The BVO 2m yagi series de DJ9BV

Traduction francaise, David, F5SDD

If you are interested in weak signal operation you need to have both reasonable power and listening capabilities. The power thing is not that much easy, especially during portable operation so I decided to emphasize my ability to hear weak signals and concentrate my little RF power where the DX is located.

After searching a long time for good designs I found the BVO series by DJ9BV published in 'DUBUS Technik V' in 1998.

These antennas are the most recent result of a long history of antenna design and they have been used across europe by many hams with good results.

The antenna I have used before was a BV 2.1 l, a redesigned DL6WU antenna likewise by DJ9BV which I found very satisfying (I realized my 1st QSO to G on 2m with ~20 W, not very easy here in JN48).

These antennas provide excellent datas and the datas are published and confirmed by many other hams in different countries.

To keep it still transportable I chose the smallest design - the BVO-2wl. With one antenna you get good gain (12 dbD), the lenght of 2 l (4,26m) is still handy. In the future I'm planning to upgrade to the next bigger version - the BVO-3wl a real winner in the world of DXing - it is possible with one (!) of it realized as a cross plane yagi to do regular EME !

BVO2-2wl

Mechanical construction:

Elements

The original antenna datas using solid 5mm aluminium rods for the parasitaric elements. For this diameter I couldn't find any element holders so I ask the designer DJ9BV for recalculating the antenna using 4mm elements. In this case one can use the well proved method with the nylon rivets used already in the former BV yagi series with great success.

The elements were hold in place by two rivets, so the aluminium rods are mounted isolated through the boom. A method that provides long term good and reliable results, is well and often tested and is a quite simple method for fixing the elements. For more details take a look at detailed photos of element mounting or visit the Long Yagi work shop of G3SEK. where he also described this method of element mounting, among others.

Driven element

Due to the near 50 Ohm intrinsic impedance the realization of the driven element can be done in differrent ways. The designer DJ9BV recommends the classic folded dipole with a 4:1 transformation as a mechanically and electrically reliable solution. This has be done very successful also in the former BV-yagi series. Although there could be some matching difficulties in highly optimized/frequency range limited yagis (like the BVO series).

The near 50 Ohm impedance allows also another more simple style of DE: as pointed out in the DUBUS magazine 4/98 by Graham, F/G8MBI you could also use a simple straight split dipole and connect the feeding line directly to the antenna. This solution has several advantages:

• no additional losses in a transforming balun

• no less garantee for a balanced, straight pointing antenna (unequal current distribution also possible with a folded)

• no interaction between the balun in the yagi structure and the elements

• less weight, wind load and mechanical complexity

• less difficulties in achiving a good match in 'narrow banded' (=highly optimized) designs

The two legs of the dipole are fixed isolated on the boom with a gap of 10 mm. There you can directly connect your feeding line and run it on the boom towards the pole where it can go down without influencing the antenna. If you need a RF-socket for testing purposes etc.you can connect it with very short wires.

Start with a longer dipole of maybe 970mm and test it. It is easier to cut than adding length... .

Symmetric DE and unsymmetric feeder

At VHF and UHF the outer of the cable offers a high impedance path and especially if the feeder is taped to the boom tightly and frequently, there will be no or very little current on the braid. If you have problems you can spend a balun or you ground the coax to the boom at distances of lambda/4 and odd multiples.

For home builders I would never recommend T-match/Gamma-match. They are difficult to handle, limited in frequency and they require high quality materials and are mechanically more complex. Probably you will get undetected losses and you tried so hard to catch the µ-volts.

Boom material and support

For weight reason I choosed a square aluminium tube 20 x 20 x 1,5 mm, like I did with all my 2m yagis before. It is a good compromise in terms of stability and weight. Holding the antenna in place you can use a cord support as a riser or a 'trombone' style aluminium support (not with a cross yagi of course, in this case only a cord support!)

Element boom correction

For different boom diameters you have to add the following amount to the free space lenght:

The mechanical datas of BVO2-2wl (4mm parasitic elements and 8mm driven element)

For the non-metric world using 5mm = 3/16" diameter take a look at DUBUS Technik V or send me an e-mail for the measurements.

Electrical datas (at 144,1 MHz) by means of NEC II (DJ9BV)

• very low internal losses (< 0,1 db/6K)

• 45 - 55 Ohm input impedance (180 - 220 folded dipole)

• F/R (90° - 270°) > 23 db

• Gain: 12,1 dBD

• E-Aperture: 35,4 °

• H-Aperture:38,8°

• E stacking distance: 3,36 m

• H stacking distance: 3,16 m

• stack gain: 5,97 db

You may have a look on the horizontal and the vertical diagrams, calculated with AO 6.59 de K6STI.

Other users of BVO2-2wl yagi

BVO2-3wl

the mechanical datas of BVO2-3wl ( with 4mm elements and 8mm DE )

for the non-metric world using 5mm = 3/16" I took the datas of DUBUS Technik V in a table:

Take a look at another BVO2-3wl realized by Juergen, PE1GEP, who built a pair of cross yagis for EME use.

Electrical datas (at 144,1 MHz) by means of NEC II (DJ9BV)

• very low internal losses (< 0,1 db/6K)

• 45 - 55 Ohm input impedance (180 - 220 folded dipole)

• F/R (90° - 270°) > 23 db

• Gain: 13,4 dBD

• E-Aperture: 30,8 °

• H-Aperture: 32,8°

• E stacking distance: 3,9 m

• H stacking distance: 3,7 m

• stack gain: 5,98 dB

You may have a look on the horizontal and the vertical patterns, calculated with AO 6.59 de K6STI.