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

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.

************************************************** ***********

You might even think you have one way propagation....

denny -the devil made me do it- k8do

Denny wrote:
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.

************************************************** ***********

You might even think you have one way propagation....

denny -the devil made me do it- k8do

Yes, from the dawn of time people have invented supernatural causes for
phenomena they're unable to explain due to inadequate knowledge or
analytical skills.

Roy Lewallen, W7EL

Roy Lewallen wrote:
Yes, from the dawn of time people have invented supernatural causes for
phenomena they're unable to explain due to inadequate knowledge or
analytical skills.

Like the miracle of reflected EM waves devoid of energy? :-)
--
73, Cecil http://www.w5dxp.com