[Antennas] DCTL Antenna Question

2004年2月27日 19:15:17 -0800

At 04:28 PM 2004年02月27日, Durwydd MacTara wrote:
>Why? Since the Velocity factor and capacitances are different for the two 
>feedlines, I figured the formulae would also differ, especially for a 
>resonant antenna...

Harvey, have you (or WA6QBU or others) built a DCTL using 450 ohm line for 
the antenna portion? Since this antenna is a capacitively loaded 
quarter-wavelength dipole wrapped into a loop (with added capacitive 
end-to-end loading), I'd also expect that having a different capacitance 
per foot with 450 line might have some effect on the resonance.
The antenna and the open stub capacitance will be about 24% less for the 
same physical lengths (assuming 300 ohm VF=0.8 and 450 ohm VF=0.91). Also, 
the shorted stub will probably have about 32% higher inductance using a 450 
ohm line with the same physical length.
For certain, the SWR on the feedline isn't too important, but it might 
affect the ability to match the line if the antenna matching unit has a 
limited range (such as the built-in tuners in many transceivers).
73, Terry
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