Topband: modeling question

[i4jmy@iol.it]

Hi Ricky,

in the specific case, a choke balun used with a yagi hasn't enough 
inductive reactance to isolate the dipole from the line and its 
presence is almost ininfluent.
The coax is infact at a close potential with the boom when the 
structure is used as shunt feed vertical and the line is installed as 
explained in the previous mail.
Not only the frequency is lower and the choke inductive reactance is 
less, but an high voltage point is also an high impedance point where 
only an extremely high reactance (a resonance in practice) could really 
isolate the dipole from the line.
A choke conveniently works with yagis because the fed is low impedance 
(hi current/low voltage) but resonant traps and not chokes are needed 
to isolate antenna portions at voltage points. 
A choke for higher bands can't have enough inductance and the stray 
capacitance to be parallel resonant on a low band like 160m therefore 
the voltage developed across its ends is not so high.

73,
Mauri I4JMY





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