The AM Forum

I'm constructing phased dipoles spaced approximately 1/2 wavelength apart driven by separate amplifiers.  Yes, that's right, each has its own amplifier.  The distance is approximate, the trees are located where the trees are located. 

Instead of the usual appropriate lengths of coax, why not an adjustable phase shifter on the input of the amplifiers?  Lewallen recommends a 90 degree phase shifting L network feeding 1/4 wavelength feed lines in his section on phased verticals in the ARRL Antenna book.  I would like to make it adjustable.   Just turn the knob(s) until my trusty assistant positioned 10 wavelengths or more away tells me I've reached max.   

Is this just an adjustable L network?  Can someone point me to a reference?

Jon

 

I wouldn't use a single l network to get 90 degrees because the network will have high loss. Someone correct me but isn't 90 degrees the 3 dB point of the network.

That wouldn't be good.   This is the input side of the amplifiers but I don't have twice the power I need to drive them.   

Jon

Jon,
One easy way to accomplish this is:

Antenna one is fed with 1/2 wavelength feedline.
Antenna 2 is fed with a 1/4 section. This goes to a relay box where you switch in/out a
1/2 wave section.
This will cause antenna 2 to be + or - 90 degrees.
Direction will be off the antenna at -90 degrees.

I like Buddly's idea, but if that isn't the way you want to go, use a T-network vs. an L- network. An L-network is dependant upon the resistances to be matched. Q and phase shift are determined soley by the ratio of resistances. A T-network is indepentand of these variables. Thats why they are used in broadcash array's. Don't forget to factor the coax lenghts as well if they are different. They will introduce phase shift as well.

Mike, 2ZE