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Versione italiana

HF ANTENNAS


PRELIMINARY REMARKS

Among the problems arising when one is starting with amateur radio activity, would no doubt be the installation of an antenna. In fact the antenna is the bulkiest, but necessary, part of a radio station. Often there are problems with the neighbours who consider the antenna ugly or, even worse, a source of trouble for their household appliances, particularly in the case of a bulky directional aerial. Fortunately, in most countries, the law permits a radio amateur, within limits, to assert one's rights to install a suitable antenna, being this last necessary for his activity, besides ratified by international rules on the matter.

A great variety of aerials exist, from simple wires to more complicated structures, sometimes very cumbersome, allowing best efficiency of the station. In regard to my activity, the first aerials used were simple wires suspended between the top of a roof and nearby trees; the equipment built in 1947 and 1951 used such antennas. With the equipment built in 1973 I first used a "Levy" antenna, that is a center feed dipole, made of aluminium tubing. Beginning in 1983, I have devoted a great amount of time developing various projects of reduced size antennas, some of wich are briefly illustrated below.

WARNING: For the antennas shown on this page a minimal efficiency 50% and for the smallest one 25% is indicated; that determines how much of the applied power is used to radiate, the remaining part being dissipated as heat in the antenna itself. That is true if the antenna, having so small dimensions, has been mounted in ideal position so showing a radiation resistance near to the calculated one. An antenna of great dimensions cannot be always mounted in ideal position, so its efficiency can drop significantly. Remember that each point of the S scale of the receiver is equivalent to 6 dB so the 50% is equivalent to 1/2 point of the S scale and the 25% is equivalent to 1 point of the S scale. So I want to make clear that these antennas can't be considered absolutely inefficient.

All these projects are widely described in various issues of RadioRivista (see Articles) to allow building them by other amateurs.
RadioRivista is the official magazine of the ARI, the Italian Radio amateurs Association.


MONOBAND LOOP FOR 14 MHz

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Fig. 1
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Fig. 2
The picture in fig. 1 shows the last version of this loop, as built in 1986; the prototype of the same size, but with cylindrical coils, was built in 1983. The dimensions are 60 x 60 centimeters, the efficiency is about 50% compared with a full-size dipole, it can operate quite well also inside of an attic, provided that it lies more than 50 centimeters away from the wall and no metallic parts are used in the roof structure. The bandwidth of this antenna is very narrow, that's why a motorized tuning condenser has been mounted, so doing a remote control facility. The prototype was described in RadioRivista, in the issue of February 1985, while the last version appears in the issue of June 1989. In this last issue another loop is described, also for 14 MHz, whose outline is in the picture of fig. 2. Its prominent feature is that no motorized condenser is necessary, the bandwidth being large enough to operate over the entire 14 MHz band, only with the aid of a standard transmatch. The efficiency is about 75%; it was used, with very good results, from 1987 to 1989, when it was discontinued to create space for further experiments.


MONOBAND LOOP FOR 7 MHz

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Fig. 3
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Fig. 4
Two versions of this loop have been realized, the first (fig. 3), one's size is 125 x 125 centimeters, was realized in 1984 and is described in RadioRivista, in the issue of April 1987; in the same issue a remote tuning unit was described that will be explained further. The second (fig. 4), even smaller, only 80 x 60 centimeters, was realized in 1987 and is described in RadioRivista, in the issue of June 1989; a subsequent modification (already shown on the picture), is described in the issue of October 1996. The efficiency of the first version is about 50%, while for the second it is estimated about 25%; in spite of that, it is still possible to use this last one for long distance communications. Also these loops show a very narrow bandwidth, thus a motorized tuning condenser is necessary; they can operate also inside of an attic with the same restrictions as for the above-mentioned 14 MHz loop.


VERTICAL DIPOLE FOR 7 MHz

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Fig. 5
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Fig. 6
The prototype, as appears in fig. 5, was realized in 1989 and is described in RadioRivista, in the issue of February 1991. Even if the radiator is only 1.85 meters long, the efficiency is estimated to be nearly 50%, compared with a full-size dipole. Its bandwidth is large enough to permit operation over about 80 kHz in the 7 MHz band, with the aid of a standard transmatch. The project was subsequently revised and a final version is shown in fig. 6, that refers to the one built by the radio amateur IK 5 SES. The radiator length was increased to 2 meters.


VERTICAL DIPOLE FOR 3.5 MHz

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Fig. 7
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Fig. 8
This antenna was realized in 1993 and is described in RadioRivista, in the issue of October 1996. The pictures refer to the one built by the radio amateur IK 5 PWN. The radiator is 3.5 meters long and the efficiency is estimated to be nearly 50%. The band of 3.5 MHz being somewhat wide, it was necessary to develop a remote tuning facility, appearing in fig. 8. This project is a development of that of the above-mentioned 7 MHz dipole.


VERTICAL DIPOLE FOR 1.8 MHz

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Fig. 9
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Fig. 10
This antenna is probably the most interesting of the ones seen up to now, since it allows operation over the 1.8 MHz band, without the need of wide spaces; in fact, a dipole for this band would be about 80 metres long. The fig. 9 shows the prototype, working since 1991 with very good results. It has been installed in a particular way, with the bottom coil located inside the attic, thus allowing operation also over 3.5 and 7 MHz bands by shunting the coil, at the cost of little reduction in efficiency. Fig. 10 refers to the one built later by the radio amateur IK 5 PWN. The radiator is 6 meters long and efficiency is estimated to be nearly 50%, allowing easy long distance communications also with only 100 watt output power. Bandwidth is very narrow but no remote tuning is necessary, because the 1.8 MHz band, at least in Italy, is restricted as well. Also this project is a development of that of the above-mentioned 7 MHz dipole; the prototype is described in RadioRivista, in the issue of October 1992, while the one built by IK 5 PWN appears in the issue of October 1996.
Go to Activity on 137 kHz
See also LW Antenna Details


TWIN CROSSED LOOP FOR 14 TO 28 MHz BANDS

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Fig. 11
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Fig. 12
At last, an antenna that covers the bands from 14 to 28 MHz, with the aid of a remote controlled transmatch. Fig. 11 shows the antenna itself, while fig. 12 refers to the transmatch, with removed cover, and its control unit. This antenna consists of two crossed loops, that permit, by means of a remote controlled switch, a quick change from horizontal to vertical polarization. The dimensions of the loops are 150 x 70 centimeters, the efficiency varies from 50 to 75%, according to the band in use, operation is possible also over 7 and 10 MHz bands, but with a noticeable reduction of efficiency. The antenna is described in RadioRivista, in the issue of October 1996, while the transmatch was described in the issue of February 1991, being projected for another previous multiband loop.

REMOTE TUNING UNIT

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Fig. 13
This device was realized to allow remote tuning of the various antennas seen up to now that, because of the narrow bandwidth, need a retouch of tuning also for small changes of frequency. Its prominent feature is the possibility of tuning over an occupied frequency, without causing noticeable interference. It is described in RadioRivista, in the issue of April 1987 as already mentioned.


See also LW Receiving Loop

© Cesare Tagliabue I5TGC