RE: Re: NEC-LIST:Proper Antenna Patterns

Chuck, et al:

If an antenna in the real world is mismatched at the feed, this can cause
currents to flow on the outside of the shield of the coax from the source/
sink (transmitter or receiver) to the antenna. These extra currents (which are
usually not modeled in NEC) do contribute to the pattern measured. I measured
the pattern impact of the currents on the outside of a coax feeding a
half-wave dipole (i.e., single feedpoint or port) using the XELEDOP full-scale
pattern measurement system developed by SRI (then Stanford Research Institute)
back in the 1960s. It took me some minor sleuthing to see what was causing the
pattern perturbations I had measured, but I confirmed my suspicions that it
was feedline radiation by installing a balun and watching the effect go away!

So I think we need to be careful about generalizations which may apply to the
model, and its assumptions, but which may not apply in a real-world
implementation.

George

Original Message:
-----------------
From: Chuck Counselman ccc_at_space.mit.edu
Date: Thu, 11 Oct 2001 11:42:53 -0400
To: nec-list_at_gweep.ca
Subject: Re: NEC-LIST:Proper Antenna Patterns

At 8:36 AM -0500 10/11/01, KLysiak wrote:
>The complex (real and imaginary parts or amplitude and phase) antenna
>patterns for transmit and receive antennas change with termination
>impedance.

If the antenna in question has one single feedpoint, or "port," then
neither the transmit nor the receive pattern of the antenna depends
on the impedance seen by the antenna looking through this port to a
source or load. By this statement I mean that the
_angular_dependence_ of the complex amplitude of the transmitted or
received signal is independent of the source or load impedance. Of
course it remains true that impedance mismatch between the
source/load and the antenna will affect the signal transmitted or
received, but the latter effect is purely multiplicative, and
independent of direction.

Perhaps you are thinking of the problem of an array antenna having a
plurality of feedpoints. In this case the pattern _can_ depend on
how the feedpoints are excited, because the current or voltage at one
feedpoint depends on the the current or voltage at another.

>Normally NEC uses an ideal conjugate matched source on transmit
>antennas.

NEC uses what the user tells it to use (with certain degenerate
exceptions; e.g., I've discovered that NEC-4 does not use a voltage
source of zero voltage if that's what you specify; instead it uses
1+j0 volts. With these exceptions, if you specify (say) a voltage
source with zero series impedance, that's what you get. You do not
get an "ideal conjugate matched source" unless that's what you
specify via EX and LD cards/commands or equivalents.

>If the antenna is fed with something other than an ideal conjugate
>load (i.e. 50 ohm coax) its pattern will change.

Untrue.

Don't take my word for it. Do the experiment. Run NEC and look at
the results.

>How can this be properly modeled in NEC? In particular, if we want to model
>a LPDA that has internal transmission lines which we want to feed with a 50
>ohm source. How do we a 50 ohm source to a segment that has an internal
>transmission line on it?

Use an LD command to put a 50-ohm resistance in series with a voltage
source in the relevant segment.

-Chuck.

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