Richard Fry wrote:
. . .
Statements on this newsgroup about the free space gain of a very short
doublet antenna being not much less than a full-size 1/2-wave dipole
might lead some people to conclusions different than shown in the FCC
chart.

Only if some erroneous statements were made here that I don't recall
seeing or if a person doesn't look at the chart carefully.

The chart shows that in the absence of ground loss (the "theoretical"
curve), the difference in gain between a very short vertical and 0.25
wavelength high vertical is 0.4 dB (300 vs 314 mV/m). This is
approximately the number I've quoted before on a number of occasions
(0.45, or less than 0.5 dB). I don't recall seeing anyone here ever
claim that this will be the gain difference of real antennas with loss
(the other curve) -- as the antenna gets shorter, the radiation
resistance drops, and so the fraction of power lost in the ground system
increases. The curve clearly shows how strong this effect is when using
the ground system adopted by AM broadcasters, and in fact the ground
system resistance can be inferred from the graph. This was first
described and shown in detail by Brown, Lewis, and Epstein in their
classic 1937 paper. Jerry Sevick, W2FMI, built and measured a number of
very short 40 meter verticals with elaborate ground systems to show that
the loss could be kept very modest in a real installation. His
experiments were published in QST in the '70s.

One shouldn't overlook another potentially large cause of loss when
using a very short antenna, either -- the matching network. Short
dipoles and verticals have a large amount of capacitive reactance which
requires an inductor to match, and loss in the inductor can become large
compared to the antenna's feedpoint resistance. But like ground system
loss, this isn't a loss inherent in the antenna itself.

The actual performance of a vertical can be made to approach the
"theoretical" curve arbitrarily closely with a sufficiently elaborate
ground system. The system the broadcasters have chosen isn't pefect --
it's simply one that's been deemed to be good enough for the job at
hand. Ground systems can be made which have lower loss.

What you have to always keep in mind is that if you put 100 watts into a
lossless antenna, 100 watts must be radiated. The lowest possible gain
of such an antenna is 0 dBi, which is what you get if that 100 watts is
spread equally in all directions. An infinitesimally short dipole has a
directional pattern which is similar to that of a quarter wave dipole,
just a little fatter. That's why its gain is a little less.

Even though we can't ever build a lossless antenna, it's useful to
understand them. It allows us to separate, and trade off as we wish, the
various factors affecting gain and loss.

Roy Lewallen, W7EL

The link below leads to a graphic on the FCC website showing the groundwave
field strength produced by 1kW from vertical radiators working against a
radial ground system of 120 quarter-wave radials; the typical AM broadcast
setup. The field strengths in the table do not include the affect of ground
conductivity. They are related only to the "efficiency" (an FCC definition)
of the radiator against the stated ground plane.

Field strengths in the FCC chart range are empirical values, and range from
~190 mV/m for a 0.05 wavelength vertical to ~641 mV/m for a 0.625 wavelength
vertical. This is a considerable range of field strength values for
matched, simple antennas all driven with the same tx power.

Statements on this newsgroup about the free space gain of a very short
doublet antenna being not much less than a full-size 1/2-wave dipole might
lead some people to conclusions different than shown in the FCC chart.

"FWIW."

http://www.fcc.gov/mb/audio/decdoc/s...3190fig8/2.gif

RF

Roy Lewallen wrote:
. . .
. . . An infinitesimally short dipole has a
directional pattern which is similar to that of a quarter wave dipole,
just a little fatter. That's why its gain is a little less.
. . .

I really meant "half wave dipole", although the statement is still true
as written.

Roy Lewallen, W7EL

"Roy Lewallen" wrote in message
...
Roy Lewallen wrote:
. . .
. . . An infinitesimally short dipole has a directional pattern which is
similar to that of a quarter wave dipole, just a little fatter. That's
why its gain is a little less.
. . .

I really meant "half wave dipole", although the statement is still true as
written.

Roy Lewallen, W7EL

The input impedance of a 0.05 wavelength monopole, over a perfectly
conducting ground plane, is
0.836 - j1331. Using a shunt "L" series "C" matching network -- inductor Q
of 300 -- indicates a
network loss of 9 dB.

Frank

"Roy Lewallen" wrote
The chart shows that in the absence of ground loss (the
"theoretical" curve), the difference in gain between a
very short vertical and 0.25 wavelength high vertical is
0.4 dB (300 vs 314 mV/m). This is approximately
the number I've quoted before on a number of occasions (0.45, or less than
0.5 dB).
___________________

However even when using the theroretical plot, the field difference between
0.25 lambda and 0.5 lambda radiators is 1.65 dB (380 vs 314).

Going from 1/4 wave to 5/8 wave the field ratio is even mo almost 3 dB
(440 vs 314).

I have seen it written on this NG that an improvement of antenna gain of as
little as 1dB is worthwhile.

RF

PS: I see I read the wrong side of the chart in my earlier post. Thanks
for your mercy g. Well, the point was right, even if the numbers were
wrong. /RF

Richard Fry wrote:
"---range from about 690 mV/m for a 0.05 wavelength vertical to about
641 mV/m for a 0.625 wavelength vertical.

On page 871 of the 1955 edition of Terman`s "Electronics and Radio
Engineering" is Table 23-1, "Directive Gain of Simple Antennas Relative
to Isotropic Radiator".

The directive gain of an elementary doublet, assumed to be
infinitesimally short, is given as 1.5. A resonant 1/2-wave wire is
given a gain of 1.64 in the same table.

Terman says on page 870:
"Directive gain depends entirely on the distribution in space of
radiated power."

So, the miniscule doublet puts 1.5 x as much power in its best direction
as does an isotropic. This is 1.76 dBi according to Kraus.

From Terman`s dB table on page 8, a power gain of 1.5 is a little less
than 2 dB gain. Kraus says 1.76 dB. A power ratio of 1.64 is more tha 2
dB, but less than 2.5 dB gain. The gain difference between a tiny dipole
and a 1/2-wave dipole is almost insignificant. Certainly it is less than
1 dB.

Kraus`s 3rd edition of "Antennas" has Figure 6-2 pn page 192 which gives
gains of common antennas:

An isotropic (uncommon) has a directivity of h1.00, and a gain of 0 dBi.

An elementary dipole has a directivity of 1.5, and a gain of 1.76 dBi.

a 1/2-wave dipole has a directivity of 1.64 and a gain of 2.15 dBi

A short monopole gas a directivity of 3 and a gain of 4.8 dBi.

A 1/4-wave monopole has a directivity of 3.28 and a gain of 5.2 dBi.

A 1/2-wave monopole has a directivity of 4.8 and a gain of 6,8 dBi.

With specific gain figures, we don`t need to characterize figures as
large or small. I just hope I copied them correctly. Better yet, get
your own copy of Kraus. It`s in print and well worth the price.

Best regards, Richard Harrison, KB5WZI

"Richard Harrison" wrote:
The directive gain of an elementary doublet, assumed
to be infinitesimally short, is given as 1.5. A resonant
1/2-wave wire is given a gain of 1.64. The gain difference
between a tiny dipole and a 1/2-wave dipole is almost
insignificant. Certainly it is less than 1 dB.
______________

This is all true, of course, but might lead some to the invalid conclusion
that it also applies to monopoles (whips) of 1/2 wavelength or less working
against a ground plane, such as in mobile operations or back yard verticals.

When a vertical radiator works against a perfect ground plane, the
electrical length of that radiator effectively is doubled. So in reality an
electrical "1/4-wave vertical" has twice the gain of a *1/2-wave* dipole
because of the image effect of the ground plane, and the fact that all
radiation is confined to one hemisphere (above ground). Note in your quote
from Kraus, p 192 that the short monopole and the 1/4-wave monopole each
have 3dB more gain, respectively, than the free space elementary (short)
dipole and the 1/2-wave dipole he also lists there.

Using your statements above the line, one might think that it rather
pointless to use anything longer than a 1/4-wave vertical. But going from a
1/4-wave to a 1/2-wave vertical in fact will add ~1.6dB of gain at the peak
of the pattern envelope, and a 5/8-wave vertical will add almost 3dB. These
are worthwhile improvements in system performance. Broadcast engineering
consultants have recognized this, and used it to advantage for decades.

Better yet, get your own copy of Kraus. It`s in print and
well worth the price.

Good advice (I already have it).

RF

Richard Fry wrote:
. . .
Using your statements above the line, one might think that it rather
pointless to use anything longer than a 1/4-wave vertical. But going
from a 1/4-wave to a 1/2-wave vertical in fact will add ~1.6dB of gain
at the peak of the pattern envelope, and a 5/8-wave vertical will add
almost 3dB. These are worthwhile improvements in system performance.
Broadcast engineering consultants have recognized this, and used it to
advantage for decades.
. . .

It's important to realize that the graphs you posted are for surface
wave field strengths. This is equivalent to far field strengths at zero
elevation angle over perfect ground.

Amateurs seldom communicate by surface wave, except for local contacts.
When the vertical is surrounded by real ground, attenuation of the sky
wave at lower angles occurs. One of the results of this is that the
antennas which concentrate energy more at lower angles end up losing a
greater fraction of the total radiated energy. This tends to decrease
the gain difference between a 5/8 and 1/4 wave vertical, for example,
over a typical sky wave path.

In the case of VHF/UHF mobile operations, which are essentially line of
sight, the finite size of most ground planes (e.g. a car top) can affect
the pattern considerably, again altering the gain difference between
various heights of verticals.

While there's an extensive body of well established and proven knowledge
in the broadcast industry, we have to be careful in applying it to
typical amteur communications. Often, the conditions are different (as
in this discussion, of surface vs sky wave propagation; or fixed vs
variable frequency operation), and the important criteria are different
(a few percent difference in coverage area is important to a broadcaster
because of its impact on advertising revenue, but a fraction of a dB is
seldom important to an amateur; a broadcast phased array can take a long
time to design and adjust, but amateurs want to switch or change
directions). So we can't just assume that an antenna or method that's
best for a broadcaster is best for us.

Roy Lewallen, W7EL

"Roy Lewallen" wrote:
Amateurs seldom communicate by surface wave, except for local contacts.
When the vertical is surrounded by real ground, attenuation of the sky
wave at lower angles occurs. One of the results of this is that the
antennas which concentrate energy more at lower angles end up losing a
greater fraction of the total radiated energy. This tends to decrease the
gain difference between a 5/8 and 1/4 wave vertical, for example, over a
typical sky wave path.
__________________

I investigated your concept statements using NEC-2 models of 1/4-wave and
5/8-wave verticals in the 40m band (7.3MHz), working against the same
infinite ground plane of "Average" parameters.

* The 5/8-wave vertical has a peak gain of 0.2dBi,
16 degrees above the horizon.

* The 1/4-wave vertical has a peak gain of -6.4dBi,
26 degrees above the horizon, and its entire radiation
envelope is always within that of the 5/8-wave.

I don't know which range of elevation angles is considered most useful for
skywave paths on 40m, but it would appear that with equal tx power, a
5/8-wave vertical always will have a usefully better skywave than a 1/4-wave
vertical over a typical ground plane -- and probably by more than 3dB.

If you could check my conclusions on this I'd be grateful.

RF

It sounds like you might have made the mistake of connecting a wire
directly to Sommerfeld or reflection coefficient ground. Doing this with
NEC-2 (or EZNEC) produces a resistance of unpredictable and meaningless
value at the connection point, lowering the indicated field strength by
an unpredictable amount. (EZNEC gives you a warning message when you try
to do this.)

EZNEC provides an option not available in NEC-2, a "MININEC-type"
ground. This functions as a perfect ground when calculating impedances
and currents, but uses the user-specified ground constants (conductivity
and dielectric constant) when calculating the pattern. It simulates an
antenna with a lossless ground system, allowing you to separately see
the effect of ground conductivity on the pattern without the magnitude
of the field being affected by changes in the ground system loss. The
best you can do with either EZNEC or NEC-2 if you want to include ground
system loss is to include radial wires just above the ground in the
model and connect the vertical to them. Then, however, any differences
you see will hold only for that particular ground system -- and, the
above-ground approximation isn't a terrifically accurate representation
of a buried system.

Using the MININEC-type ground with EZNEC (and only 10 segments, so this
can easily be done with the demo program) and starting with the Vert1.ez
example file, 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 (easily done by first
changing Units to Wavelengths) produces a maximum gain of 1.19 dBi at 15
degrees elevation angle. The 1/4 wave trace protrudes outside the 5/8
wave trace only from about 25 to 41 degree elevation.

But more interesting is the gain difference at various low elevation
angles. The comparison is easily done with EZNEC v. 4.0 by saving the
trace from one antenna, then superimposing that pattern on the pattern
of the second antenna. By clicking the name of the superimposed pattern
in the 2D plot window, a new entry appears in the data box showing the
difference between the two at the angle of the cursor.

It turns out that the 5/8 wave really shines at really low angles when
the ground is poor, but isn't so impressive when the ground is very good
-- at least at 7 MHz. Over average ground, the gain difference is at or
just above 3 dB up to about 10 degrees. (My explanation of the reason
for the difference over real ground was overly simplistic. I apologize.)
Above 10 degrees, the difference decreases. Over poor ground, the gain
difference is about 4.5 dB up to 5 degrees, and over 4 at 10. So if you
have poor ground, you can really benefit from a higher radiator. Over
very good ground, though, the difference is about 2 dB up to 5 degrees
elevation, only 1.2 at 10 degrees, and less than a dB at 12 degrees and
above. So it might or might not be worthwhile to extend the height of a
tower for that amount of benefit.

Those figures depend on frequency, too, and the pattern shape varies
considerably with frequency and ground characteristics. So 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.

Roy Lewallen, W7EL

Richard Fry wrote:
__________________

I investigated your concept statements using NEC-2 models of 1/4-wave
and 5/8-wave verticals in the 40m band (7.3MHz), working against the
same infinite ground plane of "Average" parameters.

* The 5/8-wave vertical has a peak gain of 0.2dBi,
16 degrees above the horizon.

* The 1/4-wave vertical has a peak gain of -6.4dBi,
26 degrees above the horizon, and its entire radiation
envelope is always within that of the 5/8-wave.

I don't know which range of elevation angles is considered most useful
for skywave paths on 40m, but it would appear that with equal tx power,
a 5/8-wave vertical always will have a usefully better skywave than a
1/4-wave vertical over a typical ground plane -- and probably by more
than 3dB.

If you could check my conclusions on this I'd be grateful.

RF