A method for bringing two Dissimilar Beams
with each other using an Oscilloscope
Set up: Equal lengths of feed-lines from both driven elements should be
constructed such that they can both reach the inputs of a dual trace
A means of a signal source that is a mile or so away that is in direct line
of sight of both antennas and located at about mid height between the two
antennas. A mobile rig with a horizontal dipole can serve, or another hams QTH
Procedure: Have your friend place a carrier on one of the bands of interest
and point your stack at this signal source. The dual trace scope should pick up
the two signals from the beams at this point. You can adjust the scope patterns
to lie on top of each other or use the time measurement cursors to measure the
time interval between the two peaks of the sine waves. This is the time
misalignment that can be adjusted out to bring the two signals into direct
alignment with each other.
By observation of the two signals you can determine which signal should be
delayed to bring them into alignment. Its sometimes easier to figure the number
of degrees you need to delay. A full sine wave is 360 degrees, a half is 180
degrees and from zero crossing to a peak is 90 degrees, etc.
If the signals are exactly 180 degrees out of phase with each other, you may
simply reverse the balun or coax connection to one of the beams. If one is say
leading by 45 degrees in time you can simply build a delay line with coax to and
add it to the coax of the "leading wave-form" Yagi.
Lets do an example: A 10m signal is used, but any frequency for the beams can
With the above set up it is found that a difference of 30 degrees if found
from the lower beam (leading) and the upper beam (lagging). The lower beam is receiving the signal before the upper beams driven element
receives the wave. A coax delay line must be constructed to delay the lower beam
by 30 degrees at 10 meters. A one wavelength section of coax at 10 meters is 360
degrees long. There is also a velocity factor (VF) parameter that must be used
to calculate the physical length of coax we need. If we use RG213 by looking at
the charts in the handbook we see that the velocity factor is .66 the speed of
The Length we need is 30 degrees So we need to cut a piece of coax that is:
L = 30/360 X VF X WL (wavelength)
L = .08 X .66 X (983.6/28.4Mhz) = .08 X .66 X
This is the
length of coax we need to add.
L = length in feet
VF = Velocity factor of our coax
WL= Wavelength of the signal in space or 983.6 ft/ frequency
Adding this additional length to the leading
antennas feed line will bring the oscilloscopes patterns into alignment.