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A method for bringing two Dissimilar Beams
into phase 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 oscilloscope.

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 in town.

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 be used.

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 light.

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 34.63 = 1.82 feet 

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.