Hi David,
You cannot ignore the ground plane at all. It would be better to think
of the transmit antenna as two separate antennas at 1m above and, 1m
below, the level of the ground plane and then you can ignore it to some
extent. You will always see the combination of these two sources.
Simply put a horizontally polarized wave will be shorted out by the
ground plane (i.e. phase reversal resulting in zero field at the plane).
Where the receive antenna picks up the most signal will depend upon the
frequency. At low frequencies this will be a maximum well above the
ground plane. The angle of the main lobe will vary with frequency, as
will the number of lobes.
To consider the transmit antenna alone, you have an array of two
dipoles, space 2m apart. You can probably find many examples of antenna
patterns for this case, but realize that the electrical spacing (i.e. in
terms of wavelengths) is varying with frequency.
There are other effects to consider also, such as the change in
impedance, hence change in the antenna match and hence in the radiated
power. This impedance change is due to the coupling of the antenna with
the ground plane (its image). This affects the receive antenna, equally
if it is at the same height, but this is a height dependent effect.
This is not a simple problem and is the reason that the original FCC
site attenuation curves deviated from theory below about 80MHz as these
effects are then dominant. This gets more confusing with more complex
antennas too. This is just the 30 second snap shot it hopefully it will
give you some insight to the effect of the ground plane.
Regards,
Colin..
-----Original Message-----
From: Spencer, David H [mailto:david.spen...@usa.xerox.com]
Sent: Wednesday, October 17, 2001 9:09 AM
To: EMC-PSTC Internet Forum
Subject: Horizontal Antenna
All,
I have a question regarding a horizontal transmit antenna over a ground
plane. My questions implies a dipole.
The questions:
What impact does the ground plane have on a horizontally transmitted
signal?
If the transmit antenna is 1 meter above the ground plane is the only
signal received by a second antenna, say 10 meters away, the direct
wave?
This assumes no other reflecting surfaces within the area.
If there is a reflected signal from the ground plane is does that signal
incur a phase reversal?
Under what conditions do phase reversals of a reflected signal occur?
Finally, what is the angle of the direct wave (main lobe) from a
horizontally polarized antenna? . And, assuming there is a second lobe
at 1
meter, what angle would that second lobe be at, assuming horizontal
polarity?
Thanks for any info
Regards,
David Spencer
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