Rhombics
 

On Sun, 1 Oct 2006 15:41:31 -0500, "Richard Fry"
wrote:

"Richard Clark" wrote
I would further note that their data all exhibited values
that lay below 100% (if by 100% that is meant to be
theoretical.)
________

My statement was "relative field." Relative field is defined as E/E(max).

RF

So, am I to take this response to mean that you have no further
comment upon your statement:
In fact for distances just into the far-field region for the radiator
defined in (2) above (far field usually being defined as further than
2H^2/lambda from the antenna), h-plane relative field is virtually 100%,
regardless of ground conditions.

as rebutted by mine:
As I pointed out once before, NEC (EZNEC in particular) will exhibit
fields from any antenna that are consistent with Brown, Lewis, and
Epstein's field data to within 1dB.
By way of elaboration, this INCLUDES at zero degrees.

As I read the two, they are in contradiction, but it appears you
perceive none. If you do not, I return to older queries:
What is the comparison being made?

To what purpose?

73's
Richard Clark, KB7QHC

Rhombics
 

"Richard Clark" wrote:
So, am I to take this response to mean that you have no further
comment upon your statement:
In fact for distances just into the far-field region for the radiator
defined in (2) above (far field usually being defined as further than
2H^2/lambda from the antenna), h-plane relative field is virtually 100%,
regardless of ground conditions.

as rebutted by mine:
As I pointed out once before, NEC (EZNEC in particular) will exhibit
fields from any antenna that are consistent with Brown, Lewis, and
Epstein's field data to within 1dB.
By way of elaboration, this INCLUDES at zero degrees.
________

AFAIK, neither NEC 2 nor NEC 4 from any source will show a composite
elevation pattern over a defined ground at a specified distance to include
the space wave and the surface wave in a single display. One must merge
them using his/her own understanding and resources.

RF

Rhombics
 

On Sun, 1 Oct 2006 16:16:44 -0500, "Richard Fry"
wrote:

AFAIK, neither NEC 2 nor NEC 4 from any source will show a composite
elevation pattern over a defined ground at a specified distance to include
the space wave and the surface wave in a single display. One must merge
them using his/her own understanding and resources.

This seems to ramble well off the earlier path encapsulated by Denny
and apparently subscribed to by Owen, in regard:
But to bring us back to the major complaint which seems to be that the
Nec engine doesn't model the last few degrees over ground very well, so
that the zero angle is discarded by the software... Richard seems on a
mission to prove the NEC engine wrong - well, I agree, the NEC engine
does have limitations for low angle signals which is why the authors
have installed an angle cut off... Per Richard's citations
To which I object to, to no notice (I wasn't surprised however).

This subsequent restriction to GRAPHICS (and not to issues of NEC per
se) merely highlights my earlier comments that such "merging" serves
no apparent purpose of the Ham's activities in MF/HF, especially when
such GRAPHICS would only support a region of one to a dozen or so
miles. There is nothing remarkable or noteworthy in this indulgence.

To bring us back to the major complaint.... That complaint is without
foundation. EZNEC easily models the last few degrees over ground very
well (and to all constraints offered). Having said this, I, for one
(out of at least three) can see a contradiction. I, for one (out of
at least three) can support my contentions with data taken directly
from EZNEC whereas all other discussion to this point has been
particularly void of informed content.

So, I return to those same, earlier queries:
What is the comparison being made?

To what purpose?

73's
Richard Clark, KB7QHC

Rhombics
 

Sal M. Onella wrote:
"How are they at VHF/UHF?"

Arnold B. Bailey in "TV and Other Receiving Antennas" wrote on page 528
specifications for a 200 MHz rhombic antenna. It may be scaled for
another frequency. For 20 MHz, multiply dimensions by 10. It is four
straight horizontal wires (no. 10 AWG) each 36 ft. long, separated 18
ft. at mid point, overall length 31 feet. Transmission line is 300-ohm
balanced twinlead. Reaistance at center frequency is 600 ohms as is the
termination resistance. Gain at centerfrequency is 14.5 dBd (or less).
Frequency bandwidth for 1 dB down is 30%. For 3 dB down, it is 100%.
Polar pattern shows lots of side lobes.

Best regards, Richard Harrison, KB5WZI

Rhombics
 

"Richard Clark" wrote
On Sun, 1 Oct 2006 16:16:44 -0500, "Richard Fry"
wrote:

AFAIK, neither NEC 2 nor NEC 4 from any source will show a composite
elevation pattern over a defined ground at a specified distance to include
the space wave and the surface wave in a single display. One must merge
them using his/her own understanding and resources.

This seems to ramble well off... etc etc

Not to say that what I posted is untrue or irrelevant, however.

This subsequent restriction to GRAPHICS (and not to issues of NEC per se)
...

But the GRAPHICS are only plotting what NEC calculated, are they not?

RF

Rhombics
 

Richard Fry wrote:

AFAIK, neither NEC 2 nor NEC 4 from any source will show a composite
elevation pattern over a defined ground at a specified distance to
include the space wave and the surface wave in a single display. One
must merge them using his/her own understanding and resources.

Neither NEC-2 nor NEC-4 shows a display of any kind. Output is solely in
the form of tables of numbers. Surface wave data, when produced, is
always combined with the sky wave as a total field value. It's chosen
with an option on the RP input "card". The NEC-2 manual as well as NEC-2
itself -- including source code which enables you to see exactly how the
calculations are done -- are freely available on the Internet. So
there's no need to speculate about what it does or doesn't do.

EZNEC demo, standard, and plus program types don't include the surface
wave in far field outputs, which as I explained in an earlier posting in
another thread(*), isn't of much practical use to most amateurs.
However, when needed the data can be obtained from the near field
analysis (which is actually a total field analysis and isn't restricted
to the near field). EZNEC pro program types (EZNEC-M and EZNEC/4) allow
direct inclusion of the surface wave in the far field analysis, mostly
for the use of AM broadcast consultant customers. It provides tabular
and graphical outputs in the form of an azimuth plot at any desired
horizontal distance from the antenna and height above ground. Like NEC,
the output is the total field, or in other words the sum of surface and
sky wave, and the results should be virtually identical to those from NEC.

(*) I recommend that anyone interested in this topic read the posting I
made just a few days ago, Sept. 28, on this newsgroup under the thread
"Elevation Patterns of Ground Mounted Vertical Monopoles". It can easily
be located at http://groups.google.com if it's not still on your
newsreader client.

Roy Lewallen, W7EL

Rhombics
 

Richard Clark wrote:
On Sun, 1 Oct 2006 16:16:44 -0500, "Richard Fry"
wrote:

AFAIK, neither NEC 2 nor NEC 4 from any source will show a composite
elevation pattern over a defined ground at a specified distance to include
the space wave and the surface wave in a single display. One must merge
them using his/her own understanding and resources.

This seems to ramble well off the earlier path encapsulated by Denny
and apparently subscribed to by Owen, in regard:
But to bring us back to the major complaint which seems to be that the
Nec engine doesn't model the last few degrees over ground very well, so
that the zero angle is discarded by the software... Richard seems on a
mission to prove the NEC engine wrong - well, I agree, the NEC engine
does have limitations for low angle signals which is why the authors
have installed an angle cut off... Per Richard's citations
To which I object to, to no notice (I wasn't surprised however).
. . .

I'm not aware of any such problem with NEC, or any "angle cut off"
intentionally included in NEC(*). As Richard says, EZNEC has no problem
extending analysis of any kind -- near field, far field, or (in pro
programs) far field with surface wave, down to ground level. The value
of zero for far field sky wave (that is, far field at distances beyond
which the surface wave has decayed to essentially zero) at zero
elevation angle for horizontally polarized waves over any ground and for
vertically polarized waves over non-perfect ground is a rigorously
correct result. It follows directly from calculation of ground
reflection coefficients, the simple formulas for which you can find in
Kraus' _Antennas_ and many other references.

I'd be very interested if such a limit exists, and would be very
grateful to anyone who could point to the place in the NEC code where it
occurs, or provide an example of a model producing a result where its
effect is evident. I strongly suspect that whatever effect is being
seen, it's due to misinterpretation or other causes and mistakenly
attributed to a limit which doesn't exist.

(*) There are many places in the NEC and EZNEC code where protection is
provided against divide-by-zero errors, which limits internal
calculations and perhaps a minimum or maximum field strength or angle.
However, these usually limit a minimum divisor value to something on the
order of 10^-10 to 10^-20 or so, beyond the point at which a calculated
result is significant or, often, even valid.

Roy Lewallen, W7EL

Rhombics
 

"Roy Lewallen" wrote:
EZNEC pro program types (EZNEC-M and EZNEC/4) allow direct inclusion of
the surface wave in the far field analysis, mostly for the use of AM
broadcast consultant customers. It provides tabular and graphical outputs
in the form of an azimuth plot at any desired horizontal distance from the
antenna and height above ground. Like NEC, the output is the total field,
or in other words the sum of surface and sky wave, and the results should
be virtually identical to those from NEC.
______________

This is as may be, and can be useful when properly understood and applied.

But is it not true that for the ground-mounted monopoles defined in my
previous posts, no form of NEC and its Windows shells will show in a
_single_ evaluation whether in tabular or graphical form, the net values of
the surfacewave+spacewave elevation pattern at a user-specified distance
over real earth, for all elevation angles in a given azimuth slice?

RF

Rhombics
 

On Sun, 1 Oct 2006 17:24:17 -0500, "Richard Fry"
wrote:

This subsequent restriction to GRAPHICS (and not to issues of NEC per se)
...

But the GRAPHICS are only plotting what NEC calculated, are they not?

The point of the matter (re-iterated by Roy) is that the graphical
representation is one for wide application (99.9999%) and the
remaining (0.0001%) still have access to the DATA. There is nothing
lost but convenience. True, one may come to some erroneous conclusion
on abstracting the near-in characteristics to the far field
representations. I would challenge them more for their
mis-application than their literal mis-understanding, however. There
is not much demand for 160M operation to the horizon that goes wanting
elaborate modeling. Simple experience eclipses that easily.

To cut to the nut of the matter:
But to bring us back to the major complaint which seems to be that the
Nec engine doesn't model the last few degrees over ground very well
is simply wrong in the first degree (also re-iterated by Roy).

I've been modeling for results at the horizon for as long as I've held
a copy of EL/EZNEC. This and the Brown, Lewis, Epstein data confirm
to within 1dB as I've reported on more than one occasion. This is a
validation in the absolute, not relative sense; and to a probable
higher degree of accuracy than the average Ham pursues or could
obtain.

73's
Richard Clark, KB7QHC

Rhombics
 

Richard Fry wrote:
______________

This is as may be, and can be useful when properly understood and applied.

But is it not true that for the ground-mounted monopoles defined in my
previous posts, no form of NEC and its Windows shells will show in a
_single_ evaluation whether in tabular or graphical form, the net values
of the surfacewave+spacewave elevation pattern at a user-specified
distance over real earth, for all elevation angles in a given azimuth
slice?

NEC can do that in a single run. You'll need an RP "card" for each point
on the curve, each specifying the horizontal distance and zenith angle
of the observation point. The reason that so many "cards" are necessary
is that to maintain a fixed distance from the antenna, the horizontal
distance must decrease as the elevation angle increases. And
specification of distance in NEC (and EZNEC pro) is done as horizontal
distance, not radial distance. See the NEC-2 manual for detailed
information.

In the more than ten years EZNEC pro has had ground wave analysis, I
believe your recent request is the only one I've ever gotten for such a
display. Apparently EZNEC pro users, like me, don't see any practical
use for one.

Roy Lewallen, W7EL