On Thu, 23 Apr 2009 14:02:28 -0400, Scott McGrath wrote:
>In the case of a broadcast system you are setting up a single frequency
>system and you want all the components to have an impedance match as you
>want as much of the power as humanly possible to be radiated by the
>purpose built antenna, You do not want the RF bouncing around the
>transmission system creating heat.
Few broadcast systems are "single frequency." They may have a single
carrier, but most are concerned with good phase response over some finite
bandwidth depending on what modulation scheme is employed. With FM,
that's at least 200 kHz. With TV, it's 6 MHz. With AM, it's at least 20
kHz. With all of these systems, non-ideal amplitude and phase response
creates distortion.
I grew up a few miles from an AM station with a 4-tower array on 930 kHz,
and during my college years, worked for them. Directional AM antennas are
designed to generate a null in their pattern in the direction of another
station on the same frequency, and nulls are the result of precise
cancellation between two or more wavefronts. When I drove through one of
the nulls in their pattern, the carrier went away but the sidebands were
loud and clear. A bit further up or down the road, the sidebands went
swishy. Just one example of non-flat amplitude and phase response
creating distortion. Multi-path is another example. One of the reasons
that few TV stations on the low-band VHF channels (2-6) use multi-bay
antennas is that it's difficult to get good phase response over that
relatively high percentage bandwidth (more than 10% on Ch 2).
To have good phase response, the load Z needs to be flat over the full
modulated bandwidth. That's another BIG reason why low SWR is critical in
broadcast. Yes, heat and voltage are important too.
73,
Jim Brown K9YC
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