TWO TWEENBOOM ANTENNAS FOR 50 MHZ

Slobodan Bukvić, YU7XL, yu7xl@mts.rs; http://www.qslnet.de/member1/yu7xl/

If the length of a yagi antenna exceeds 6 – 7 meters, some mechanical problems occur, because the antenna boom will bend if not supported. More aluminum and more physical work is required to make it stronger. On the other hand, a 50 MHz yagi antenna of this length offers not so much gain. If you want more gain, you have to make antenna longer, or to  stack two antennas at a very big distance. Everything you do is painfull!

Maybe the best choice is to build a tweenboom antenna. Such an antenna is constructed in the way that two booms support each other. Actually, the tweenboom antenna consists of two antennas stacked at small distance. One more boom is added; it passes along the centerline between the two antennas and serves for reinforcement (and for aiming, too). It carries two elements only: reflector and radiator.

Tweenboom antenna has several benefits, if compared to classic yagis. Its horizontal radiation pattern is wider, but vertical pattern is narrower, what is ideal for DX-ing. Furthermore, closely stacked antennas (especially in the form of the twins), have usually bigger bandwidth. Because their booms strengthen each other, you can use smaller boom tube diameters. Maybe, at first sight, this antenna construction looks more complex then classic yagi, but it actually is not. Up to about 10 meters of boomlength, it is relatively easy to build. However, a different way in building should be taken, meaning that all antenna parts have to be mounted on a vertical mast from the beginning.

Their associated gain is not 2.5 – 3.0 dB over a single antenna, as for normally stacked antennas, but around 1.5 dB. However, the distance for best stacking, is  as much as 8 - 11 meters. The distance between the two halves in a tweenboom (in its middle) is usually 2.4 metres only.

Twinboom antenna has one more benefit: low antenna temperature. This ability is not considered as important on 50 MHz (and lower frequencies), where Sky and Earth temperatures are very high. Anyway, do not forget that the main point of low antenna temperature is: more energy is taken from side lobes and redirected into the main lobe. The result is: more gain. Therefore, if you compare two antennas with same gain but different temperatures, you will see that the lower temperature antenna has wider radiation patterns.

Having this in mind, the statement „gain is the only important...“ is not quite thrue. Well, low antenna temperature is much more important on higher, UHF/SHF frequencies, and for reception only (because the Sky and Earth temperatures are much, much lower), but their impact on antenna radiation pattern should not be neglected on lower frequencies, either, no matter if recepcion or transmission considered.

Here are two twins for 50 MHz. Below you can see the sketch (figure 1) of the first one, the 10- ele twin, which is 1.18 λ long. The second one has 12 elements and it is 1.58λ long. Its outlook is the same, just 2 yagi directors are added.

ANTENNA  X61012XL1Q

            The X61012XL1Q is a 10 element twinboom antenna, 1.18 λ long. On the sketches, figures 1 and 2, you can see how it looks like. The figure 4 gives its positions, and the figure 3 showing dimensions and shape of the radiating element. Note the 10 mm gap, where feeding cable is connected. In these points, the impedance is 200 Ώ, so that you have to use coax cable 50 Ώ, with simmetrizing  ½ λ balun.

            All elements are of aluminum tubing Ø12 mm. Elements are isolated from the boom.

            Antenna booms are of aluminum Ø25 – 35 mm.

            Antenna supports (see figure 2) are of aluminum tubing Ø12 mm.

            Beside this, you will need some aluminum plates 10 x 10 cm, 5 mm tick, with clamps, and that is all.

            The table 1 shows positions and dimensions of antenna elements. The table 2 tells us the performances of the antenna. And finally, the table 3 gives an interesting comparison: twinboom antenna X61012XL1Q versus classic yagi Y6912XL6. Both antennas have the same gain, but the length of the twin is 7.085 metres, and the length of the classic yagi is 11.705 metres. That means the classic yagi should be 3.62 metres longer (165%) than the twinboom – in order to produce the same gain.

ANTENNA X61212XL1Q

Everything previously said for the X61012XL1Q is valid for this, X61212XL2Q, a 12 element twinboom antenna. It is a bit longer, 9.465 metres (1.58λ), accordingly a bit more work should be taken.

The twinboom antennas can stack as any other antenna can. Best stacking distances: vertically 10.25 metres, horizontally 11.0 metres. Enough to discourage the ones that would like to try?