Roy Lewallen wrote:
[...]
modeling the particular situation would be a good idea before doing
any expensive and extensive tower lengthening. In all the cases I
looked at, however, the 5/8 wave vertical did show some gain over a
quarter wave vertical up to at least 14 degrees. Whether the difference
is worth the added height is up to the individual.

Another height-related factor that may be worth considering is the
effect of surrounding the lower part of the antenna by nearby buildings.
Inside a typical 2-floor home are 3-D grounded meshes of electrical
wiring and (in many countries) central heating pipes. The wiring mesh
typically extends up to 6m/20ft above ground, which may be a significant
fraction of the antenna height.

For example, in the suburban situation here, the lower part of my
vertical for 40m was almost completely surrounded by these "scattering
objects" at distances ranging from 0.5 to 2 wavelengths. I never got
around to modeling the effects of these objects... though someone could
easily try it.


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

"Roy Lewallen" wrote:
... the gain of a resonant (~0.24 wavelength) high vertical at
7 MHz with "average" ground is -0.0 dBi at an elevation angle of 26
degrees. Changing the height to 0.625 wavelengths produces
a maximum gain of 1.19 dBi at 15 degrees elevation angle...

Thanks for your comprehensive, civil analysis. It appears either that my
incarnation of NEC-2 doesn't deal with this situation properly, or I didn't
use it right (the latter is more likely). I'll have a look into it.

In all the cases I looked at, however, the 5/8 wave vertical
did show some gain over a quarter wave vertical up to at least 14 degrees.
...Over average ground, the gain difference is at or just above 3 dB up to
about 10 degrees.

.... which supports my contention earlier in this thread: The peak gain
increase between a 1/4-wave and a 1/2-wave or 5/8-wave vertical is 3dB above
the gain differences of those antennas as dipoles of _twice_ that length in
free space.

Repeating the reasons for this...

* the electrical length of the vertical is doubled by its image below the
ground plane (a 1/4-wave vertical monopole becomes an electrical
1/2-wave dipole)

* the peak "free space" gain of the monopole and its image is increas-
ed 3dB, because all radiation from it is confined to one hemisphere
(above the ground).

RF

Richard Fry wrote:
* the peak "free space" gain of the monopole and its image is increas-
ed 3dB, because all radiation from it is confined to one hemisphere
(above the ground).

Remember, that's for perfect ground only (and maybe salt
water ground). If one buries half of a dipole in earth
ground, one loses most of that 3 dB to the ground.

For instance, EZNEC reports: The max gain of a 40m 1/4WL
vertical with 8 horizontal radials one foot above average
ground is -0.29 dBi. Raising the radials to one wavelength
above ground increases the max gain to +3.23 dBi. (Of course,
the 3D radiation patterns are not exactly the same but the
correlation to that 3 dB of image power is in there because
of decreased ground losses at increased height.)

Problem: Most everyone with a 1/4WL vertical and four buried
radials is throwing away about half of his/her source power.
Solution: Put up a horizontal dipole. :-)
--
73, Cecil http://www.qsl.net/w5dxp

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"Cecil Moore" wrote
Richard Fry wrote:
* the peak "free space" gain of the monopole and its image is increas-
ed 3dB, because all radiation from it is confined to one hemisphere
(above the ground).

Remember, that's for perfect ground only (and maybe salt
water ground). If one buries half of a dipole in earth
ground, one loses most of that 3 dB to the ground.
____________

According to the empirical results of AM broadcast radiators, and also Roy
Lewallen's EZNEC numbers in his last post in this thread, the ground plane
itself doesn't need to be perfect, or maybe salt water to realize the gain
improvement. It's just that a very low resistance connection to it must
exist for the vertical to work against.

RF

Richard Fry wrote:
According to the empirical results of AM broadcast radiators, and also
Roy Lewallen's EZNEC numbers in his last post in this thread, the ground
plane itself doesn't need to be perfect, or maybe salt water to realize
the gain improvement.

Point is, it has to be near perfect. 4 buried radials just
won't hack it.
--
73, Cecil http://www.qsl.net/w5dxp

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I've been away today and just discovered several postings I'd like to
comment on. I'll be in and out for the next couple of days, so won't be
able to carry on a very immediate conversation. I'll comment on this one
first.

The attenuation characteristics I've been talking about (attenuation of
primarily the low angle signal) isn't affected substantially by normal
ground systems. It's due to reflection from the ground well beyond the
boundary of typical radial wires. Increasing the number of radials will
improve the efficiency, which increases the size (actually, strength) of
the resulting pattern but not its shape. The ground reflection
phenomenon alters the shape, and this shape modification is a strong
function of the ground constants and frequency. Nothing you do with a
ground system of normal diameter will bring back that low angle
radiation. You need to move to the beach or go maritime mobile to get it.

When you model using a MININEC-based program or use EZNEC's MININEC-type
ground, you're seeing only the pattern modification due to ground
reflection. The ground system loss (that is, I^2 * R loss for currents
returning to the feedpoint via the ground) for that ground type is zero,
unless you intentionally insert a resistive load at the base of the
antenna to simulate ground system loss.

Roy Lewallen, W7EL

Cecil Moore wrote:
Richard Fry wrote:

According to the empirical results of AM broadcast radiators, and also
Roy Lewallen's EZNEC numbers in his last post in this thread, the
ground plane itself doesn't need to be perfect, or maybe salt water to
realize the gain improvement.


Point is, it has to be near perfect. 4 buried radials just
won't hack it.

Richard Fry wrote:

... which supports my contention earlier in this thread: The peak gain
increase between a 1/4-wave and a 1/2-wave or 5/8-wave vertical is 3dB
above the gain differences of those antennas as dipoles of _twice_ that
length in free space.

Things seem to be getting a little confused here.

When you replace a free space environment with a perfect ground plane,
the *average* field strength of *all antennas* increases by 3 dB for a
given power input because of the reduced volume. This shows up as a 3 dB
gain increase when the gain is referenced to a free-space antenna such
as an isotropic source. No antenna is given any additional advantage
over any other - they all get the same amount of increase. So if I read
the above statement correctly, it's not true. The gain increase between
a 1/4 and 1/2 or 5/8 wave antenna over a perfect ground is the *same* as
the gain increase between a 1/2 and 1 or 5/4 wave dipole in free space.
Not 3 dB greater. If you'll look at the patterns of the antennas, you'll
find that the pattern of a 1/4 wave vertical over perfect ground is
identical in shape to half the pattern of a 1/2 wave free space dipole,
but 3 dB stronger. Likewise for any other vertical and its twice-as-long
free space dipole counterpart.

When the perfect ground is replaced by real ground, an attenuation
factor is introduced which actually changes the pattern shape. This
pattern shape change is different for each height of vertical because it
depends on the angle at which the radiation from each part of the
antenna strikes the ground. The different antenna heights have different
current distributions and so different fractions of the total radiation
hits the ground at different angles. The effect of the attenuation at
each elevation angle depends on the ground constants and the frequency.

You're probably more used to looking at surface wave attenuation, where
this ground reflection effect doesn't exist. Instead, there's a single
frequency and ground dependent attenuation that's essentially the same
for all antenna heights. What I'm talking about here is sky wave
radiation which consists of both a directly radiated "ray" (which
undergoes no attenuation other than that caused by its expanding volume
with distance) and a "ray" reflected from the ground. It's the
attenuation and phase shift of this second "ray", which depends on the
elevation angle, ground constants, and frequency, which causes the
pattern shape modification and attenuation of low angle signals. If you
look into the way NEC-2 operates you'll see that it does just this
calculation. The relationship of the reflected ray before and after
striking the ground is described by a fairly simple reflection
coefficient, which is quite different for horizontally and vertically
polarized waves. If you assume a current distribution, it's not
difficult to calculate the pattern manually. The reflection coefficients
can be found in Kraus.


Repeating the reasons for this...

* the electrical length of the vertical is doubled by its image below the
ground plane (a 1/4-wave vertical monopole becomes an electrical
1/2-wave dipole)

I don't think that's a good use of the term "electrical length". It is
true that the radiation pattern of a 1/4 wave vertical over perfect
ground (but not imperfectly conducting ground) is the same as that of a
half wave dipole in free space. Also, its feedpoint impedance assuming
no loss is exactly 1/2 that of a 1/2 wave dipole in free space.


* the peak "free space" gain of the monopole and its image is increas-
ed 3dB, because all radiation from it is confined to one hemisphere
(above the ground).

Yes, but this is altered if the ground isn't perfect. When the ground
isn't perfect, the shape of the pattern of the monople is no longer the
same as half a free space dipole, so the gain difference is no longer a
constant 3 dB at all angles. Some of the radiated energy is lost in the
ground reflection, and the fraction which is, depends on the angle at
which it strikes the ground.

Roy Lewallen, W7EL

A good fraction of your power is lost to the ground even if you have a
perfectly lossless ground system. To find the amount, use EZNEC (the
demo program is perfectly adequate), select MININEC-type ground and
appropriate ground constants. Set the plot type to 3D and run a pattern
calculation. The "Average Gain" reported at the bottom of the main
window is the loss due to ground reflection. Short of making a ground
screen several wavelengths in diameter, all you can do to improve this
is to move to a location where the ground is more conductive.

For example, the example file Vert1.ez shows a loss of over 5 dB in the
total radiated signal.

Ground reflection loss applys to horizontal antennas, too. But with
horizontal antennas it primarily attenuates the very high angle
radiation, while with verticals it gets the low angle radiation. You can
easily see its effect by superimposing a plot calculated with perfect
ground over a plot calculated with MININEC-type (if there's a direct
ground connection) or Real, High Accuracy(*) (if there isn't) ground.

(*) For NEC-2 users, this is EZNEC-speak for Sommerfeld ground.
Reflection coefficient ground doesn't provide any real advantage with
modern computers, so it's no longer an option in EZNEC. In v. 3.0 and
earlier versions it was called "Real, Fast Analysis" ground.

Roy Lewallen, W7EL

Cecil Moore wrote:
Richard Fry wrote:

* the peak "free space" gain of the monopole and its image is increas-
ed 3dB, because all radiation from it is confined to one hemisphere
(above the ground).


Remember, that's for perfect ground only (and maybe salt
water ground). If one buries half of a dipole in earth
ground, one loses most of that 3 dB to the ground.

For instance, EZNEC reports: The max gain of a 40m 1/4WL
vertical with 8 horizontal radials one foot above average
ground is -0.29 dBi. Raising the radials to one wavelength
above ground increases the max gain to +3.23 dBi. (Of course,
the 3D radiation patterns are not exactly the same but the
correlation to that 3 dB of image power is in there because
of decreased ground losses at increased height.)

Problem: Most everyone with a 1/4WL vertical and four buried
radials is throwing away about half of his/her source power.
Solution: Put up a horizontal dipole. :-)