below 300 MHz, as far as possible.

Also, amplifiers will always produce 'Harmonic distortion' ie cause distortion at frequencies that are multiples of the main amplified signal.

Hence if a 60 MHz (VHF Band I) digital signal is being amplified, its second harmonic will be 120 MHz (60 x 2), its third harmonic will be 180 MHz and fourth harmonic will be 240 MHz all in Band III. Hence a 60 MHz signal's harmonics will cause interference with other TV signals at 120, 180 and 240 MHz.

Similarly, a 200 MHz signal will cause harmonics at 400 MHz. However, if Digital signals commence at 300 MHz, the second harmonic will be at 600 MHz, and its 3rd Harmonic at 900 MHz, where there is usually no digital cable TV signal.

For a small LCO's network, it is always preferred to carry digital signals at the lower frequency band since signal losses on co-axial cable are much lower.

As a good technical compromise between using the lowest frequency band and using a band which has lower interfering signals, digital cable TV is almost always now carried above 300 MHz



Dish Doctor
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