[Amps] High SWR

[Scott McGrath]

Perhaps I was oversimplfying - I spend much of my day distilling complex 
technical issues to bullets the non-technical can understand.  I meant 
single frequency as opposed to frequency agile transmittters like modern 
HF rigs with a 25:1 frequency range

- Scott

Jim Brown wrote:
> 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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