Hi Richard,

On 8 jun, 13:41, "Richard Fry" wrote:
One exception being the typical MW broadcast monopole vertical used with
120 buried radials, each 1/4-wave or more in length. This configuration
produces its maximum radiation in the horizontal plane (ie, zero degrees
elevation). If this was not true, then AM broadcast stations would have
very few daytime listeners.

Day-time listening does not require maximum radiation at 0 deg.
elevation, but does require radiation at 0 deg elevation. So at that
point I thing you should reconsider your statement.

The ground wave fields 0.3 mile from such antenna systems have been
accurately measured as far back as 1937 by Brown, Lewis and Epstein of RCA
Labs, and for a vertical radiator of 60 to 90 degrees in height shown to be
within a few percent of the peak, free-space field produced by a 1/2-wave
dipole, at the same tx power. The frequency used in this set of tests was 3
MHz.

I especially mentioned "short wave communication" and "average soil"
to exclude the AM an LW broadcast propagation. In that case the pseudo
Brewster angle can be that low, that (near) fields form the ground
current do not cancel the direct field from the vertical radiator.

Ground wave propagation loss including earth curvature will cause the
h-plane field from these verticals to go to zero beyond some distance* from
the transmit antenna site, but that does not mean that zero h-plane field
was "launched" by this vertical radiator in the first place.

*and that distance can be over 200 miles for a high power AM station on a
low frequency

I agree that vary close to the transmitter (you are in the transition
field zone), the field strength will behave as over a perfect
conducting ground plane.

There is another sign that even an AM broadcast station does not have
maximum radiation at 0 deg. elevation. When you check your field
strength (E or H) graphs up to 60 miles (where earth curvature can be
neglected) you will notice that the field strength falls of faster
then with 1/distance. That would not be the case when max radiation is
at 0 deg. elevation.

My information shows 20 dB below 1/r (1 MHz, 100km, "good ground"),
41dB below 1/r (1 MHz, 100km, "good ground"), source: "The services
textbook of radio, 1958, volume 5 page 442. For average soil, and
especially rocky like ground, the field strength will be significantly
lower.

Probably your material does also treat "height gain factors" Then you
will see that above a certain height, field strength increases with
increasing height (indication of more gain under that elevation).

Best regards,

Wim
PA3DJS

"Wimpie" wrote
There is another sign that even an AM broadcast station does not have
maximum radiation at 0 deg. elevation. When you check your field
strength (E or H) graphs up to 60 miles (where earth curvature can be
neglected) you will notice that the field strength falls of faster
then with 1/distance. That would not be the case when max radiation is
at 0 deg. elevation.
_____________

No matter the relative value of its h-plane radiation, the resulting ground
wave field from an AM broadcast station is subject to losses not only by
1/distance, but also as a function of earth conductivity and the frequency.
Only for free space paths does the field decay by 1/distance, alone.

If the h-plane ERP of an AM monopole did not match the peak ERP of a
1/2-wave dipole when driven with the same tx power, then the monopole could
not generate the same field as the peak, free-space field of that dipole,
when the monopole is measured in its far field, but close enough to it for
groundwave propagation loss due to the conductivity of the path to be small.

This was proven in the Brown, Lewis and Epstein study in 1937, where at 0.3
miles for 60-90 degree verticals they measured an equivalent field strength
of better than 190 mV/m at 1 mile for 1 kW of radiated power. The peak,
free-space field from a 1/2-wave dipole for those conditions is about 195
mV/m.

RF

Hi Richard,

My previous posting didn't reach the group, so I made a new one.
On 8 jun, 16:25, "Richard Fry" wrote:
[deleted}

No matter the relative value of its h-plane radiation, the resulting ground
wave field from an AM broadcast station is subject to losses not only by
1/distance, but also as a function of earth conductivity and the frequency.
Only for free space paths does the field decay by 1/distance, alone.

I fully agree with this. It is the reason that an AM broadcast
transmitter has its main lobe (Take of Angle) at non zero elevation.
When you would measure the field strength at, for example, 30 degr
elevation, you would find the 1/distance decay.

When you would measure the field strength of an AM broadcast
transmitter over seawater, the 1/distance decay holds to about 100km.
Above that you have to correct for earth curvature.

If the h-plane ERP of an AM monopole did not match the peak ERP of a
1/2-wave dipole when driven with the same tx power, then the monopole could
not generate the same field as the peak, free-space field of that dipole,
when the monopole is measured in its far field, but close enough to it for
groundwave propagation loss due to the conductivity of the path to be small.

You should distinguish between transition region and far field
(Fraunhofer) region. Close to the transmitter the field generated by
the ground current coincide with the field from the vertical radiator
therefore 3 dB gain increase in field strength occurs. Further away
from the antenna, both amplitude and phase of ground current will
change with respect to the field from the radiator, partly destructive
interference occurs.

Only a surface with infinite conductivity will support the surface
wave up to infinity, hence giving main lobe at zero elevation (for
vertical radiators up to 5/8 lambda). Even a vertical radiator (for
example 1/2 wave dipole) not touching ground will show max radiation
at zero elevation for perfect conducting ground plane (there is no
(pseudo)Brewster angle for surface with infinite conductivity).

This was proven in the Brown, Lewis and Epstein study in 1937, where at 0.3
miles for 60-90 degree verticals they measured an equivalent field strength
of better than 190 mV/m at 1 mile for 1 kW of radiated power. The peak,
free-space field from a 1/2-wave dipole for those conditions is about 195
mV/m.

The objective of the Brown, Lewis and Epstein study was to get as much
as power radiated instead of dissipated in the ground, especially with
relative small antennas (financial aspect). They did not investigate
Take of Angle.

BTW, they did a good job as the field strength from a 1/2-wave dipole,
free space, 1kW input is about 138mV/m (rms) at 1 mile. Their
measurements show 190mV/m (almost 3 dB gain).

If you have access to a full wave EM simulator that supports
dielectric layers, you could run a simulation of the far field pattern
for a vertical radiator.

Best regards,

Wim
PA3DJS

"Wimpie" wrote
When you would measure the field strength of an AM broadcast
transmitter over seawater, the 1/distance decay holds to about 100km.
Above that you have to correct for earth curvature.

This is approximately, but not precisely true, as the conductivity of sea
water is not perfect. Divergence from the inverse distance field is present
at distances up to 100 km, as well.

You should distinguish between transition region and far field
(Fraunhofer) region. Close to the transmitter the field generated by
the ground current coincide with the field from the vertical radiator
therefore 3 dB gain increase in field strength occurs. Further away
from the antenna, both amplitude and phase of ground current will
change with respect to the field from the radiator, partly destructive
interference occurs.

I did state that BL&E's measurements were made in the far field. The
longest radiator length in their study was 90 feet, and the test frequency
was 3 MHz. Using the equation commonly accepted for the transition to the
far field zone (D = 2*L^2/lambda), D = 49.4 feet for these conditions. BL&E
took their measurements at 0.3 miles (1,584 feet).

Only a surface with infinite conductivity will support the surface
wave up to infinity, hence giving main lobe at zero elevation (for
vertical radiators up to 5/8 lambda).

This is a common belief based on a mathematical analysis for an infinite
distance from the radiator. But the main lobe _as launched_ from a vertical
monopole up to 5/8-wave in height, and with its base on the earth _always_
is directed in the horizontal plane, regardless of the r-f loss in the
ground system used with the vertical, or earth conductivity in the near
vicinity of the site.

The BL&E study was done in the sandy soil of New Jersey (about 4 mS/m), yet
for a 1/4-wave radiator with 113 0.41-wave radials they measured fields at
0.3 miles that were within a few percent of their theoretical maximum for a
perfect 1/4-wave monopole radiator over a perfect ground plane.

A calculation of the elevation field at an infinite distance over other than
a perfect, infinite ground plane shows zero field in the horizontal plane,
and a peak in relative field at some positive elevation angle. That is the
field that survives to infinity, but does not mean that this is the shape of
the field that is radiated by the monopole in the first place, and that
exists at far-field distances closer to the radiator. Closer to the
radiator, the h-plane field is not zero -- which in fact is the basis for
the daytime coverage of all MW broadcast stations.

BTW, they did a good job as the field strength from a 1/2-wave dipole,
free space, 1kW input is about 138mV/m (rms) at 1 mile. Their
measurements show 190mV/m (almost 3 dB gain).

That is due to the fact that radiation from a 1/4-wave monopole with its
base on the earth is restricted to one hemisphere, which for a perfect
ground plane produces 3 dB gain over a 1/2-wave dipole in free space. To
account for this, multiply your 138 mV/m by SQRT(2). This produces the
same 195 mV/m value that I posted earlier as the theoretical, h-plane
maximum for a 1/4-wave monopole for these conditions.

RF