Rhombics
 

Before I go to the trouble of putting up a rhombic, I've been using NEC
to get an idea of the gain, radiation angle etc for various leg lengths.
It all looks very promising on the computer but I'd be interested in
real-world experiences. For example, how well does the real antenna
approach the PC simulation when various factors like wire sag, uneven
ground, presence of trees and shrubbery?
Alan
VK2ADB

Rhombics
 

On Fri, 29 Sep 2006 16:56:07 +1000, Alan Peake
wrote:

Before I go to the trouble of putting up a rhombic, I've been using NEC
to get an idea of the gain, radiation angle etc for various leg lengths.
It all looks very promising on the computer but I'd be interested in
real-world experiences. For example, how well does the real antenna
approach the PC simulation when various factors like wire sag, uneven
ground, presence of trees and shrubbery?

Hi Alan,

Rhombics were the stars of antenna developement in the late 1920s for
RCA. For very long wavelengths, certainly few other practical designs
could achieve the same gains. For shorter wavelengths, other designs
replaced the Rhombic (poor return on real estate in comparison).

If the wire sags such that most of it is on the ground, you suffer.
This is a judgement call otherwise and sag is quite within the ability
to model if judgement demands.

The presences of trees and shrubbery is something all designs suffer.
Unless you are speaking of an antenna in a forest, the Rhombic would
probably do quite well (after all, it is wavelengths long, as are few
trees) until you start getting into short wavelengths.

Rhombics are few and far between these days. Reasons are principally
wavelength based in comparison to available real estate. Few have the
real estate for long wavelengths. If you are working at a short
wavelength, there is a better design to do the same job.

73's
Richard Clark, KB7QHC

Rhombics
 

Alan, the rhombic is a relatively inefficient antenna that 'may' give
you significant gain in point-to-point service, which is why it was
popular in the early days of commercial and governmental radio
services...
For ham radio it is less attractive due to the real estate it requres,
due to the cost of the poles (it usually needs to be supported at a
number of points along each leg to stop excessive whipping in the
wind)... Less attractive because it requires retuning of the matching
unit for small changes in frequency...Less attractive because it
confines your communications to a narrow angular beam...
Being a long ways from the other population centers of the world you
may be in a situation where you find it effective... Most of us
don't...

cheers denny / k8do

Rhombics
 

Denny wrote:
Alan, the rhombic is a relatively inefficient antenna that 'may' give
you significant gain in point-to-point service, which is why it was
popular in the early days of commercial and governmental radio
services...
For ham radio it is less attractive due to the real estate it requres,
due to the cost of the poles (it usually needs to be supported at a
number of points along each leg to stop excessive whipping in the
wind)... Less attractive because it requires retuning of the matching
unit for small changes in frequency...Less attractive because it
confines your communications to a narrow angular beam...
Being a long ways from the other population centers of the world you
may be in a situation where you find it effective... Most of us
don't...

cheers denny / k8do


I have to disagree with some of your statements Denny..

I used rhombic's over the years and they can be very efficient if
properly fed they do not require constant tuning. Their bandwidth can
be quit good. and you only need 4 pole if properly installed. I use
large rhombics before that went through 100 plus M.P.H. winds without
any problems. They are relatively expensive to install properly . the
matching situation can be handled easily with Baluns or open wire
feeders. only problem you may have is comming up with the terminating
resistor.. Their kinda hard to find cheaply today.

Remember that they are very good point to point radiators and rx
antennas if you calculate things right for your desired path .. they
also can provide multi lobes to favored directions if desired. and that
is somewhat dependent on your operating Frequency and the size of your
Rhombic.

If I had the room I'd consider rhombics for Sure.. W6AM used them for
years and as one who worked that station from many places on this earth
I can tell you he was never the weakest signal on any band.
73 Dave kc1di

Rhombics
 

I used a rhombic in the 70s while on contract TDY at Hill AFB, Utah [AGA5HI -
USAF MARS].

It was point to point from Utah to SE Asia [Vietnam War] and approximately 5
wavelengths per leg on 19 MHz and about 40 feet high [guess ... near top of
telephone poles].

It supported the SE Asia phone patch net [USAF MARS]. The USAF had LOTS of land
for the antenna.

We later switched to a Log Periodic as more versatile.

/s/ DD

Alan Peake wrote:

Before I go to the trouble of putting up a rhombic, I've been using NEC
to get an idea of the gain, radiation angle etc for various leg lengths.
It all looks very promising on the computer but I'd be interested in
real-world experiences. For example, how well does the real antenna
approach the PC simulation when various factors like wire sag, uneven
ground, presence of trees and shrubbery?
Alan
VK2ADB

Rhombics
 

"Alan Peake" wrote in message
...
Before I go to the trouble of putting up a rhombic, I've been using NEC to
get an idea of the gain, radiation angle etc for various leg lengths. It
all looks very promising on the computer but I'd be interested in
real-world experiences. For example, how well does the real antenna
approach the PC simulation when various factors like wire sag, uneven
ground, presence of trees and shrubbery?
Alan
VK2ADB


Alan,
Rhombics perform better than what simulations and modeling show.
Put them up and see for yourself.

73 Yuri

Rhombics
 

Alan, VK2ADB wrote:
"It all looks very promising on the computer but I`d be interested in
real-world exerience."

Don`t worry. I`ve erected many rhombics. They were all astisfactory and
very forgiving. Most were about 4 or 5 wavelengths on a side (leg) and
they were about twice as long as wide. They all worked well over a wide
range of frequencies. It`s just a terminated transmission line with a
big spread in the middle so it will radiate. Matching is a cinch.

Best regards, Richard Harrison, KB5WZI

Rhombics
 

"Yuri Blanarovich"
Rhombics perform better than what simulations and modeling show.
___________

I can vouch for that. In 1961 I was stationed at Dharhan AFB, Saudi Arabia,
and operated many times from HZ1AB there -- at the time the only licensed
amateur radio station in Saudi not connected with the royal family.

We used a Collins exciter driving a heavily modified BC-610 to a terminated
rhombic aimed down the eastern seaboard of the US. We had no trouble
reaching the States (and hearing 1,000s of DX-ers wanting to talk to us).

OTOH, military links I took care of using KWS-1s into 6-element rotatable
"beams" didn't do as well.

RF

Rhombics
 

On Fri, 29 Sep 2006 11:23:27 -0400, kc1di wrote:



Denny wrote:
Alan, the rhombic is a relatively inefficient antenna that 'may' give
....
I used rhombic's over the years and they can be very efficient if

Although the term "efficiency" seems to be used with gay abandon in
this place, the rhombic is not close to 100% efficient, a portion of
the transmitter power (approaching 50%) is dissipated in a
non-radiating loss.

That loss does not have an adverse impact on the gain in the desired
direction, and so it should not be regarded as a disadvantage of
itself.

Owen
--

Rhombics
 

On Fri, 29 Sep 2006 16:56:07 +1000, Alan Peake
wrote:

Before I go to the trouble of putting up a rhombic, I've been using NEC
to get an idea of the gain, radiation angle etc for various leg lengths.
It all looks very promising on the computer but I'd be interested in
real-world experiences. For example, how well does the real antenna
approach the PC simulation when various factors like wire sag, uneven
ground, presence of trees and shrubbery?
Alan
VK2ADB

The rhombic can deliver you a frequency agile antenna with gain, and
low angle major lobe if of sufficient length and at sufficient height.
Side lobes are not pretty, space requirements are huge at HF and the
antenna is not readily rotatable, construction is simple, but serious.

You are on hectares (doesn't sound as good as acres, does it?). Space
is not a big issue, and every ham that can accomodate a good size
rhombic should have one (or more) as a talking point. You could deal
with the fixed heading disadvantage two ways: place the shack in the
middle of the rhombic and switch feed / load ends, or go the whole hog
and erect a set of rhombics to cover your desired / preferred paths.

Keeping in mind your exposure to high winds and snow (ice loading),
the construction needs to be robust. If for example you want coverage
down to 20m you should be aiming legs of close to 100m. Sag of 5% of
span is easily accomodated if the end heights are at 20m of more, but
becomes a problem as you lower the end height much. You could model
the effect of the combination of sag and low end height in NEC by
breaking the leg wires into several sections following the approximate
catenary (or parabola for ease). I haven't done it, but I suspect
uncertainty about the ground conditions and ground profile will
introduce more model error than modest sag.

Owen
--

Rhombics
 

Owen Duffy wrote:
On Fri, 29 Sep 2006 16:56:07 +1000, Alan Peake
wrote:

Before I go to the trouble of putting up a rhombic, I've been using NEC
to get an idea of the gain, radiation angle etc for various leg lengths.
It all looks very promising on the computer but I'd be interested in
real-world experiences. For example, how well does the real antenna
approach the PC simulation when various factors like wire sag, uneven
ground, presence of trees and shrubbery?
Alan
VK2ADB

The rhombic can deliver you a frequency agile antenna with gain, and
low angle major lobe if of sufficient length and at sufficient height.
Side lobes are not pretty, space requirements are huge at HF and the
antenna is not readily rotatable, construction is simple, but serious.

You are on hectares (doesn't sound as good as acres, does it?). Space
is not a big issue, and every ham that can accomodate a good size
rhombic should have one (or more) as a talking point. You could deal
with the fixed heading disadvantage two ways: place the shack in the
middle of the rhombic and switch feed / load ends, or go the whole hog
and erect a set of rhombics to cover your desired / preferred paths.

Keeping in mind your exposure to high winds and snow (ice loading),
the construction needs to be robust. If for example you want coverage
down to 20m you should be aiming legs of close to 100m. Sag of 5% of
span is easily accomodated if the end heights are at 20m of more, but
becomes a problem as you lower the end height much. You could model
the effect of the combination of sag and low end height in NEC by
breaking the leg wires into several sections following the approximate
catenary (or parabola for ease). I haven't done it, but I suspect
uncertainty about the ground conditions and ground profile will
introduce more model error than modest sag.

Owen
--

Dollar for Dollar DB for DB i would errect a Curtain antenna. It
outperforms Rhombics in all aspects of design and construction.
Considering that the 4 poles of a Rhombic can be used to build an
Array that will cover the globe, have equal or greater gain that would
make the curtain antenna a better choice. I suppose thats why just
about ever shortwave station in the world uses them!

If you want an opinion of a station who has used and is still using
stacked rhombics ask Ian VK3MO. He will tell you thats his 6 wavelength
perside rhombic is a terrible general purpose antenna. Since his
antenna is fixed on new York he finds that quite frequently the
propagation path rarely comes in on the direct computed bearing. He
loses 10 to 20 db having his sharp rhombic. His rhombic does not have
sufficient azimuth diversity since the 3db horizontal beamwidth is so
narrow.

You also wont have the problem of finding a termination resistor, a
decent globar one anyway. See w8ji.com or look at TCI's web page. Its
one hell of a antenna. I am surprised nobody in ham radio has installed
one, considering the large number of 200 ft towers in the world. When
radio Switzerland closed down there was a special even ham station on
air using a rotable 300ft high curtain, oh what a signal 24 dbi
produces with a 100 watts! Now if you know of a stacked array that uses
4 mono band or other stacked antennas that cal deliver greater than
20dbi thats not a laser beam let us all know. I tried to come with
such a stack using 6 log periodics using 42 ft booms on a 200 foot
tower. It deliver such gain on the higher frequencies but fell short on
13mhz. The curtain will do it with a lot less expense.

Bob



Bob

Rhombics
 

Keeping in mind your exposure to high winds and snow (ice loading),
the construction needs to be robust. If for example you want coverage
down to 20m you should be aiming legs of close to 100m.

Yes, it looks like 80m per leg is reasonable - 4 wavelengths at 20m
I did as you suggested in terms of modelling the sag and it didn't seem
to upset the pattern greatly.

I can put the antenna over the house but I'm not sure if the extra
feeder loss would outweigh the advantages. Might not be too bad with 600
ohm or greater feeder.
Alan

Rhombics
 

Dollar for Dollar DB for DB i would errect a Curtain antenna. It
outperforms Rhombics in all aspects of design and construction.

Well, I don't know much about curtains but VK3ATN apparently had one up
and preferred his rhombics. Don't know the full details though.


If you want an opinion of a station who has used and is still using
stacked rhombics ask Ian VK3MO. He will tell you thats his 6 wavelength
perside rhombic is a terrible general purpose antenna. Since his
antenna is fixed on new York he finds that quite frequently the
propagation path rarely comes in on the direct computed bearing. He
loses 10 to 20 db having his sharp rhombic. His rhombic does not have
sufficient azimuth diversity since the 3db horizontal beamwidth is so
narrow.

That's a problem, which is why I had thought of only 4 wavelengths per
side. NEC says it would be about 15 degrees wide on 20m.


You also wont have the problem of finding a termination resistor, a
decent globar one anyway.

I was going use it unterminated to begin with. Aimed at Europe, the
other end points at Central America so I'd be surprised if I had
problems covering both those areas at the same time.

Alan

Rhombics
 

"Richard Harrison" wrote in message
...
Alan, VK2ADB wrote:
"It all looks very promising on the computer but I`d be interested in
real-world exerience."

Don`t worry. I`ve erected many rhombics. They were all satisfactory and
very forgiving. Most were about 4 or 5 wavelengths on a side (leg) and
they were about twice as long as wide. They all worked well over a wide
range of frequencies.

How are they at VHF/UHF? For Field Day, I usually do 2M & 440 FM voice with
very large vertically-polarized yagis. What am I going to get with a
rhombic? Horizontal polarization, I'll bet, except maybe the whole rhombic
can be rotated 90 degrees to transmit/receive a vertically-polarized signal.

I really want to try some experimenting, since 5 - 10 wavelengths of 440 is
doable in my backyard using sticks guyed up with twine. What about UHF
television reception? This could get interesting.

Rhombics
 

feeders. only problem you may have is comming up with the terminating
resistor.. Their kinda hard to find cheaply today.

Might not be a problem for 100W - I'm on solar power and don't want to
have to fire up the generator to power a linear :)


Remember that they are very good point to point radiators and rx
antennas if you calculate things right for your desired path .. they
also can provide multi lobes to favored directions if desired. and that
is somewhat dependent on your operating Frequency and the size of your
Rhombic.

I found a strange thing with NEC in terms of multiple lobes. At 20m, NEC
gave me a nice main lobe with the antenna at 15m but at 10m, there were
two lobes about 30 degrees apart. I played with the height to see how it
changed the lobe pattern but the results didn't make sense. The two
lobes remained until the antenna went to a certain height, then one
extra millimetre in height, and it went straight back to one main lobe.
Alan

Rhombics
 

feeders. only problem you may have is comming up with the terminating
resistor.. Their kinda hard to find cheaply today.

Might not be a problem for 100W - I'm on solar power and don't want to
have to fire up the generator to power a linear :)


Remember that they are very good point to point radiators and rx
antennas if you calculate things right for your desired path .. they
also can provide multi lobes to favored directions if desired. and that
is somewhat dependent on your operating Frequency and the size of your
Rhombic.

I found a strange thing with NEC in terms of multiple lobes. At 20m, NEC
gave me a nice main lobe with the antenna at 15m but at 10m, there were
two lobes about 30 degrees apart. I played with the height to see how it
changed the lobe pattern but the results didn't make sense. The two
lobes remained until the antenna went to a certain height, then one
extra millimetre in height, and it went straight back to one main lobe.
Alan

Rhombics
 

Richard Fry wrote:
"Yuri Blanarovich"

Rhombics perform better than what simulations and modeling show.

___________

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

Problem with the model??
Alan

Rhombics
 

Richard Harrison wrote:
Alan, VK2ADB wrote:
"It all looks very promising on the computer but I`d be interested in
real-world exerience."

Don`t worry. I`ve erected many rhombics. They were all astisfactory and
very forgiving. Most were about 4 or 5 wavelengths on a side (leg) and
they were about twice as long as wide. They all worked well over a wide
range of frequencies. It`s just a terminated transmission line with a
big spread in the middle so it will radiate. Matching is a cinch.

Best regards, Richard Harrison, KB5WZI


Well, I'll give one a try. I can get four 15m poles at a reasonable
price (I've planted hundreds of pine trees on the property but the
tallest is only 25' so far and not where I need them - should have
planned that a bit better!!)
What sort of feed arrangement did you use?
Alan

Rhombics
 

On Sat, 30 Sep 2006 16:31:29 +1000, Alan Peake
wrote:




feeders. only problem you may have is comming up with the terminating
resistor.. Their kinda hard to find cheaply today.

Might not be a problem for 100W - I'm on solar power and don't want to
have to fire up the generator to power a linear :)

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
--

Rhombics
 

Alan, VK2ADB wrote:
"What sort of feed warrangement did you use?"

600-ohm open-wire line. Many of the rhombics I erected were made from
U.S. Army WW-2 Signal Corps kits. These used three cables in the diamond
shaped curtain which came together at the end supports but were spread
apart by several feet at the side supports. This construction tended to
further reduce impedance variations as frequency changed. We didn`t
bother with this refinement with receiving antennas.

Actual antwenna driving point impedance tended to exceed 600 ohms so
width of the attachment point to the anteena was wide for the cable size
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.

The cable used in the kits was made from (3) AWG 12 Copperweld wires
twisted together. There was also stainless steel wire for a dissipation
line at the far-end of the rhombic. We had to replace this with larger
wire as we were broadcasting with 100 KW transmitters, far more power
than anticipated by the Signal Corps. Under some conditions and at some
frequencies, it is possible to dissipate up to 50% of the power fed into
the rhombic in its dissipation line.

It is possible to operate without a dissipation line or resistance. The
rhombic becomes bidirectional without the termination. I know from
experience during a period when the Signal Corps dissipation lines
melted from the broadcast power and our fan mail came from South America
as well as Central Europe.

Best regards, Richard Harrison, KB5WZI

Rhombics
 

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

Rhombics
 

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

Rhombics
 

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

Rhombics
 

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

Rhombics
 

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
--

Rhombics
 

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

Rhombics
 

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

Rhombics
 

"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

Rhombics
 

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

Rhombics
 

"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

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