I am seeking opinions as to the performance of such an antenna

The opinion is it will be a great antenna.

The modeling facts a
It will give you approx. 11 dbi gain, and 13 db f/b. It will have a feedpoint
Z at resonance (7.025 MHz) of 33.3+j0.

The gain will vary from 11.24 dbi at 7 MHz to10.2 at 7.2
The f/b will vary from 12 db at 7 MHz to 9 at 7.2 (peaking at 13.6 db at 7.05
MHz)
The feedpoint Z will vary from 30-j6 at 7 MHz to 50+j33 at 7.2

The parameters of this particular antenna a
ht above ground 23 meters
Length of one side of driven ele = 10.3 meters
Length of reflector = 10.66 m
spacing = 6.7 m
angle between legs=120 degrees

There are of course hundreds (no, thousands) of different combinations of
these parameters that will give different performance characteristics. This
one is not necessarily optimized for any particular objective.but it is close
to what you said you want to build. Naturally I highly recommend that you
model your own antenna and see what changes affect what !! I'd be happy to
give you my 4NEC2 file that I used to determine the above. It and the program
are free. It would be a great starting point for you.

Interesting observation - I have made similar trial runs for others who have
inquired on this newsgrouop, and the tower reflector, and always offer to send
my file and offer to help get them started in modeling (10-20 minutes on the
phone with both of us looking at the same data in the program). To date no
one has either acknowledged my postings or accepted my offer!! Is modeling
that intimidating? I learned what little I know by hacking around for hours.
I could teach someone what I know in 10 minues if they were interested and
computer literate, short circuiting many hours off the learning curve.
Modeling is fascinating. All the tradeoffs that Roy just aluded to can be
studied easily on your own PC without taking the word of others. You can
change the angle, the height, the ground under, the dimensions,the size of
wire, the spacing. You can examine the gain, f/b, pattern, imedances, takeoff
angle, bandwidth.
And I will GIVE you the simple little file that you load into your own free
copy of the modeling program that allows you to do all of this.

Rick K2XT

Rick wrote:
I am seeking opinions as to the performance of such an antenna

The opinion is it will be a great antenna.

The modeling facts a
It will give you approx. 11 dbi gain, and 13 db f/b. It will have a feedpoint
Z at resonance (7.025 MHz) of 33.3+j0.

The gain will vary from 11.24 dbi at 7 MHz to10.2 at 7.2
. . .

Try deleting the parasitic element and see what the gain of just the
driven element is. The difference between this and the Yagi gain is the
gain relative to a single element at the same height. This is a more
meaningful measure of gain than dBi for this application.

Roy Lewallen, W7EL

On Mon, 12 Nov 2007 18:54:14 -0800, Roy Lewallen wrote:


Try deleting the parasitic element and see what the gain of just the
driven element is.

Ok, I did. The gain at 7 MHz went from 11.2 to 7.22 dbi, at 30 degrees
elevation angle.

Rick K2XT

On Nov 13, 8:05 am, (Rick) wrote:
On Mon, 12 Nov 2007 18:54:14 -0800, Roy Lewallen wrote:
Try deleting the parasitic element and see what the gain of just the
driven element is.

Ok, I did. The gain at 7 MHz went from 11.2 to 7.22 dbi, at 30 degrees
elevation angle.

Rick K2XT

That seems more reasonable... Heck, a NBS yagi with 3 elements
only gives about 9.4 dbi plus or minus.
MK

wrote:
Heck, a NBS yagi with 3 elements
only gives about 9.4 dbi plus or minus.

Free space?
--
73, Cecil http://www.w5dxp.com

wrote:
On Nov 13, 8:05 am, (Rick) wrote:
On Mon, 12 Nov 2007 18:54:14 -0800, Roy Lewallen wrote:
Try deleting the parasitic element and see what the gain of just the
driven element is.
Ok, I did. The gain at 7 MHz went from 11.2 to 7.22 dbi, at 30 degrees
elevation angle.

Rick K2XT

That seems more reasonable... Heck, a NBS yagi with 3 elements
only gives about 9.4 dbi plus or minus.
MK

That's the gain of an NBS Yagi in free space. It's more like 13 or so
over ground, depending on height and ground characteristics. Comparing
the gain of one antenna over ground to another (for example a dipole) in
free space is a trick often used by antenna manufacturers to fool naive
people. Both must be in the same environment for a comparison to be
meaningful.

Roy Lewallen, W7EL

On Nov 13, 5:16 pm, Roy Lewallen wrote:


That's the gain of an NBS Yagi in free space. It's more like 13 or so
over ground, depending on height and ground characteristics. Comparing
the gain of one antenna over ground to another (for example a dipole) in
free space is a trick often used by antenna manufacturers to fool naive
people. Both must be in the same environment for a comparison to be
meaningful.

Roy Lewallen, W7EL

I missed the part where he said it was modeled 23 m over ground..
But then you have to wonder about the ground quality used.
Thats why I always quote antenna gain using free space numbers..
I often forget that some people don't always do this... :-/
Gives a level playing field that works for anyone no matter where they
are.
I never quote gain numbers including ground effects unless I
specifically
mention it due to some reason. And thats very rare. So I tend to
constantly
think in terms of free space gain most of the time if I'm thinking or
talking
about antennas not mounted in any specific location.
One can always add the effects of ground themselves if they desire
that
for their local QTH.
MK

wrote:

I missed the part where he said it was modeled 23 m over ground..
But then you have to wonder about the ground quality used.
Thats why I always quote antenna gain using free space numbers..
I often forget that some people don't always do this... :-/
Gives a level playing field that works for anyone no matter where they
are.
I never quote gain numbers including ground effects unless I
specifically
mention it due to some reason. And thats very rare. So I tend to
constantly
think in terms of free space gain most of the time if I'm thinking or
talking
about antennas not mounted in any specific location.
One can always add the effects of ground themselves if they desire
that
for their local QTH.
MK

This isn't quite true.

A large fraction of the amateur population think only of Yagis when they
think of antennas, and often assume that whatever is true for Yagis is
true for all other antennas. It isn't.

Consider two horizontal antennas, such as a dipole and Yagi, modeled (or
constructed) in free space and at some height over ground. The free
space gain difference will be the same as the over-ground gain
difference only if the two have the same elevation pattern. Otherwise,
different fractions of the total field will reflect from the ground at
various angles, resulting in a different over-ground gain than
free-space gain. So knowing the free space gain doesn't necessarily tell
you what the gain difference will be when mounted over the ground.

As it turns out, both dipoles and moderate sized Yagis have broad
elevation patterns. As a consequence, the free space and over-ground
gain difference is about the same. So if you know one you know, pretty
closely, the other. And people often assume this is generally true for
all antenna types. But it isn't. A W8JK type array, for example, has a
decidedly narrower elevation pattern than a single dipole, so the
over-ground gain difference between the two isn't the same as the free
space gain difference. Worse yet, the maximum gain occurs at a different
elevation angle, so even if you point both a W8JK and dipole or W8JK and
Yagi in the direction of the same distant station, you might not see the
predicted gain difference because of the different elevation patterns.

Another problem arises when comparing antennas with different
polarizations. Vertically polarized waves are affected by ground
reflection quite differently than horizontally polarized waves. And the
nature of the ground reflection of vertically polarized waves is
affected much more by the quality of ground than horizontally polarized
waves. So the difference between horizontally and vertically polarized
antennas depends heavily not only on whether ground is present, but also
its quality and the heights of the antennas.

Not all antennas can be rotated like Yagis, either. The dB gain of an
extended double Zepp (EDZ) compared to a dipole is negative in most
directions, so that's what a user would experience when using one for
general communications. Yet you universally see the dB gain of an EDZ
with respect to a dipole quoted as a positive value. The moral of that
story is that when comparing antennas you can't rotate, a single value
for gain isn't adequate to describe the difference. You need to know the
gain in each direction for a meaningful comparison. (That's why antenna
modeling programs give you a polar plot of gain and not just a single
number.) And even if you can turn the antenna, you'll need multiple gain
values if the antennas have different elevation patterns.

When modeling -- or measuring -- antennas, if you want to know the gain
of an antenna compared to a dipole, say, the only way to reliably
determine it is to model -- or build and measure -- both and compare the
two. In the direction(s) of interest.

Roy Lewallen, W7EL