[BC] Class "D" AM Towers

[PeterH]

On Feb 15, 2010, at 7:58 PM, Steve Lewis wrote:

> Do both of these receive the benefit of the 5/8's wave lower angle of
> radiation?

On the contrary. A radiator this tall has very high high-angle  
radiation, a fact which was not known until the mid-1930s.

Any taller than 195 degrees and the station will suffer close-in  
fading, as a function of both frequency and power.

Class As are the most affected, as these stations have a very large  
primary service area and an almost completely protected secondary  
service area.

225 degrees on both 650 and 1170 were complete and utter failures.  
Both of these Blaw-Knox radiators were amputated towards a more  
forgiving effective height, about 195 degrees in WSM's case. WCAU's  
radiator no longer stands, but it is understood that the height was  
similarly reduced.

The self-interference limited the stations' service areas to about  
100 miles. Definitely not good for a Class A.

At 195 degrees, this self-interference was greatly reduced, or was  
completely eliminated.

The lower the power, the taller the radiator which is consistent with  
good practice, as lower-powered stations have a dramatically smaller  
primary service area anyway and no protected secondary service area  
at all.

200 degrees is almost always a good bet for a lower than Class A  
station, and for one or two Class A stations.

There are several successful 5 kW stations with 225 degree radiators  
in arrays, but 225 degrees is not generally employed at this power  
level.

There are numerous Class C stations with 225 degree radiators.

Some generalizations can be made ...

5/8-wave ... 225 degrees ... suitable for almost all stations of 1 kW  
or less; suitable for some 5 kW stations; unsuitable for most 10 kW  
and higher-powered stations

5/9-wave ... 200 degrees ... suitable for a few 50 kW stations;  
suitable for stations of lower power, but this height is too  
expensive when employed in complicated arrays

195 degrees ... particularly suitable for ND-U Class As which, by  
rule, must operate with 50 kW, and any taller than this height should  
be sectionalized; suitable for lower-powered stations, where the  
budget allows

Where superior performance is required, a sectionalized radiator can  
be evaluated.

180 degrees over 180 degrees, the only true Franklin, was and is  
successful on 1500 and 1530; this is the highest efficiency in  
present use, about 510 mV/m/kW at 1 km, or the equivalent of about  
100 kW out for 50 kW in, when referenced to the minimum conforming  
radiator for the class, 362.10 mV/m/kW at 1 km

180 degrees over 120 degrees was and is successful on 1040

120 degrees over 120 degrees was successful on 1200, with two  
examples having been constructed at different sites; this  
configuration has the least high-angle radiation of those  
sectionalized radiators which were mentioned

Phoenix's 1230 and 1400 certainly benefit from a tall height.  
Particularly 1400 as it has a ground system sized for 550.