On Thu, 29 Dec 2005 09:53:21 -0800, Roy Lewallen
wrote:

Bob Bob wrote:
And to complicate matters further the max antenna height being talked
about being around 3-4 wavelengths off the ground is going to break the
radiation pattern up into a number of lobes at various vertical angles.

For that reason alone I think it is still worth modelling. There will be
specific heights where the horizontal radiated component is at a maximum
and this is likely the most desirable. . .

Modeling will show that at great distances from the antenna, the
horizontally radiated field is zero from any horizontally polarized
antenna over ground.

Roy Lewallen, W7EL

Please see http://www.cebik.com/vhf/ex6.html on the topic.

Near the end of the article he gets into the horizonatally polarized
less than 1WL interrupted loops typical of 6m (halos, squares and
triangles).

For more, he also has an article on HOHPLs (horizontally oriented
horizontally polarized loops 1WL and greater).

Worth reading and observing the models used.

Allison
Kb1GMX FN42HH

True Roy

I should have said something like "approaches" the horizontal

Apologies for the mis-speak

I am also going to have to look into the theory of how much VHF
diffracts and bends so one can choose the right angles grin..

Cheers Bob

Roy Lewallen wrote:

Modeling will show that at great distances from the antenna, the
horizontally radiated field is zero from any horizontally polarized
antenna over ground.

Roy Lewallen, W7EL

Modeling will show that at great distances from the antenna, the
horizontally radiated field is zero from any horizontally polarized
antenna over ground.

Roy Lewallen, W7EL

"Bob Bob" wrote
True Roy. I should have said something like "approaches"
the horizontal. Apologies for the mis-speak.
___________

Pending some clarification from one of you (or anyone else) as to what is
meant here, I point out that h-pol radiation directed toward an elevation
at/near ground level has been, and still is the basis for very successful
commercial broadcasting by FM and TV stations.

RF

Roy Lewallen wrote:

Modeling will show that at great distances from the antenna, the
horizontally radiated field is zero from any horizontally polarized
antenna over ground.

I need to clarify this. It's based on a theoretical model which doesn't
apply in many practical cases. It assumes that the signal reaches the
destination by two paths, direct and via reflection from the ground;
that the ground is perfectly flat at the point of reflection; and the
destination is very far from the source. The lower the elevation angle
being observed, the farther the destination has to be for this effect to
occur. For example, consider the field strength at an elevation angle of
one degree from an FM transmitter whose antenna is 1000 feet above the
average terrain. Neglecting Earth curvature (which probably shouldn't be
neglected in this case), at a very distant point, the reflected signal
which will interfere with the direct signal strikes the ground at a
point about 11 miles from the transmitter. The receiver would have to be
more than 22 miles away (and of course, higher in elevation than the
transmitting antenna) for reasonable cancellation to occur. At a half
degree elevation angle, the receiver would have to be twice that
distance; at a quarter degree, four times, and so forth. Full
cancellation at zero elevation angle would occur only at an infinite
distance. In many practical situations, you can safely assume that the
potentially interfering ground reflection takes place beyond the
receiver, so a better estimation of received signal strength can be
obtained by looking at the free space pattern.

Roy Lewallen, W7EL

Roy, I prefer my radiation patterns to be functions of the antenna
itself. After all, that's what it's all about.

To determine the field strength at a distance I prefer not to take any
notice of Eznec's misleading ideas on the state of the ionosphere or
groundwave loss and do my own calculations. Or I can follow up Eznec
by using a program dedicated to either groundwave or ionospheric
propagation.

I agree preferences will differ. But it is important to understand
exactly what radiation patterns do or do not mean.

Please do not take this note as any criticism of Eznec. It is a
valuable (and free) calculating resource. But, as you demonstrate, it
is difficult to describe in a few words exactly what it does.

Please simplify!
----
Reg.

Reg Edwards wrote:
Roy, I prefer my radiation patterns to be functions of the antenna
itself. After all, that's what it's all about.

That's just fine. All antenna modeling programs I know of allow
selection of a free space environment, which gives you just what you want.

To determine the field strength at a distance I prefer not to take any
notice of Eznec's misleading ideas on the state of the ionosphere

EZNEC makes no assumptions about the state of the ionosphere, or even
its existence.

or
groundwave loss and do my own calculations.

The only assumption EZNEC makes in its far field analysis about ground
wave loss is that at an infinite distance it's infinite. The
professional EZNEC programs, which do directly report far field ground
wave signal strength when requested, use the Norton approximations
(implemented in NEC) which are widely accepted.

Or I can follow up Eznec
by using a program dedicated to either groundwave or ionospheric
propagation.

I agree preferences will differ. But it is important to understand
exactly what radiation patterns do or do not mean.

With that I agree wholeheartedly.

Please do not take this note as any criticism of Eznec. It is a
valuable (and free) calculating resource. But, as you demonstrate, it
is difficult to describe in a few words exactly what it does.

Please simplify!

First a note, EZNEC is not free. It's copyrighted, commercial software.
Only the EZNEC demo program is free and can be copied and distributed
freely.

Like other powerful tools, EZNEC requires some effort on the part of the
user to understand its use. If you're not willing to make that effort, I
suggest that you not use it, but find (or write) a program that suits
you and your limited willingness to learn. [Give NEC-2 or MININEC a
try!] All types of EZNEC, including the demo program, include a
comprehensive and extensively indexed manual. I also recommend the ARRL
antenna modeling course, which numerous users have made positive
comments about. But both of those are useful only to people who want
more than to get a quick answer without taking any time to consider what
the answer means.

I'll leave it to you and others to produce simple programs which do one
specific, simple thing. There's a place for those, but also a place for
more versatile programs like EZNEC which unavoidably require a bit more
effort to fully use.

Roy Lewallen, W7EL