EZNEC Vertical interpretation
 

John Ferrell wrote:
I don't find the Helix feature in my copy of EZNEC+ V4.

You'll find it in the EZNEC manual index under "Helix Creation".
(There's lots of other good information in the manual, too.)

I believe I can get the same results by inserting an appropriate load.
It will just be a little more cumbersome.

It'll be less cumbersome, but you probably won't get the same results.
The amount difference will depend on the geometry of the helix. However,
a lumped load is adequate to illustrate the phenomena you're investigating.

I was not aware that the feeding of the radiator was affected by the
driving source. My "lab models" require a base inductance to get the
radiator into a range I can feed. I have not been including that in
the models.

I added a base inductor of 100uh(r=3) and got the results you
predicted. The 28 foot radiator is now showing -3.75 dbi gain!
Lesson learned!

The loss is due solely to the inductor's resistance -- you'll get the
same result by replacing the inductor with a 3 ohm resistor. The amount
of gain reduction due to the inductor will get worse and worse as the
antenna gets shorter and shorter, for two reasons. The first is that the
antenna's radiation resistance drops as it gets shorter, so the inductor
loss becomes a greater fraction of the total feedpoint resistance. The
second is that you need a larger inductor for a shorter antenna and,
assuming a constant Q, that means more inductor resistance.

. . .

Roy Lewallen, W7EL

EZNEC Vertical interpretation
 

"John Ferrell" wrote
I am attempting to compare the gain of a full size (1/4 wave) vertical
to various shorter verticals at 160 meters.

I am looking at the gain in dbi on the 2D plot. I see 1.39 dbi for the
full 1/4 wave and I see 1.52 dbi for the 28 foot version.

I don't believe the shorter antenna has less gain. Where am I going
wrong?
_____________

Assuming that your vertical is ground-mounted, getting the "right answer"
for this question depends on the quality of your wire models, and especially
your assumptions about the ground plane, and the way your vertical connects
to it.

A classic 1/4-wave vertical monopole working against a very good radial
ground system such as used in AM broadcast should produce a peak gain of
around 5 dBi. The theoretical max for such a system with perfect conditions
is 5.15 dBi.

Electrically short(er), base-insulated vertical monopoles, for a given
frequency, have lower radiation resistance at the feedpoint near the tower
base. So, given the same r-f resistance in the path to the ground plane,
the radiation efficiency of the shorter system will drop.

RF

EZNEC Vertical interpretation
 

Richard Fry wrote:
. . .
A classic 1/4-wave vertical monopole working against a very good radial
ground system such as used in AM broadcast should produce a peak gain of
around 5 dBi. The theoretical max for such a system with perfect
conditions is 5.15 dBi.
. . .

Unfortunately, "perfect conditions" includes a perfectly conducting
ground far beyond the radial field, a luxury neither we nor the
broadcasters have. The maximum far field (sky wave) gain of a ground
mounted quarter wave vertical over average ground, with a completely
lossless ground system, is on the order of 0 dBi, and this occurs at
roughly 25 degrees above the horizon (both depending on frequency as
well as ground characteristics). See the EZNEC example file Vert1.ez as
an illustration. Its MININEC type ground simulates a lossless ground system.

Roy Lewallen, W7EL

EZNEC Vertical interpretation
 

Thank you, I get that!
On Thu, 20 Apr 2006 13:12:20 -0700, Roy Lewallen
wrote:



The loss is due solely to the inductor's resistance -- you'll get the
same result by replacing the inductor with a 3 ohm resistor. The amount
of gain reduction due to the inductor will get worse and worse as the
antenna gets shorter and shorter, for two reasons. The first is that the
antenna's radiation resistance drops as it gets shorter, so the inductor
loss becomes a greater fraction of the total feedpoint resistance. The
second is that you need a larger inductor for a shorter antenna and,
assuming a constant Q, that means more inductor resistance.

. . .

Roy Lewallen, W7EL
John Ferrell W8CCW

EZNEC Vertical interpretation
 

The Vert1 example does illustrate the point well by varying the ground
options.
On Thu, 20 Apr 2006 16:16:21 -0700, Roy Lewallen
wrote:

Richard Fry wrote:
. . .
A classic 1/4-wave vertical monopole working against a very good radial
ground system such as used in AM broadcast should produce a peak gain of
around 5 dBi. The theoretical max for such a system with perfect
conditions is 5.15 dBi.
. . .

Unfortunately, "perfect conditions" includes a perfectly conducting
ground far beyond the radial field, a luxury neither we nor the
broadcasters have. The maximum far field (sky wave) gain of a ground
mounted quarter wave vertical over average ground, with a completely
lossless ground system, is on the order of 0 dBi, and this occurs at
roughly 25 degrees above the horizon (both depending on frequency as
well as ground characteristics). See the EZNEC example file Vert1.ez as
an illustration. Its MININEC type ground simulates a lossless ground system.

Roy Lewallen, W7EL
John Ferrell W8CCW

EZNEC Vertical interpretation
 

Found the helix. I will have to puzzle it out. I have a lot to digest.
I am glad I asked the question. The answers turned on a lot lights.

On Thu, 20 Apr 2006 19:22:17 GMT, "Cecil Moore"
wrote:

"John Ferrell" wrote:
I don't find the Helix feature in my copy of EZNEC+ V4.
I believe I can get the same results by inserting an appropriate load.
It will just be a little more cumbersome.

The helix feature is found under the Wires window under the create
menu. But it is somewhat complicated. A lumped inductance load
will get you started.

The problem with feeding an impedance of 1.2-j1400 ohms is
getting power into the antenna without dissipating most of it
in the matching network.
John Ferrell W8CCW

EZNEC Vertical interpretation
 

"Roy Lewallen" wrote:
The maximum far field (sky wave) gain of a ground mounted
quarter wave vertical over average ground, with a completely lossless
ground system, is on the order of 0 dBi, and this occurs at roughly 25
degrees above the horizon (both depending on
frequency as well as ground characteristics).
_____________

The above is an understandable conclusion using NEC analysis, however it is
not supported empirically. If it was, AM broadcast stations would perform
very much differently than they do.

The measured data in Brown, Lewis & Epstein's 1937 benchmark paper "Ground
Systems as a Factor in Antenna Efficiency" proved that the *radiated*
groundwave field from a vertical monopole working against 113 buried radials
each 0.41 lambda in length was within a few percent of its calculated peak
value for a radiation pattern with maximum gain in the horizontal plane.
The path length for the measurement was 0.3 miles, which was in the far
field of the vertical monopole configurations measured.

BL&E's measurements, and the results of thousands of measurements made of
the groundwave fields of MW broadcast stations using such radial ground
systems ever since demonstrate that their peak gain always lies in the
horizontal plane.

It is true that, as a groundwave propagation path becomes longer, the field
measured at increasing elevations above the earth at distant ranges might be
higher than measured at ground level at those ranges. But that is not
because more field was launched by the original radiator toward those higher
elevations -- it is because the the groundwave path has higher losses, which
accumulate as that path lengthens. Therefore a NEC plot showing the
conditions reported in the quote above do not accurately depict the
elevation pattern as it is launched from the radiator, and the groundwave
field it will generate.

There are software programs designed for calculating MW groundwave field
strength given the FCC "efficiency" of the radiator and the conductivity of
the path. The radiator efficiency is the groundwave field developed by the
radiator with a given applied power at a given distance (1 kW @ 1 km).
These values must meet a certain minimum level for the class of station. I
think in all cases, they must be within ~0.5 dB of the theoretical value for
a radiation pattern with its peak gain in the horizontal plane. In the case
of directional MW antennas, this performance must be proven by field
measurements.

Finally, standard equations show a peak field of ~137.6 mV/m at 1 mile from
a 1/2-wave dipole radiating 1 kW in free space. The calculated groundwave
field at 1 mile radiated by 1 kW from a 1/4-wave vertical MW monopole over
a perfect ground plane is ~195 mV/m. This is the same field as generated by
the free space 1/2-wave dipole, when all radiation is confined to one
hemisphere (137.6 x 1.414).

The groundwave fields measured from thousands of installed MW broadcast
antenna systems confirm that their intrinsic radiation patterns are within a
fraction of a decibel of that perfect radiator over a perfect ground plane,
no matter what is the conductivity at the antenna site (N.B. Reg).

RF

EZNEC Vertical interpretation
 

"Richard Fry" wrote
The groundwave fields measured from thousands of installed MW
broadcast
antenna systems confirm that their intrinsic radiation patterns are
within a
fraction of a decibel of that perfect radiator over a perfect ground
plane,
no matter what is the conductivity at the antenna site (N.B. Reg).

========================================

So you cleverly avoid the problem of how to calculate the effects of
ground conductivity by laying down more than enough radials of more
than sufficient length to hide the effects.

B,L & E sure had the right idea.

But hardly a satisfactory economic engineering solution.
----
Reg.

EZNEC Vertical interpretation
 

"Reg Edwards" wrote
So you cleverly avoid the problem of how to calculate the effects
of ground conductivity by laying down more than enough radials
of more than sufficient length to hide the effects.

B,L & E sure had the right idea.

But hardly a satisfactory economic engineering solution.
_________________

So you think it is preferable to install a less than optimal radial ground
system, as long as you can calculate how deficient it is?

MW broadcast stations want/need maximum groundwave coverage. Installing a
"BL&E" radial ground system will provide that for all conductivity
conditions at the tx antenna site -- which equates to a very satisfactory
and economic engineering solution for this application.

RF

EZNEC Vertical interpretation
 

Richard Fry wrote:
"Roy Lewallen" wrote:
The maximum far field (sky wave) gain of a ground mounted
quarter wave vertical over average ground, with a completely lossless
ground system, is on the order of 0 dBi, and this occurs at roughly 25
degrees above the horizon (both depending on
frequency as well as ground characteristics).
_____________

The above is an understandable conclusion using NEC analysis, however it
is not supported empirically. If it was, AM broadcast stations would
perform very much differently than they do.

NEC analysis has been supported many times by measurement and observation.

The measured data in Brown, Lewis & Epstein's 1937 benchmark paper
"Ground Systems as a Factor in Antenna Efficiency" proved that the
*radiated* groundwave field from a vertical monopole working against 113
buried radials each 0.41 lambda in length was within a few percent of
its calculated peak value for a radiation pattern with maximum gain in
the horizontal plane. The path length for the measurement was 0.3 miles,
which was in the far field of the vertical monopole configurations
measured.

Yes. The question is what is the calculated value. B, L, and E
normalized their measurements to the unattenuated field strength at one
mile for 1000 watts radiated power. I couldn't find anywhere in their
paper where they explained how they determined the ground attenuation
between the antenna and their observation point.

BL&E's measurements, and the results of thousands of measurements made
of the groundwave fields of MW broadcast stations using such radial
ground systems ever since demonstrate that their peak gain always lies
in the horizontal plane.

No, the field strength is strongest at low elevation angles only close
to the antenna, as you further explain below.

It is true that, as a groundwave propagation path becomes longer, the
field measured at increasing elevations above the earth at distant
ranges might be higher than measured at ground level at those ranges.
But that is not because more field was launched by the original radiator
toward those higher elevations -- it is because the the groundwave path
has higher losses, which accumulate as that path lengthens. Therefore a
NEC plot showing the conditions reported in the quote above do not
accurately depict the elevation pattern as it is launched from the
radiator, and the groundwave field it will generate.

Of course the standard far field analysis doesn't accurately depict the
field close to the antenna -- it's a plot of the field at points very
distant from the antenna, as clearly explained in the manual. NEC allows
you to include the surface wave if you want, and it accurately shows the
total field including the surface wave at a distance of your choice.
(Accurate, that is, up to a hundred km or so, beyond which the deviation
of the flat ground model from the curved Earth begins affecting results.)

Don't feel bad -- Reg has a lot of trouble understanding this, too.

There are software programs designed for calculating MW groundwave field
strength given the FCC "efficiency" of the radiator and the conductivity
of the path. The radiator efficiency is the groundwave field developed
by the radiator with a given applied power at a given distance (1 kW @ 1
km). These values must meet a certain minimum level for the class of
station. I think in all cases, they must be within ~0.5 dB of the
theoretical value for a radiation pattern with its peak gain in the
horizontal plane. In the case of directional MW antennas, this
performance must be proven by field measurements.

Finally, standard equations show a peak field of ~137.6 mV/m at 1 mile
from a 1/2-wave dipole radiating 1 kW in free space. The calculated
groundwave field at 1 mile radiated by 1 kW from a 1/4-wave vertical MW
monopole over a perfect ground plane is ~195 mV/m. This is the same
field as generated by the free space 1/2-wave dipole, when all radiation
is confined to one hemisphere (137.6 x 1.414).

The groundwave fields measured from thousands of installed MW broadcast
antenna systems confirm that their intrinsic radiation patterns are
within a fraction of a decibel of that perfect radiator over a perfect
ground plane, no matter what is the conductivity at the antenna site
(N.B. Reg).

No, the measured fields from quarter wave broadcast antennas are
considerably less than 195 mV/m for 1 kW at one mile, unless perhaps
there's only salt water between the antenna and measurement point. As
you explained above, the surface wave is attenuated with distance. What
you seem to be missing is that the attenuation is strongly dependent on
ground conductivity (between antenna and measurement point, not just at
the antenna site) and frequency, so the actual field strength at one
mile for 1 kW radiated will always be considerably less than the perfect
ground case. The 195 mV/m and associated values for various antenna
heights is the "unattenuated" or "inverse" field, which doesn't include
the surface wave attenuation beyond simple inverse distance field
strength reduction. It's the field strength you'd get if the ground
between antenna and measurement point were perfect, not what you get
over real ground. I'm not very conversant with FCC antenna measurement
methodology, but somewhere the measured field strength is normalized to
the unattenuated field strength by fitting to a ground attenuation
curve, which in turn depends on frequency and ground conductivity. (I've
been told that this is the way broadcasters determine ground
conductivity -- by seeing how far the measured field strength deviates
from the unattenuated value.) I believe that the surface wave
attenuation curves used by the FCC are from the 1937 I.R.E. paper by
K.A. Norton. That paper is also the basis for NEC's surface wave
calculations.

Roy Lewallen, W7EL