[BC] The AM bandwidth issue

[Thomas G. Osenkowsky]

One issue not addressed here is transmission bandwidth. The 73.44 ("NRSC")
specifications apply to audio frequencies and their attendant sidebands.
This assumes, however, symmetrical sidebands. Theory tells us that if we
modulate a 1000 Watt 1000 kHz carrier with a pure 10 kHz sine wave we will
have 250 Watts or 50% field intensity in each sideband (990 and 1010 kHz).

Let us examine a theoretical non-directional antenna. Up to 225 electrical
degrees the radiator will be more efficient at the upper sideband,
conversely less efficient at the lower sideband. This is due to the fact
that the electrical height increases with increasing frequency and decreases
with decreasing frequency. The carrier and sideband frequencies will have
different impedances. The matching network at the tower base has phase shift
which will alter the impedances as will the transmission line. The
transmission line will have a greater attenuation at the upper sideband and
a lower attenuation at the lower sideband. These differences are generally
considered negligible.

When the transmission line enters the transmitter room it may be directly
connected to the transmitter or to a transfer switch if an auxiliary
transmitter and/or dummy load are used. In AM, these are seldom 50 Ohm
concentric as is common in FM. The transmitter has an RF output network
which matches 50 Ohms to the impedance of the RF output source (i.e. plate
of the final RF amplifier). It is at this location where symmetrical
sidebands are important. Symmetrical sidebands at the ATU input terminals or
the transmitter antenna terminals may NOT produce desirable results. As
frequency changes, so does the component reactance. Each transmitter has its
own desired rotation and the manufacturer should be consulted for what
rotation is desired at the antenna terminals (a convenient measuring
location).

In a directional array, there is an additional consideration: pattern
bandwidth. When you change frequency, the electrical height of each radiator
changes as does the electrical spacing. With changing component reactances,
radiator self impedance, mutual impedances between radiators and array
spacing, it is important that the sideband integrity not be greatly
disturbed by different field attenuation at sideband frequencies caused by
pattern shape and size variations. This is especially important in minima
and null azimuths. If we attenuate the carrier to a greater extent than the
sideband(s), overmodulation will result. This is evidenced by audio
distortion on the receiver while passing through a pattern null. Ideally,
you should hear a drop in signal but not distortion.

Few stations have ideal pattern and impedance bandwidth. Many would require
physical relocation of towers to accomplish this. The bottom line is that
bandwidth restrictions are not necessarily adhered to by restricting
transmitted audio bandwidth alone.

Tom Osenkowsky, CPBE

>But weren't the NRSC modifications supposed to solve that problem? By
>cleaning up the splatter and monkey chatter? Wasn't that the goal?
>>And since the NRSC was designed to make the best
>>of bad receiver bandwidth, it was only a "holding" place.