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Re: On phased arrays with azimuth and elevation beamforming

on 8/29/00 8:18 AM, Bob Bruninga at bruninga@nadn.navy.mil wrote:

> I too have dreamed of this capability.  But it all boils down to having an
> electronic module that can change the phase of the RF signal going to each
> antenna.  So far, I have not found anything practical that we can afford.

Actually, Bob, a mixer circuit specially tuned can act as a phase shifter.
Look in the Mini-Circuits catalog and you will see that they sell phase
shifters for a relatively low price.

Another way to do it that is not all that expensive is to lay out an easy
circuit using the following components:

1 90 degree Hybrid Power Splitter (Mini-Circuits sells these too)
2 Varactor diodes (variable capacitance diodes)

The Hybrid has an input port, a 0 degree output, a 90 degree output and
what's call in "isolation" port.  In normal use, the isolation port is
terminated in 50 Ohms and you get a split signal at the outputs with 90
degrees out of phase.

In our application though, the input port remains the input, the isolation
port becomes a single output port and the 0 and 90 degree ports each have a
varactor tied between them and ground.  Now, you'll need to develop the bias
circuitry for the varactors, but that is simple.  As you vary the bias
voltage on the varactors, you will vary their capacitance.  The varying
capacitance will cause a phase shift of the signal from the input to the
output.  This works quite well, but is only for low power use unless you
have some high power varactors (rare) and a high power splitter.

For higher power signals at microwave frequencies, it is a trivial thing to
build a 90 degree quad hybrid on a circuit board.  You could do the same
thing I suggested above, but this time, you could use some high quality
variable capacitors that could handle the higher power instead of the
varactor diodes.  Yes, it would be a more mechanical than electrical
adjustment, but it would work nonetheless.  The tough part may be to find
high power caps that work at microwave frequencies.

As you go higher up to like 10 GHz and so forth, it becomes even easier as
you can easily use a line stretcher to adjust the phase of your signal.

Lastly, if you really want to get down and dirty and be able to do phase
shifting at lower frequencies, you could get two multi position RF switches.
Then you could switch in different lengths of coax that would give varying
(but discrete) values of phase shift.  The most expensive part would be the
relays, but you can find them at hamfests if you look hard.

So there's some practical ideas for making a farily cheap phase shifter.



Jon Ogden
NA9D (ex: KE9NA)



"A life lived in fear is a life half lived."

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