NEC-LIST:RE: Proper Antenna Patterns

Jerry,
        Thanks for correcting me in the NEC input impedance. This clarifies part of
my misunderstanding.

-----Original Message-----
From: Gerald Burke [mailto:burke2_at_llnl.gov]
Sent: Thursday, October 11, 2001 11:48 AM
To: KLysiak
Subject: Re: Proper Antenna Patterns

>Gentlemen,
> The complex (real and imaginary parts or amplitude and phase) antenna
>patterns for transmit and receive antennas change with termination
>impedance. Normally NEC uses an ideal conjugate matched source on transmit
>antennas. If the antenna is fed with something other than an ideal
conjugate
>load (i.e. 50 ohm coax) its pattern will change.
> 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?
>
>Keith A. Lysiak, Ph.D.
>Southwest Research Institute

Keith,

By default NEC uses a zero impedance source. If it was conjugate
matched then half of the power would be dissipated in the source
load. With a zero impedance source all of the power goes into the
antenna, no mater what its impedance is. If you want to model a 50
ohm source, you can put a 50 ohm load on the source segment, in
series with the voltage source. The pattern will only change by a
constant factor in amplitude and phase if there is a single feed
point. The amplitude change will be reflected in the gain values.
Phase is usually not of interest, but maybe it is in your direction
finding applications.

Jerry Burke
LLNL

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