On Mon, 12 Jul 2004 07:31:01 GMT, Richard Clark
wrote:
[Earlier stuff snipped]
|
|Following up with a series of 2 wavelength measurements, it is
|interesting to note that of the series of 19 tests, fully 13 of them
|evidenced HIGHER gain than those from the 20 wavelength series of
|measurements.
|
|The step from 2 wavelength to 5 wavelength showed gains consistent
|with doubling the length of the antenna size for many separations
|(e.g. 3dB gain, or thereabout). However, it appears that beyond 5
|wavelengths (considering my next cardinal point was a doubling to 10
|wavelengths) no further gain was observed as a general characteristic.
|
|If I were to judge this at the 180 degree spread and compare against
|ALL other designs; then the absolute greatest gain for a V design was
|observed to be slightly less than 4dB. In fact, the 2, 5, 10, and 20
|wavelength designs configured as simple dipoles barely differed one
|from the other (1dB at most, and typically 9.9dBi).


I must confess that I've tried to follow your path but clearly I'm
lost.

May I suggest that analysis at "180 degree spread", which I take to
mean a dipole, has no relationship to a vee configuration, other than
the wire length. If you will take each leg length and vary the apex
angle to the optimum, you will (should) find that the gain *does*
continue to increase with increased leg length, albeit at a
sub-proportional rate.

For each leg length there is an optimal apex angle. Leaving the angle
fixed and varying the length is *not* a fair test of gain vs. length.

Here are the results I obtained from a quick MultiNEC (NEC-2)
analysis. This at 144 Mhz with height = 20', Sommerfeld Gnd, with
average dirt. #12 AWG Al wire, 20 segments/WL, elevation angle = 4
deg. Source on the middle of a short (3 segment) wire.

Len. (WL) Ang. (deg) Gain (dBi)

10.200 30 19.84
20.193 20 21.97
30.188 16 23.06
40.185 14 23.70
50.182 12 24.10

These data seem resonable and consistant with my expectations.

Wes

On Mon, 12 Jul 2004 10:24:01 -0700, Wes wrote:

Here are the results I obtained from a quick MultiNEC (NEC-2)
analysis. This at 144 Mhz with height = 20', Sommerfeld Gnd, with
average dirt. #12 AWG Al wire, 20 segments/WL, elevation angle = 4
deg. Source on the middle of a short (3 segment) wire.

Len. (WL) Ang. (deg) Gain (dBi)

20.193 20 21.97

Hi Wes,

For this particular design (except mine is elevated 5 wavelengths), I
pushed for 1 degree resolution, with a split source, with tapered
segments (1023 all told) and I still fall short, but also well ahead
of my earlier reports:
18.5dBi @ 3 degrees w/3 degree lobe width

73's
Richard Clark, KB7QHC

This analysis should be done in free space, not over ground. Propagation
to the moon should avoid reflection from the ground, and in any case
EZNEC's flat, infinite-extent ground model isn't representative of what
the signal would encounter in real life.

One other comment. As a dipole gets longer, the lobes move closer and
closer to the direction of the wire. Bending the dipole into a vee shape
aligns pairs of the lobes so they point in the same direction, i.e.,
along the vee axis. That's why the optimum angle becomes less and less
as the wires get longer, and why a dipole model isn't necessarily
representative of what a long vee will do.

Roy Lewallen, W7EL

Richard Clark wrote:

For this particular design (except mine is elevated 5 wavelengths), I
pushed for 1 degree resolution, with a split source, with tapered
segments (1023 all told) and I still fall short, but also well ahead
of my earlier reports:
18.5dBi @ 3 degrees w/3 degree lobe width

73's
Richard Clark, KB7QHC

On Mon, 12 Jul 2004 23:02:30 -0700, Roy Lewallen
wrote:

Hello Roy,

There are any number of problems with your comments:

This analysis should be done in free space, not over ground.

The application demands ground as an inescapable reality of design.
Designing in free space, unless you expand upon your commentary, is
meaningless. In other words, the product of a free space analysis
offers no more insight that this blighted version. Your comments that
follow fairly shout this as a wholly undecipherable problem.

Propagation to the moon should avoid reflection from the ground,

This has already been offered as a cautionary. However, as a
cautionary it says nothing about the impact of application aside from
the introduction of noise (ground temperature) which lies outside of
EZNEC's constraints. There would undoubtedly be issues of ducting,
much less diffraction at the air/space boundary - and these too are
within the domain of propagation modelers which is not what I perceive
EZNEC to be. However, propagation modelers do work from antenna
characteristics and it would seem this work is adequate to that
(anticipated) task. The propagation modelers I am used to seem to
expect ground considerations rolled into the antenna characteristics.

and in any case EZNEC's flat, infinite-extent ground model isn't representative of what
the signal would encounter in real life.

No doubt, but this still says nothing on which to hang a hat. The
valuations offered range from 10 to 22dBi. Are these values off by
3dB, 10dB, 100dB? Rather than having a good low angle response, the
actual response is straight up? What is the context of your warning?
If they are not representative do we have an unimaginable response?
If the EZNEC is sufficient for Rhombics at HF, certainly at VHF the
wavelength horizon is much further off and earth appears that much
flatter. Earth curvature exists for all applications and your
warnings would suggest no model is useful.

73's
Richard Clark, KB7QHC

Roy Lewallen wrote:
This analysis should be done in free space, not over ground.
Propagation to the moon should avoid reflection from the ground, and in
any case EZNEC's flat, infinite-extent ground model isn't
representative of what the signal would encounter in real life.

Very long rhombics and vees (for any frequency) cannot ignore the
existence of ground. On the contrary, they rely on it, so a model
including ground reflection is correct for this antenna. This includes
the use of these antennas for EME in former days.

Apologies to Richard for not responding to the antenna files that he
kindly sent across, a few days ago (other deadlines). The discussion has
moved on in the meantime, so it makes more sense for me to jump in again
here.

One other comment. As a dipole gets longer, the lobes move closer and
closer to the direction of the wire. Bending the dipole into a vee
shape aligns pairs of the lobes so they point in the same direction,
i.e., along the vee axis. That's why the optimum angle becomes less and
less as the wires get longer,

That would have been my first point in response to Richard's findings.
As a single end-fed wire gets longer, its radiation pattern becomes
predominantly X-shaped, with the four main lobes moving closer and
closer to the line of the wire itself. The optimum angle for a V-beam is
chosen to make the main lobes of the two wires overlap exactly, so they
reinforce as strongly as possible in the forward direction. If the V
angle is kept constant and only the leg length is increased, the maximum
obtainable forward gain will not be realized.

I don't have any of the classic references for optimum design of V-beams
or rhombics to hand; but unless Richard and I are both doing it totally
wrong, the V-beam does seem to show some reluctance to increase in gain
by a whole 3dB for each doubling of the leg length (and the rhombic
would do the same).

However, that doesn't detract from the reputation of the rhombic in
particular as "the king of HF antennas" - if you have the real estate
and can tolerate the fixed direction, the rhombic can give several dB
more forward gain than almost any other practical antenna.


All of this prompted me to try to model the 50-wavelength-per-side
rhombic that we used for EME, way back when. My recollection is that the
included angle was 12deg, but the original notes are long gone.

EZNEC+ v4 predicts the spectacularly narrow main lobe that one would
expect, and it also confirms the well-known finding that if you
terminate the rhombic at the far end, the pattern changes from
bi-directional to unidirectional but the forward gain also drops by
about 3dB. However, 30-40 years ago it was believed that it is not
important to terminate an extremely long rhombic "because most of the
forward-traveling wave has been lost to radiation before it arrives at
the far end." The model categorically negates that belief - even at
50wl/side, termination has much the same effect as for shorter rhombics.

Unfortunately the segmentation density in my model (7.5 segs/wl) is too
sparse to be confident about the absolute value of the gain. Normally
one should both increase and decrease the segmentation density to
confirm that the predictions remain stable; but this is not possible
because this enormous antenna has already used 1499 out of the 1500
allowable segments.

Since I can't be confident about the gain predictions, there is no point
in quoting and discussing them here - we already chase enough wild geese
in this ng.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Sorry, I don't have the time or inclination to get into another
protracted "last man standing is the winner" diatribe. I'll leave it to
the readers to evaluate what I've written and decide whether or not it
makes sense, or whether they'll choose instead to be persuaded by your
objections. Either is fine with me.

Roy Lewallen, W7EL

Richard Clark wrote:
On Mon, 12 Jul 2004 23:02:30 -0700, Roy Lewallen
wrote:

Hello Roy,

There are any number of problems with your comments:


This analysis should be done in free space, not over ground.


The application demands ground as an inescapable reality of design.
Designing in free space, unless you expand upon your commentary, is
meaningless. In other words, the product of a free space analysis
offers no more insight that this blighted version. Your comments that
follow fairly shout this as a wholly undecipherable problem.


Propagation to the moon should avoid reflection from the ground,


This has already been offered as a cautionary. However, as a
cautionary it says nothing about the impact of application aside from
the introduction of noise (ground temperature) which lies outside of
EZNEC's constraints. There would undoubtedly be issues of ducting,
much less diffraction at the air/space boundary - and these too are
within the domain of propagation modelers which is not what I perceive
EZNEC to be. However, propagation modelers do work from antenna
characteristics and it would seem this work is adequate to that
(anticipated) task. The propagation modelers I am used to seem to
expect ground considerations rolled into the antenna characteristics.


and in any case EZNEC's flat, infinite-extent ground model isn't representative of what
the signal would encounter in real life.


No doubt, but this still says nothing on which to hang a hat. The
valuations offered range from 10 to 22dBi. Are these values off by
3dB, 10dB, 100dB? Rather than having a good low angle response, the
actual response is straight up? What is the context of your warning?
If they are not representative do we have an unimaginable response?
If the EZNEC is sufficient for Rhombics at HF, certainly at VHF the
wavelength horizon is much further off and earth appears that much
flatter. Earth curvature exists for all applications and your
warnings would suggest no model is useful.

73's
Richard Clark, KB7QHC

On Tue, 13 Jul 2004 05:49:08 GMT, Richard Clark
wrote:

|On Mon, 12 Jul 2004 10:24:01 -0700, Wes wrote:
|
|Here are the results I obtained from a quick MultiNEC (NEC-2)
|analysis. This at 144 Mhz with height = 20', Sommerfeld Gnd, with
|average dirt. #12 AWG Al wire, 20 segments/WL, elevation angle = 4
|deg. Source on the middle of a short (3 segment) wire.
|
|Len. (WL) Ang. (deg) Gain (dBi)
|
|20.193 20 21.97
|
|Hi Wes,
|
|For this particular design (except mine is elevated 5 wavelengths), I
|pushed for 1 degree resolution, with a split source, with tapered
|segments (1023 all told) and I still fall short, but also well ahead
|of my earlier reports:
| 18.5dBi @ 3 degrees w/3 degree lobe width

Keep trying, you're getting closer [g].

On Tue, 13 Jul 2004 01:07:20 -0700, Roy Lewallen
wrote:

Sorry, I don't have the time or inclination to get into another
protracted "last man standing is the winner" diatribe. I'll leave it to
the readers to evaluate what I've written and decide whether or not it
makes sense, or whether they'll choose instead to be persuaded by your
objections. Either is fine with me.

Roy Lewallen, W7EL

Hi Roy,

Are you really that bored by your customers? You could have said 80%
less to equal the substance above, but instead you had time enough to
choose the editorial over the technical.

73's
Richard Clark, KB7QHC

On Tue, 13 Jul 2004 09:53:16 -0700, Wes wrote:


Keep trying, you're getting closer [g].

Your advice contradicts other advice we have been offered [g].

On Mon, 12 Jul 2004 23:02:30 -0700, Roy Lewallen
wrote:

|This analysis should be done in free space, not over ground. Propagation
|to the moon should avoid reflection from the ground,

Not true at all. "Ground gain" is routinely relied upon in EME,
allowing marginal antenna systems to succeed on a rising/setting moon
scenario.

http://www.bigskyspaces.com/w7gj/smallemestn.htm

http://www.qsl.net/oz1rh/gndgain/gnd...m#_Toc10586457

|and in any case
|EZNEC's flat, infinite-extent ground model isn't representative of what
|the signal would encounter in real life.

Not exactly, but often good enough, especially for comparative
purposes. The reflector at the other end isn't a smooth ball of green
cheese either but it still looks pretty smooth at rf. [g]

N7WS