RE: NEC-LIST:Using Current Sources

> -----Original Message-----
> From: Tim Brown [mailto:t.brown_at_eim.surrey.ac.uk]
> Sent: Monday, July 29, 2002 11:26 AM
> To: nec-list_at_gweep.ca
> Subject: NEC-LIST:Using Current Sources
>
>
> Dear All,
>
> I was wondering if any of you out there would expect differences from
> using current sources in NEC instead of the conventional voltage
> sources. Theoretically there should be no difference since the voltage
> source with zero impedance in series with an element is
> exactly the same
> as a current source in parallel with an element with infinite
> impedance.
>

Incorrect. A voltage source has a zero source impedance while the current
source has an infinite source impedance. For example, a voltage source
cannot feed a short, and a current source cannot feed an open.

> I've been simulating an 8 element dipole array with two rows
> of 4 elements
> like the plan view below (so the dipoles are coming out of the
> email!). The bottom row of elements are in quadrature with
> the top row.
>
> X X X X (This row in zero phase)
> X X X X (This row 90 degree phase)
>
> Now, if this simulation is carried out with current sources, we get a
> contradiction with the voltage sources and there is no real
> valid reason
> why we should. The measurements support the voltage source
> simulations as
> I'd expect them to. Does anyone else have any light on this?
> Or are the
> current sources actually feeding the elements as they should
> be? Thanks,

        If you have two (or more sources) in a linear circuit, by the
principle of superposition, you can analyse the circuit by turning off all
sources but one. You find the voltages and currents for that source. You
then repeat for each sources individualy. At the end, you add up all
voltages and currents to find the final solution. When you turn off a
source, the voltage source becomes a short while the current source becomes
an open. Obviously the two sources are not equivalent and chosing one source
over the other could change your circuit dramatically.
        I'm not sure about the details of your measurements but here is a
possible explanation for your observations. In your measurements, you
couldn't have an ideal voltage or current source since all sources have an
internal impedance. A high-impedance source (high wrt the load impedance)
resembles a current source while a low-impedance (wrt load impedance) source
is more like a voltage source. Therefore, if your array impedances are high
wrt to your sources impedances, your voltage source simulations would give
better agreement. In general, you'd want your simulations and measurements
to have same source impedances and then, you woudn't have to worry about
what source type to use.

Gilbert Morin

> Tim.
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