High-efficiency antenna system at 2m for contests and tropics

The author of this article is Robi, S53WW. You can find the original in English on its website: http://lea.hamradio.si/~s53ww/. You could already read two great articles by Robi on this portal (preamplifiers S53WW and XVRT Javornik) – and the third is also excellent. Thank you Robi!

1. A few words about 2m contesting in the EU

Each serious 2m contestman must come before this question – what type of antenna is best for my contest QTH? A quick reply is printed – with as much profit as possible! The answer is correct only for those stations, which are 800km or more away from the locations with the greatest activity. These are mainly DL and OK, then locations extending outside Central Europe. But if your contest QTH starts in several directions, which bring a significant amount of points (a large number of QSO*QRBs), then the most important feature of the antenna used is the width of the main radiation lobe for -3 dB.

The figure shows a more or less common directional connection diagram for a typical contest QTH in S5 (Alps in azimuth 300°, The Balkan Mountains and the Adriatic Sea in azimuth 120 – 180°). You can tell from the statistics on the left side of the picture, I guess 60% points are obtained from three directions, which have a width of about 30° (this statistic is calculated by the program VHFSTAT.EXE of data in EDI format, conversion from EDI to QSO/TST format is possible with VHFLIB.EXE. According to this diagram, it is possible to definitively determine, that it is necessary to have two antenna systems – one with a lobe width of 30° and the other with a lobe width of 75°. At this point the question arises of interference caused by nearby stations when using a 75° lobe antenna. It is important to use an RX with excellent dynamic range (which of course excludes preamps). The deployment of home made RX appears to be the best, but also the XVRT+HF RIG composition with a noise figure around 2,0 dB is OK. Total assembly noise (coaxial cable attenuation plus RX noise) doesn't have to be under 2,0 dB, if the gain of the transverter exceeds the dynamic range of the receiver (turbo deluxe XVRT and super HF RIG are demoted to mid-range – otherwise completely linear preamplifier – is included for them!).

A common solution is to install two or more antenna systems. It has a second one for effective use (third etc.) antenna system to have the same performance as the main one (similar gain and performance +/- 3 dB). If more than two antenna systems are used, is a normal RX/TX arrangement (while the power of one TX is divided between two antennas and one RX is switched between two antennas), which is not an optimal solution today. The time to switch between three or more antennas is too long and during that time we can completely overhear the caller. At least two independent receivers are required for effective use of three or four antenna systems (of course with the second operator). If all RX operators can also broadcast, it is the most optimal solution, although not essential.

The minimum gain of the antenna for the tropo contest is approx 16 dBi. This gain is achievable with a long yagi (4 lambda), but its lobe width for -3 dB (in the E plane) is less than 30°. As well as achieving azimuth coverage in latitude 50 – 70° and profit more than 16 dBi? The answer is I-stack (vertical system) short yagi antennas.

2. I-stack fours 4 the. yagi with maximum gain

After a thorough review of our contest QTH (S59DEM/JN75DS) according to the direction diagram of the connections, I decided to build a vertical system of four short yagis, which have a main lobe width for -3 dB asi 50°. To achieve the desired gain, I chose the yagi version with maximum gain. The maximum gain is when the antenna impedance is below 50 ohms. With a fixed impedance of the antenna considered 12,5 ohm can be obtained 50 ohm using a folded dipole and a simple coil balun. In that case, it is possible to build an antenna, which has an impedance 50 ohms, profit 11,3 dBi, lobe width 50° in the E plane and length 1600 mm (0,8 lambda). Simulated It's at in the H plane you can see the directional characteristics in the attached images.

Fig.2 – characteristic of stacking two 4 el.yagi with maximum gain versus vertical distance. Boom is from Al profile 20×20 mm, elements from Al rod diameter 5,0 mm are isolated from the boom (boom-e correction is 2,0 mm). Plastic holders (home made) were used as element insulators. This technology turned out to be mechanically less stable, so the elements and insulators were later glued with boom two-component glue.

The downside of short maximum gain yagins is the narrow bandwidth (144 – 145 for PSV 1:1,5) and a poor front-to-back ratio (for example 12,5 dB for this design). Since the use of one antenna is not intended, but of the antenna system, the F/B ratio problem can be effectively solved by appropriate stacking. The proposed system also improves PSV (thanks to the compensation of the reactive component) – PSV 1:1 in its entirety 143 – 146 MHz. Individual antennas can be horizontally spaced lambda/quarter time and fed in the correct phase. In that case, F/B is the ratio 25 to 30 dB . Vertical stacking is in the picture 3.

Fig.3 – dimensions of an I-stack of four 4 el.yagin. notice, that the vertical separation of the antennas is at least 2.1m and can be increased to 2,4 m with a gain increment of 0.1dB/0.1m. The positions can be seen in the upper part of the sketch, in which the mast intersects with the boom. The top and bottom antennas are attached to the mast between D1 and D2, the middle antennae are shifted by 520 mm forward. For this reason, the cables feeding the middle antennas should be shorter by the electrical length lambda/4 (i.e, that the cables should have a length of 520mm*speed_factor (340 mm for cables with solid PE dielectric and 360 mm solid PTFE).

Dipól (in fig.4) is a classic folded dipole with the same dimensions as the DJ9BV antennas, but with length 900 mm a Wonderful article it is formed by a coil of RG-188/RG-316 cable (obr.5). The dipole is connected to the boom directly (the dipole tube is in the middle along the length 25 mm pressed and drilled. The boom is fixed with a suitable screw.). The same technique is used to connect the balun to the dipole.

Fig.5 – detail of coil balun: the seven-turn coil is wound from 500 mm long thin Teflon cable (e.g.. RG-188) on a plastic body with a diameter 18 mm (plastic pipe for home installations) with pre-drilled holes at a distance 22 mm, which serve to fix the cable. The N-connector is directly connected to RG-188.

Finished balun it is covered by an electrical wiring plastic box. A detail of the dipole with the balun is shown YOUR. The author with an antenna at the Ljubljana Faculty of Electrical Engineering is YOUR.

This antenna system has proven itself in contests (S59DEM/S55A/S53WW) in the last two years compared to one 15 the. DL6WU. Our main antenna system consists of two 15 the. DL6WU stacked vertically. As bad as 3 dB more gain (4×4 has a longer power supply, approx 0,5 dB and only RG-213 as stacking cables (from the old system 4×6 the.)).

Does, S53WW
http://lea.hamradio.si/~s53ww/
Slovak translation by Viliam, OM0AAO