THE HF AERIALS OF
I 5 T G C
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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; all these projects are widely described in various issues of "RadioRivista", 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

The picture at left 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 wand 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 at right. Its prominent feature is that no motorized condenser is necessary, the bandwidth being large enough to operate over the entire 14 Mc 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

Two versions of this loop have been realized, the first (picture at left), 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 (picture at right), 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 Mc loop.




VERTICAL DIPOLE FOR 7 Mhz

The prototype, as appears in the picture at left, 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 Kc in the 7 Mc band, with the aid of a standard transmatch. The project was subsequently revised and a final version is shown in the picture at right, that refers to the one built by the radio amateur IK 5 SES. The radiator length was increased to 2 meters; the description of this version is in progress and will be published in a future issue of RadioRivista.




VERTICAL DIPOLE FOR 3.5 Mhz

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 Mc being somewhat wide, it was necessary to develop a remote tuning facility, appearing in the picture at right. This project is a development of that of the above-mentioned 7 Mc dipole.




VERTICAL DIPOLE FOR 1.8 Mhz

This antenna is probably the most interesting of the ones seen up to now, since it allows operation over the 1.8 Mc band, without the need of wide spaces; in fact, a dipole for this band would be about 80 metres long. The picture at left 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 Mc bands by shunting the coil, at the cost of little reduction in efficiency. The picture at right 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 watts output power. Bandwidth is very narrow but no remote tuning is necessary, because the 1.8 Mc band, at least in Italy, is restricted as well. Also this project is a development of that of the above-mentioned 7 Mc 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.
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TWIN CROSSED LOOP FOR 14 TO 28 Mhz BANDS

At last, an antenna that covers the bands from 14 to 28 Mc, with the aid of a remote controlled transmatch. The picture at left shows the antenna itself, while the picture at right 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 Mc 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

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.




Cesare Tagliabue I 5 TGC
Updated: September 2007


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