Open-sleeve principles VHF-Duoband principles Duoband-Yagi 6m/10m 10m-Dipole+ 3-El.- 6 m Triband-Yagi 6m/4m/2m   Duoband-Dipol 2m/70cm 2m/70cm 2+2/3 El. 2m/70cm 4+5 El. 2m/70cm 5+8 El.

Small Dualband-Antenna for the bands 6 m and 10 m

 

This is the antenna built

 by Bruno, F6GPC

Above a 5-El.-DK7ZB-Yagi

for the 2-m-Band

 

Details of the construction

by Bruno, F6GPC

 

Important is the fixing of the open-sleeve wire with a non conductive part for avoiding vibrations.

This antenna is a byproduct of the engagement with capacitive end loaded dipoles [1]. It is based on a shortened rotary dipole for the 10-meter band. In this antenna a 3-element wire beam for the 6-meter band is nested. In this way we have a dual-band solution with one feed point, which is 50 Ohm for both bands. This antenna has been developed  with the program EZNEC 5 [3] and is well tested.

Such a design has several interesting properties. For the 10-meter band with a reduced size to 2/3 of the half-wave dipole we have practically the same gain and only slightly lower bandwidth. The feedpoint impedance is now 50 Ohm and not the 72 Ohm of a full-size dipole. As a further addition we get the attachment points for a 3-element wire beam for the interesting 6-meter band. Power is indirectly fed here through an "open-sleeve" element of 2-mm wire, which was configured to have a second resonance at 50.150 MHz. The structure and the currents in the system with feeding at 50 MHz can be seen in Figure 1.

Fig. 2

Fig. 3
It is clear that the highest current occurs in the passively coupled wire (Wire 8) parallel to the 10-meter dipole (Wire 2) and the open-sleeve element is the actual radiator for 6 m. The horizontal pattern for 50 MHz  (Fig. 2) corresponds to that of a conventional 3-element yagi with the short boom length of 0.2 lambda.This gives a thoroughly respectable gain of 4.6 dBd and a F/B of 12 dB. At 10 m we have the bidirectional directivity pattern (Fig. 3) of the figure eight, as it is typical for a half-wave dipole.  

The mechanical construction

 
The listed numbers for each section refer to Figure 1. The middle piece consists of two 25x2 mm aluminum tubes (2), which are separated by a fiberglass tube. The distance is about 20 mm and is not critical. With the help of PP holders of NuxCom [4] and two pieces of aluminum angle it is fixed by an exhaust clamp on the mast.The exact structure can be found in Figure 4. At the centerpiece of the junction box is the connection to the balun, the contact is made through the bottom of the box with self-tapping screws and solder lugs.The ends of the inner tubes are shiftable (Fig. 5). So you can move the 20-mm tubes (1, 3) for fine adjustment and tighten them then with a hose clamp. At the end of these pieces is a 12-mm cross-drilled hole and in these the 12x1 mm aluminum tubes of the end capacity (4, 6, 5, 7) are fixed. Locking takes place with self-tapping stainless steel screws (Fig. 6).  

The reflector has a distance of 580 mm (relative to the center of the 10-m-dipole-element). To be drilled into the 12-mm-tubes of the capacitive load are 3mm holes and the 6-meter reflector is fixed with not lenghtening Kevlar or Dyneema rope. The same is done with the director, which is in 615 mm distance.The open-sleeve-wire in 85 mm distance is first tied with the  insulating rope on the 12-mm tube so that the attachment point is displaceable for the adjustment of the distance. Therefore one end is connected to the inner part of an insulation screw point. The wires must have exactly 2 mm in diameter and not be insulated.

Feeding the antenna
For balancing and common wave suppression a simple W1JR-balun is used [5]. The 2x 3 turns coax cable can be applied to a toroid core. High power up to 1 kW is possible with Aircell-5 and is shown in Figure 7 on an Amidon FT240-43. Up to 300 watts, you can also take the smaller and cheaper FT140-43, which is wound with Teflon coaxial cable RG188 (Fig. 8).

 In principle, it is of no importance for the blocking effect whether the 6 windings are arranged in the same direction or with 2x3 diagonally (as shown). Successfully tested was a choke from 10 turns RG188 on a PVC pipe with 20 mm diameter, too. This fits well into an electrical wiring box. It is recommended to solder a 2-Watt-resistor (metal oxid, 100 Kohm) across the connections to avoid statics in the system. The coax socket should be fixed on a retaining plate (Fig. 9) connected with the mounting brackets. With the mounting brackets you have a grounding to the mast.

Tuning the antenna

First, only the 10-meter dipole is built and brought to the desired resonant frequency by the 20-mm tubes which are displaced on both sides by the same amount. As a starting point can be assumed a length of approximately 10 mm on each side for a change of 100 KHz. It makes sense to tune it at 28.2 MHz. It will save you the subsequent correction, because the additional 6-meter wires raise the resonance around 200 kHz up. With an end adjusting resonance of 28.4 MHz both CW and SSB can be used in the segment up to 29 MHz with an measured SWR of 1.0 at 28.465 MHz. Even at 50 MHz, if the adjustment is optimal, the SWR remains in the range from 50 to 50.3 MHz below 1.2. These works should be checked 5 m above ground.

Then hang in the 6-m-wires and measure the SWR at 50.150 MHz. With minimal change in length of the open-sleeve-device (Fig. 10) and moving around in the distance +/- 10-20 mm an accurate SWR of 1.0 can be achieved. It is important for the wire to connect it with a strip of insulating material in the middle with the junction box. Otherwise the movement in the wind leads to a fluctuation in the adjustment, because the distance from the pipe is extremely critical. Following this rule gives a long-term stable SWR. 

Little moving of the reflector and the director wires has no consequences for the impedance and the SWR. The result can be seen in the Figures 11 and 12. The measurement was done with 8 m of Ecoflex-10 and a vectorial antenna analyzer.  

Results

The antenna is located now at DL2FBY and over a year in operation. The result exceeded the expectations on both bands. With an output of 15 watts, the Yagi produces the maximum of the allowed 25 watts ERP at 50MHz with the insertion losses and the antenna gain. During ES-openings QSOs all over Europe could be made, even 2-hop contacts to 4X and JY. At 10 m corresponds the result to a high-mounted and free dipole, here is the rotatable arrangement a positive bonus.

The small size of the dualband-antenna can be seen in relation to a 5-element-Yagi for 2 m with a boom length of 1,50 m. This 2-m-Yagi was developed by DK7ZB for KONNI-Antennas in Germany.

Table 1:  Dimensions of the Aluminium-Tubes und the Copper braids

Parts (Fig. 1)

Materials and Lenghts

2

2 x 290 mm, 25 x 2 mm Alu-Tubes (or 1’)

1, 3

2x 1370 mm, 20 x 1,5 mm Alu-Tubes (or 0,75’)+ additional 150 mm for shifting

4+6, 5+7

2 x 1240 mm, Alu-Tubes 12x1,0 mm (or 0,5’)

8

2-mm-Copper-braid, Length 2830 mm

9

2-mm-Copper-braid, Length 2960 mm

10

2-mm-Copper-braid, Length 2600 mm

Sources:

[1] Steyer, M. (DK7ZB): 50 Ohm reelle Antennenimpedanz dank gewinkelter Dipole, FUNKAMATEUR (61) 2012, Heft 4, S. 390-392 

[2] Steyer, M. (DK7ZB): Minibeam für zwei Bänder, CQ DL 5/2012

[3]: Program  EZNEC+ Ver. 5.0.58, Roy Lewallen (W7EL), P.O.Box 6658, Beaverton, OR 97007, USA  (e-Mail w7el@eznec.com), http://www.eznec.com

[4] Nuxcom Antennenbau, Attila Kocis Kommunikationstechnik, Am Berg 7, 96253 Untersiemau,Tel. (0 95 65) 616472, www.nuxcom.de

[5] Krischke, A. (DJ0TR/OE8AK): Rothammels Antennenbuch, 12. Auflage, Abschnitt 18.1.2.3, DARC-Verlag Baunatal