This may improve the SWR somewhat in a 300 ohm system, but
if you are using a tuner to match 50 ohms to 300 ohms, then you
won't seem any noticable improvement. (The tuner setting will
compensate for any slight mismatch due to the velocity factor.)
I guess if one was to use a balan of some sort to have the end to the
transmitter be 50 ohms, this could of of some importance
Hi Mike,
People are just guessing or speculating, even if somewhat popular. There is
little to no stub effect at work.
Velocity factor has an effect when the wave in the twin lead is a TEM wave
(transverse electromagnetic wave).
Currents on the two parallel conductors are in phase at every point, and
virtually equal. Voltage difference, which creates the electric field
between conductors, is zero at the ends (they are shorted) and very small at
the feedpoint.
There just isn't much effect at all, although by moving the short inwards
you can throw the antenna into a transmission line mode where the twinlead
does have a stub effect.
You can do this test to confirm it.
1.) Connect both wires in parallel on each side of the feedpoint (short them
at each end) and feed it like a dipole. Find the resonant frequency.
2.) Reconnect, with no other changes, as a folded dipole and find the
resonant frequency. It will be virtually the same, and the impedance will
simply be 4x the dipole impedance.
Now if you move the short in closer and closer to the center, the stub
effect becomes larger. Eventually by moving the short inwards enough, you
shift the resonant frequency and change the impedance ratio to a new value.
Worrying about the velocity factor of twinlead in a folded dipole is one of
those things in radio-life that technically do not mean much of anything,
until we start messing the system up.
73 Tom
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