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From Longwire to Yagi Dipole (III) A
dipole is nothing less than a longwire cut in the middle and fed
by the center. Its overall length is 1/2l
or about 40 m for the 80-m band. One segment is directed to the ground, the other one
to your transceiver. Tight "flat-top" (horisontally over
ground) it is horizontally polarized and both segments are symmetrical. There are however many models of dipoles. We will review
the most well-know of them, from which most others are inspired.
Half-wave Dipole The half-wave dipole is the most common aerial that we usually find to amateurs who wish to preserve their budget, the visibility of their installation as well as the ease of the building. This is an harmonic antenna that prevents well interferences when it is tight horizontally. Its resonance length depends on the ratio of the length of the conductor to its diameter; the smaller is this ratio (using thicker wire), the shorter the antenna for a given electrical length. The aerial length calculated using the previous formula is cut at the centre and insulators are attached at the ends of segments. The impedance at the centre of this dipole being of about 70 ohms, it can be connected to either a flat twin feeder or a coaxial cable that insures a good impedance match and a maximum signal transference to the transceiver. Folded Dipole In this version the length of the wire is 1/2l as expressed in the formula but the end are folded to the center which ends are opened are connected to the feeder. The fact to fold the dipole transforms the input impedance by a factor of 22x70 = 280 ohms so that it can be fed with a 300-ohm ribbon feeder (open wire or "ladder line"), provided the wires forming the folded dipole are the same diameter.
Another solution is to use a 300-ohm ribbon feeder for both the aerial and the feeder. Only one conductor of the aerial is cut at the center, the feeder is inserted and the joints soldered. The junction should be clamped between pieces of isolating material and properly proofed. The ends of the aerial are shorted to close the circuit. In
a three-fold dipole you need a third wire at the center that will be
attached to the feeder and the ends linked to the external loop.
Here the input impedance increases to 32x70
= 630 ohms which provides a good match to an open line feeder or a
double ribbon using insolating spreaders. To read : All about Transmission lines The center part contained the feeder and the open parts of the dipole can be connected together using a home-made insulating block or better, using a T-connector that provides mechanical anchorage and watertight termination for the feed line. This kind of aerial provides optimum performance but its disadvantage is to be monoband. It can however complete an aerial installation constitued of high gain beams to work in the lower frequencies.
Multi-band dipole Most hams working on more than one band, they usually prefer using a multi-band dipole. If several dipoles are connected in parallel at their center and fed with a common feeder, they can work on several bands using a minimum of space. In this version of the half-wave dipole, each insulated end can be tight to any convenient support and the dipoles need not all be in the same plane. Using
the properties of electromagnetism, as we told before a dipole cut
to be at resonance at a certain frequency will also be at resonance
on its harmonics, for example at three half-waves higher, eliminating the need for an additional
aerial. So if you built a dipole for the 40 m band it will
be tuned for the 15 m band too. This is the way the famous G5RV
multi-band dipole works as well as all harmonic antennas. If well cut and placed high enough above
the ground (1/2l
- 1l
or at least 8 m high) to preserve the antenna takeoff angle, such a dipole provides an excellence signal.
Some hams say even that it performs better than
any vertical. I rather should say that it works another way with a different
radiation pattern, it capture surely less QRM due to its horizontal
polarization and constitutes an
excellent complementary antenna, specially for the low bands.
Another way to build a multi-band dipole when space is limited is to use traps, consisting in parallel tuned circuits inserted in the two dipole segments. Imagine you want to build an aerial suited for the 20 m and 15 m bands. On each segment (the ones running from the isolator to the feeder) insert a trap. The full length of your segment is at resonance on the 20 m band while the length going from the feeder to the trap is at resonance for the 15 m band. Traps should be designed to resonate at 21 MHz., isolating practically the end sections of the dipole from the feeder at that frequency. On 14 MHz, the traps would have a low impedance, and the whole come into use. Some trimming of the segments is however necessary compared to the calculated lengths to compensate for the effects of the traps. OCF, Windom, Carolina Windom, and W3DZZ Historically all began with the Windom, a multi-band antenna first described in 1929 by Loren G. Windom, W8GZ in QST for general coverage of HF bands. It is an hybrid antenna mixing properties of a dipole and a vertical. As shown at left, it uses a single wire feeder tapped on to an horizontal electric wire between 14-38% off-center, hence its name of off-center-fed dipole or OCF. Don't be surprise of the variable offset. It depends only on the required impedance, e.g. 300W or 450W. Since the impedance will also depends on the frequency, using the Windom across a wide frequency range becomes difficult and, on some models, WARC frequencies can be impossible to tune (VSWR > 10:1). Other drawback, both wire segments being unbalanced, the coaxial feeding the wire radiates RF up to the shack, and amateurs using this design must be careful using high power. Today the impedance matching is often achieved using directly a balun without feeder like the 40m long Windom sold by WiMo and displayed below that comes with a 6:1 ordinary balun in T-shape placed at 1/3 of the length and ended with a PL connector (SO-239). This model can be tuned on all bands from 160 to 10m including WARC.
How does it perform ? I used a G5RV dipole 31m long and a Windom 40m long, both tight 7m high. In fact the Windom radiation pattern is much more omnidirectional than the dipole one and I don't have noticed more QRM. From what I have experimented from 160 to 10 m, including WARC bands, I received better signal reports with the Windom and I confirmed countries I was never able to reach with the dipole. Tight in the E-W direction, from Europe on 20, 17 and 15m bands I reached without much difficulties and often almost at first call South America (HK, YV, PY), Middle-east (9K, A4, YI), Asia (4S, JA) and many VK stations. Globally working bare foot with 100 W PEP, DX signal reports were between 55 and 57. Of course with 100 W in a fixed wire antenna there were many DX (VR, XW, XU, etc) I was unable to work due to pileups. But results exceeded my expectations. I kept it, and sold my G5RV, Hi ! The Window is completed with a new version called the Carolina Window designed by Edgard Lambert, WA4LVB, and Joe Wright, W4UEB, that turns this potential disadvantage - the feed line radiation in the original design - into a potential advantage. The original design belongs today to Radioworks.
Along the feed line, the Carolina uses on top a matching unit 4:1 or 1:1 completed with a vertical radiator 6.6m long (20') for a 40m long antenna made of RG-8X or RG-58C followed with a current chocke balun acting as line isolator, preventing RF radiation along the coaxial to reach the shack as in the original design. Then an ordinary coaxial goes to the antenna tuner. The coaxial segment loacted between both baluns acts like a small vertical and gives to the Carolina Windom some more gain at low elevation and a more omnidirectional pattern than a dipole of the same length as it tends to fill in the deep nulls displayed at the four points of the compass as displayed below. Displaying usually an overall length of 40m (133'), all OCF and mainly the Carolina Windom work especially well between 3.5 and 30 MHz, including on WARC bands if well tuned. Its best performance are on 15 and 10 m bands. When tight horizontally in straight line at good height (over 8m high, like a dipole), users of the Carolina Windom are often very satisfied of its performance as they can work both local stations (thanks to the vertical segment) and DX, as good or better than using a dipole. Compared to a G5RV multi-band dipole used in the same conditions with the same length, users noticed that the Carolina Windom offered a signal strenght up to 10 dB stronger or 1.5 S-point. Little drawback, this vertical segment pick up a little more QRM that a true dipole tight horizontally. At last C.L. Buchanan, W3DZZ, was the first ham to be able to create a trap antenna for the first five-pre WARC amateur bands from 3.5 to 30 MHz. This is a dipole 32.4m long (108 ft) fed with a 70-ohm Twin-Lead. The dipole contains one trap on each segment at a distance of 9.75m (32 ft) from center. Traps are made of a parallel circuit constituted of a coil of 8.2 mH and a capacitor of 60 pF. After more than 60 years of trials and errors, multi-band antennas count today by tens of models and became true competitors. Of course as we explained in other pages, each trap inserted on an antenna reduces accordingly the efficiency of the system : the loss per trap is ranging between 0.2 and 10.5% (0.006 to 0.5 dB) depending of the frequency, the lowest the highest loss. Magnetic Loop This aerial is a compact antenna, mainly used to work with the longest wavelength between 0.5-2 MHz. Nowadays however we find magnetic loops for HF frequencies too.
This aerial consists of an aluminium ring of about 50 mm in diameter or made of 5 to 7 turns of wire depending the frequency (7 turns below 3 MHz, 5 turns for HF bands) tight around a framework 10 cm wide and about 1m of diameter (1/10l). A
cheap solution consists in manufacturing the
frame from a 6 mm thick plywood or any plate. The wires
should be wound very tight and should be kept that way (under
tension the wire tends to stretch slightly). The wood blocks merely
act as bracers and as support for the broom handle.
The magnetic loop forms a tuned circuit which capacitor provides a low impedance fed to the receiver. Among its disadvantages, the capacitor has to be tuned for each band but you can easily fit a slow motion drive to it or to wire a small value variable trimmer in parallel with it (10-20 pF). Then its bandwidth is shorter than the one of any other antenna. Among its advantages, the magnetic loop is highly directional and by rotating it you can virtually eliminate any interfering station. The tuning is also very sharp and its selectivity is excellent. This is a good choice if your outdoor space is limited to a balconery or a vasistas. But this antenna works also fine indoors, stand near a wall in your shack or in the attic. Due to its great mobility this kind of aerial is often used by the army too. But as we told in the first page, do not forget the potential radiation hazard if you place the loop too close from you. A good distance is over 6m. The gain of a magnetic loop is not as high, similar to the one of a good dipole, but it is more than outweighed by the much improved signal-to-noise ratio and the directional characteristics. The direction of a station can be determined within a few degrees by nulling it out to take its bearing. In that perspective it is useful to fix below the joystick (broom-handle) a recessed slot to prevent it slipping. Note that magnetic loop antennas also exist for receive purposes, like active models (powered in 12V) manufactured by Wellbrook in the U.K. Last chapter
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