[Towertalk] Re: Phasing Lines For Stacks

[Bill Tippett]

SM2EKM wrote:
>eeee hmmm, guys dont kill me now but to be in phase, and I
guess thatīs what you want, the feedlines has to be EXACTLY
the same lenght.

Hi Jim!

        This bothered me a little too but I believe I have the
answer.  Consider a CW signal split to 2 phase lines, 1 wl to
antenna A and 2 wl to antenna B.  Under steady-state, both
signals are in phase because phase repeats every 360 degrees.
However, they are time-shifted by one period (35 nanoseconds
at 28.4 MHz).  This means antenna A radiates the signal
by itself for the first period, and antenna B radiates the
signal by itself for the last period.  For all periods between,
the signal is the combination and the result is "in-phase".
Since we are talking about a CW, there is no distortion caused
by combining two time-shifted CW signals, and I doubt we would
ever notice the first or last 35 nanosecond periods when the
signals from the two antennas are separated.  

        For SSB, it is a little more complex since the signal
is time-varying with the modulation.  However, since the maximum
modulation bandwidth for SSB is around 3 kHz (333 microsecond
period), any distortion from combining signals shifted by one
period (35 nanoseconds) should not be noticeable to humans.

        EZNEC verifies that there is no difference in far-field
patterns between phasing of 0/0/0 and 0/360/720 degrees for the
3 antennas in my stack, but I am sure EZNEC assumes steady-state
conditions, and not the case of the first and last signal periods
covered above.  Hope my simple-minded explanation helps!

                                         73,  Bill  W4ZV