[BC] Class "D" AM Towers

[Alan Gearing]

Scott:

Per the FCC's Figure 8, a 0.15 wavelength tower (54 electrical degrees) 
will produce the minimum required radiation efficiency (281 mV/m) for a 
Class B and/or Class D station.  (Assuming a full quarterwave ground 
system.)  At 1560 kHz this equates to a 95' tower.  WRKM's tower 
(actually 115') equates to 56.8 electrical degrees and so meets the 
current minimum efficiency standard.  The Commission will allow 
specification of shorter towers - up to a point, but will require a 
field proof to verify that the minimum efficiency requirements are met.  
It is also possible to top-load a shorter tower (again up to a point) to 
provide a longer effective electrical length to meet the FCC's specified 
minimums.

There is some movement afoot to request the FCC to eliminate the minimum 
radiation efficiencies in the AM rules, but this is probably some ways 
off from being implemented.

Our in-house CDBS interface is not set up to search based on tower 
heights.  However, if I get some extra time I'll go into the SQL 
database engine directly and see what I find in the way of short towers 
for Class D (or Class B) stations.

Regards,

Al Gearing

Scott Bailey wrote:
> Jerry,
>     I think you got my thread all wrong. This is just a thought that I had "if"
> my tower fell and I had to fight the city and the county. Of course, I can apply
> for a wavier from the commission to operate at a lower field strength.
>    
........................................
>     I just wanted to know how short anybody has seen a "CLASS D" (not a Class C)
> tower. At 1560, I can go to 100' and be happy. I have enough property for a
> ground system. I have bought new copper, and we will lay it as soon as the
> weather gets better. Huh.....I should have been a Ham Operator. My station is
> just a labor of love, and not to be used as a big cash register.
>
>     I've seen all kind of things done to achieve a 241 mv/m to make a Class C
> work, and the FCC should allow little Class D's like mine lower our minimum
> efficiency to 241 mv/m, as long as we are covering the COL.
>
>     WRKM in Cartridge, TN is on 1350, is a Class "D" Station, and their tower is only 110'. They get out very well with what little property they have. I can hear them in Gallatin.
>
>    
.............................................
> Best,
>     Scott
>    

-- 

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Alan E. Gearing, PE
Mullaney Engineering, Inc.
PH: 301-921-0115
FX: 301-590-9757
email: AEGearing at TZSawyer.com