Dipole Antenna Radiation Patterns
"A dipole antenna in free space exhibits a feed impedance of 72 Ohms, and has a doughnutshapedradiation pattern. But, as the dipole is brought close to the earth, the radiation pattern changes, and the feed point impedance also changes. An awareness of these changes can help ease the task of setting up a dipole for communications. And, careful selection of the dipole's position above the ground can make its impedance exactly match the impedance of the feed line used.
For me, one of the
fun aspects of antennas is that they are still "magical". There is a lot
of art involved in their design, setup and operation,
compared to much of digital electronics, where it is almost literally "plug-and-play". But, at the same time, when you
are trying to make an HF contact, the "magic" can
quickly turn to frustration when things don't work as expected. Perhaps
this discussion will help reduce some of the
frustrations, let you make that rare DX contact, and bring back some of the "magic".
Much confusion exists about the
radiation patterns of antennas. In particular, the horizontal dipole is misunderstood by most who use it. It is
generally assumed that a horizontal dipole has a radiation pattern which aims most of its signal toward the
horizon, perpendicular to the line of the antenna. As we will see here, that is only true in one
particular circumstance, and, even then, not quite an accurate picture.
Note the multiple lobes at varying elevation angles above the horizontal. These lobes represent a division of the radiated energy, so that much energy is sent off at angles which may not be helpful to the particular communication in progress. In addition, the nulls represent angles at which no energy is transmitted. Depending on the height of the ionosphere, and the location of the distant target station, the null may exist at the precise angle necessary to effect the desired contact.
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There are fewer lobes and fewer nulls, but they are still present. In addition, two of the upper lobes have moved up and merged to produce a single, wide lobe aimed straight up.
Next is the dipole at 1.5
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The large vertical lobe is back, and the nulls and lobes are fewer, but still not the expected pattern. Next is the dipole at 1 wavelength high. By now, we should be noticing a sequence to the patterns. As we lower the antenna, the lobes move higher, combine and disappear.
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And, the dipole at .5 wavelength over the ground below.
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As we continue to lower the
dipole, we continue to see the now familiar raising and combining
of the lobes, until, at .25 wavelength
the lobes have combined to produce a single broad peak,aimed straight up. Clearly, this is not going to make for great DX communications, with most of the energy concentrated above the 30 degree elevation angle. But, notice that there is still considerable energy directed even as low as 15 degrees, the 6 db down point.
This is the radiation pattern which is normally used for NVIS operations. NVIS stands for Near Vertical Incidence Skywave. Below a certain critical frequency (which depends on the density of the ionization in the ionosphere), a signal directed straight upward will be reflected back down into an area near the transmitter. This can allow communications within a 100-200 mile radius of the transmitter, the area normally skipped over by signals transmitted from the dipole at lower radiation angles (generated by higher positions above ground). NVIS antennas are normally elevated at about .1 to .25 wavelength above the ground.
The next plot shows the dipole at
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Before we leave the NVIS
discussion, lets go back and look at the .75 wavelength elevation
pattern shown below:
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A dipole's impedance varies according to its height above the ground.
The following chart shows the variations in impedance as the dipole is lowered from .5 wavelength above ground to .1 wavelength.
For eachcalculation, the antenna's length was adjusted for best SWR, using 50 Ohms as the reference impedance. The calculated dipole was modeled using #12 wire for the elements.
The Best Dipole
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This is not the ideal
DX antenna, but it does provide good signal strength at reasonably low
angles (6 dB down at 20 degrees
elevation angle). It is essentially the same pattern as the .25
wavelength height dipole, but with the
benefit of being matched to 50 Ohms. In addition, it provides a very broad NVIS signal lobe. This
looks like a good compromise between both modes, and is fairly easy to set up on
frequencies above 10 MHz. At lower frequencies, the height above ground becomes a problem to achieve with
a reasonable support structure.
The HFp Dipole
80 Meters (3.6 MHz) 45.7 Ft. high
73, John, WB4YJT
©2003, The Ventenna Co. LLC (This article courtesy of Ventenna and John, WB4YJT and
slightly modified for presentation here.)
(This article courtesy of Ventenna and John, WB4YJT and
slightly modified for presentation here.)
Print Date: Apr 20, 2003
Ventenna Co. LLC
P.O. Box 2995, Citrus Heights, CA 95611