Topband: Chokes-Ferrite vs. air

[W9UCW@aol.com]

Guessing at the number of turns in an air wound choke isn't necessary. 
Calculating the size is easy. Whether a choke is wire-wound (cored or air), 
coaxial wound, or "beaded", it should have a reactance of about 4 times the 
resistance (or impedance) of the load, at the lowest frequency of use. So, 
for 50 ohms we need a choke reactance of about 200 ohms. For 160, using the 
formula for inductance;
                                             X         
                       L(uh) = 2xPIx f(mhz)
         
              where L= inductance
                         X=reactance (4x50=200 ohms)
                         f = 1.8mhz
                                     200         
               so,   L=  6.28x 1.8             = 17.68uh

I usually round off to 20uh for 160 and solve for number of turns transposing 
the inductance formula for single layer coils.
                                  RxRxNxN   
                       L(uf)=9R + 10 l

                    where L=inductance(20uf)
                               R=coil radius
                                N=number of turns
                               l=length of coil

Considering the "migration" of the center conductor of coax in an arc, one 
should use the solid dielectric type of coax and keep the diameter 
sufficiently large.
EXAMPLES:
For RG8 (213, etc) 17T on 5" dia, 9.5" long= 19.59uf
ForRG58 types,   32T on 2.5" dia., 7.1"long= 19.45uf
For rrRG174        50T on 1.5" dia., 5.5" long=22.8uh
These will produce very "clean" choke action on 160. Generally, they will be 
effective through 30mhz, although turn-to-turn capacitance may cause some 
leakage at the highest part of the range.   73's, Barry



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