Ground gain on real ground
 

I am simulating on EZNEC an antenna yielding a gain of about 13.5 dB (at zero
deg. elevation) when in free space.

The same antenna, placed on an ideal ground, shows a gain of about 18.8 dB (at
10 deg. elevation), thanks to the so-called "ground gain".

If the antenna is placed on a non-ideal ground (according to EZNEC modelling),
its gain becomes 18.4 dB.

I am surprised of the small gain difference between the ideal ground case and
the non-ideal ground case (just 0.4 dB). As a matter of fact, while the ideal
ground may be regarded as a perfectly conductive plane, the non-ideal ground is
something having a significant ohmic resistance. I would had thought that most
of the RF power incident on a plane having a significant resistance would be
absorbed (and hence not reflected).

Any comment please?

73

Tony I0JX

Ground gain on real ground
 

Antonio Vernucci wrote:
I am simulating on EZNEC an antenna yielding a gain of about 13.5 dB (at
zero deg. elevation) when in free space.

The same antenna, placed on an ideal ground, shows a gain of about 18.8
dB (at 10 deg. elevation), thanks to the so-called "ground gain".

If the antenna is placed on a non-ideal ground (according to EZNEC
modelling), its gain becomes 18.4 dB.

I am surprised of the small gain difference between the ideal ground
case and the non-ideal ground case (just 0.4 dB). As a matter of fact,
while the ideal ground may be regarded as a perfectly conductive plane,
the non-ideal ground is something having a significant ohmic resistance.
I would had thought that most of the RF power incident on a plane having
a significant resistance would be absorbed (and hence not reflected).

Any comment please?

You must be modeling a horizontally polarized antenna. The attenuation
caused by ground reflection is quite different for horizontally and
vertically polarized fields. When the field is horizontally polarized,
nearly any reasonable ground gives a near-perfect reflection at low
elevation angles. Only at high elevation angles will you see much
difference caused by ground quality.

Unfortunately for people using vertical antennas, the story is quite
different when the field is vertically polarized.

There's a good discussion of the effect of imperfect ground and
derivations of the ground reflection coefficients in any edition of
Kraus' _Antennas_.

Roy Lewallen, W7EL

Ground gain on real ground
 

When the field is horizontally polarized, nearly any reasonable ground gives a
near-perfect reflection at low elevation angles. Only at high elevation angles
will you see much difference caused by ground quality.

Good to know. I would have otherwise thought that even sea water is not a good
reflective plane, its resistance not being very low.

73

Tony I0JX

Ground gain on real ground
 

Antonio Vernucci wrote:
[I wrote:]
When the field is horizontally polarized, nearly any reasonable ground
gives a near-perfect reflection at low elevation angles. Only at high
elevation angles will you see much difference caused by ground quality.

Good to know. I would have otherwise thought that even sea water is not
a good reflective plane, its resistance not being very low.

The resistivity of sea water is high compared to metal, but very low
compared to air. But what counts is its intrinsic impedance compared to
that of free space. At 10 MHz, the magnitude of salt water's intrinsic
impedance is about 4 ohms, compared to the 377 ohms of free space. So it
does a good job of reflecting waves when air is at the other side of the
interface.

EZNEC will tell you the intrinsic impedance and other information about
any Real ground if you select Ground Info from the Utilities menu in the
main window.

Roy Lewallen, W7EL

Ground gain on real ground
 

The resistivity of sea water is high compared to metal, but very low
compared to air. But what counts is its intrinsic impedance compared to
that of free space. At 10 MHz, the magnitude of salt water's intrinsic
impedance is about 4 ohms, compared to the 377 ohms of free space. So it
does a good job of reflecting waves when air is at the other side of the
interface.

EZNEC will tell you the intrinsic impedance and other information about
any Real ground if you select Ground Info from the Utilities menu in the
main window.

Thanks Roy, very useful advice.

Tony I0JX