Topband: Fw: Re: : antenna question ...this is not going to dieqiuite yet....................

[Tom W8JI]

> 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