More like model has a problem capturing or reflecting reality.
Modeling will calculate the pattern etc., but will not properly reflect the
interaction of the antenna design with propagation medium and
terrain/surroundings, like showing effect of capture area.

73 Yuri, K3BU

"Alan Peake" wrote in message
...


Richard Fry wrote:
"Yuri Blanarovich"

Rhombics perform better than what simulations and modeling show.

___________

I can vouch for that. .............

Problem with the model??
Alan

Antenna modeling tools aren't intended to model propagation effects. But
the stronger the signal radiated in the right direction, the stronger
the received signal will be. And the strength of the radiated signal in
each direction is what the antenna modeling program shows.

As for the "effect of capture area", the "capture area" of an antenna is
just another way of stating the gain. This information is what you get
from an antenna modeling program.

Roy Lewallen, W7EL

Yuri Blanarovich wrote:
More like model has a problem capturing or reflecting reality.
Modeling will calculate the pattern etc., but will not properly reflect the
interaction of the antenna design with propagation medium and
terrain/surroundings, like showing effect of capture area.

73 Yuri, K3BU

"Alan Peake" wrote in message
...

Richard Fry wrote:
"Yuri Blanarovich"

Rhombics perform better than what simulations and modeling show.
___________

I can vouch for that. .............
Problem with the model??
Alan

Something that I saw done on commercial sites with high power
transmitters is to run a feedline down from the termination end of
the rhombic and run it underground for a while. If the loss is
sufficient, you don't even need the terminating resistor.

Owen
Not a bad idea. There's a diagram or three in Laport's "Radio Antenna
Engineering" so I'll have a bit of a read.

Alan

to appear as about 800 ohms and then the spacing tapered steadiy down
during the descent to the horizontal transmission line which was spaced
for 600 ohms.
I like the tapered arrangement. What's the minimum distance needed to
taper from 800 to 600 at freqs from say 3.5MHz to 14MHz? I did one years
ago from 130 to 200 and it was about a foot long at 1.8GHz.

Best regards, Richard Harrison, KB5WZI


Alan

On Sun, 01 Oct 2006 17:27:31 +1000, Alan Peake
wrote:



to appear as about 800 ohms and then the spacing tapered steadiy down
during the descent to the horizontal transmission line which was spaced
for 600 ohms.
I like the tapered arrangement. What's the minimum distance needed to
taper from 800 to 600 at freqs from say 3.5MHz to 14MHz? I did one years
ago from 130 to 200 and it was about a foot long at 1.8GHz.

Alan,

Are you tapering for impedance transformation, or just to reduce
transmission line losses?

I make the matched line loss on a 600 ohms open wire line with 2mm dia
copper spaced 150mm something approaching 0.1dB/100m, with 2:1 VSWR
you are talking ~0.14dB/100m. You should find the rhombic no worse
than 2:1 unless it is too short.

My loss calculator has an ideal air spaced 600 ohm copper line (copper
loss only) 2mm/150mm, the label is "Open / air dielectric (150/2.00)",
you can try the various loads from the NEC models for more exact loss
figures.

Owen
--

Antenna modeling tools aren't intended to model propagation
effects. But the stronger the signal radiated in the right direction,
the stronger the received signal will be. And the strength of the
radiated signal in each direction is what the antenna modeling
program shows.
____________

But whenever a far-field NEC model uses any kind of real ground in the
analysis, modeling the effects of the propagation environment along with the
native radiation launched by the antenna is exactly what occurs.

This is why some think that the far field radiated by a vertical monopole
less than 5/8-wave high is zero in the horizontal plane, when in fact at
every operating frequency, it is always the highest field the antenna
produces.

RF

On Sun, 1 Oct 2006 11:30:49 -0500, "Richard Fry"
wrote:

This is why some think that the far field radiated by a vertical monopole
less than 5/8-wave high is zero in the horizontal plane, when in fact at
every operating frequency, it is always the highest field the antenna
produces.

Given the anachronism of mixing the explicit far field with the
implication of an unexpressed near field in the statement above, it is
reasonable that some might "think" something. The bare statement is
rather given to pondering, wondering, and ultimately puzzling:

What is the comparison being made?

A vertical monopole to a 5/8-wave high antenna? A vertical monopole
that IS a 5/8-wave high antenna? The far field to the near field? For
either a vertical monopole OR a 5/8-wave high antenna? For the same
vertical 5/8-wave high monopole antenna?

To what purpose?

Are we being steered toward the argument that the far field
representation of radiation would force someone to renounce their
experience of a strong signal received, at ground level, and within
sight of the antenna? This may appear to offer the temptation of a
paradox, but such opportunity is so rare as to be wholly outside of
the commonplace MF/HF activity of Hams.

After-all, even considering AM frequencies (160M) and that 5/8-wave
tall structure, there is no such thing as a zero angle signal beyond
26 miles. This range would be a stretch even then, as it demands an
angle depressed BELOW zero degrees from the aeronautical beacon at the
very top of the antenna. The zero angle range must then be something
radically less. And being less, it would be of no interest to the
average Ham. Thus the paradox of confabulating two different
radiation characteristics is lost.

As often happens, ray-tracing demands an origin, and yet an antenna
radiates from the WHOLE of its surface, not the aeronautical beacon,
nor its base insulator. To obtain that zero angle radiation
characteristic demands that the entire antenna (or certainly a
majority portion of it) be in sight. Let's simply mandate the average
height of a 5/8ths-wave for the 160M band being a 50M high point
origin for this ray-tracing exercise. The zero angle would fly over
the head of every radio in the audience who lived below that height.
In fact, most ordinary Hams would be hard pressed to erect an antenna
that high in the first place. Again, common experience would dictate
a healthy signal, but it would not be a treasured DX contact, and it
would probably be considered obnoxious QRM.

Even though such a signal (seeing only half the height of the antenna)
comes from being 18 miles away (on a billiard ball smooth planet), it
demonstrates the NEAR FIELD properties of optical ray-tracing and what
would be called a zone of confusion. 18 miles away is well outside of
the RF near field, but the argument of radiation lobes's
characteristics is an optical geometry and 18 miles is sufficiently
close enough to confound the two meanings arrived at in the quote
above.

73's
Richard Clark, KB7QHC

"Richard Clark" wrote:
Given the anachronism etc etc
________________

The purposes of my post were to state that (1) many NEC evaluations show a
combination of the radiation from the antenna PLUS the propagation effects
for the chosen earth parameters and antenna elevation, and (2) even when
such an approach shows zero field in the horizontal plane for a ground
mounted vertical monopole up to 5/8-wave high, that h-plane field in reality
is NOT zero _as it is radiated_ by the antenna.

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.

RF

On Sun, 1 Oct 2006 14:50:33 -0500, "Richard Fry"
wrote:

The purposes of my post were to state that (1) many NEC evaluations show a
combination of the radiation from the antenna PLUS the propagation effects
for the chosen earth parameters and antenna elevation, and (2) even when
such an approach shows zero field in the horizontal plane for a ground
mounted vertical monopole up to 5/8-wave high, that h-plane field in reality
is NOT zero _as it is radiated_ by the antenna.

This is simply an example of misusing a tool, not the evidence that it
lacks the capacity to show characteristics as they exist. Anyone can
conspire to fail.

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

73's
Richard Clark, KB7QHC

"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