RE: NEC-LIST: Image Plane Log Periodic Array

Hi David,

You can simulate a finite ground plane using a wire mesh to represent the
surface. It works reasonably well to model dipoles and quadrafilars mounted
over a conductive plate on a surface of a satellite.

It works fairly well for simulating a single transmission frequency, but not
so well for simulating the interaction of various transmitters (passive
intermodulation effects, active intermodulation in amplifier stages, etc.)
because real (closed) surfaces conduct surface currents that create magnetic
fields.

The magnetic fields create spatial biases that may distort the
directionality of the log periodic antenna as the transmitted/received
signal varies in frequency due to modulation. That is, the antenna pattern
remains "the same" but the axis of the mainlobe may vary spatially according
to frequency.

To put it in a nutshell, if the Tx/Rx frequency varies by more than a few
percent of the bandwidth of the appropriate passbands of the log-periodic
antenna, time-varying frequency may produce a time-variation in the mainlobe
axis that is difficult to detect with a time-invariant modeling tool, such
as NEC.

Just my $0.02.

Michael Pender

-----Original Message-----
From: nec-list-admin_at_gweep.ca [mailto:nec-list-admin_at_gweep.ca]On Behalf
Of David de Haaij
Sent: Saturday, March 29, 2003 1:25 AM
To: NEC-List_at_gweep.ca
Subject: NEC-LIST: Image Plane Log Periodic Array

Morning all,

We are building an image plane log P (Log Periodic Monopole Array or
LPMA in short) on a finite ground plane. I am trying to construct a
model in NEC2, but I am currently unable to implement a finite size
ground plane. Has anybody done something similar in the past? If so,
what did you do to make it work?

Regards,
David
Grintek Antennas
South Africa

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