The AM Forum

I have constructed a pair of verticals for 75 meters and will feed them with equal lengths of 52 ohm coax.  I am trying to calculate the length of coax to insert on one side to change from broadside fire to end fire (90 degrees)  I am using  rg58
foam coax for the extra coax to be inserted with a relay.  I have calculated it several times and keep getting a different answer each time. Can someone give me the answer, I can't seem to make it come out.  The antennas are 1/2 wave apart and the velocity factor on the coax is .66. ???

The constant for 1/4 wavelength in free space is 246.  Divide your frequency in Mhz into 246.  This is your 1/4 wavelength in space in feet.  Then multiply that footage by the V-of-P which is .66 for solid dielectric coax.

I fear you may not be able to get the pattern as deep or controllable with that spacing.  1/4 wave is more efficient in that type of array.  1/2 wave spacing will give somewhat opposite effects because of the phase relationship (90 deg vs 180 deg).

However, it all will work so if you figure it as using a bit of extra coax the same as the distance between the two antennas, you will have what you want.

You can also use a simple phase shift network at the antenna base, switched in and out to achieve the same thing.  Since you will probably have an impedance matching network at each one already (unless you are shunt feeding them), the only real concern would be the room in the network enclosure.

73DG