Small Duoband-Antenna

Sometimes a small two-band directional antenna is the first choice for portable operation or for the balcony under restricted conditions. It is not always possible to accommodate a larger directional antenna. Especially with vertical polarization, the space requirement is small and there may still be an installation option within an existing antenna system. I think with the dimensions presented here, it is certainly the smallest directional antenna for the two amateur bands described so far.

The antenna presented and shown in Fig. 1 is often carried in my car to be QRV with a fishing rod as a support mast. It is a 2-element Moxon yagi for the 2-m-band, which has been supplemented by two further elements to form a dual-band directional antenna for the 70-cm-band. The additional elements are only parasitically fed, the 70 cm radiator is used as an open-sleeve element. The development goal was to have a directional antenna that was as small as possible and that needed only one feed point and only one coax cable.

Physical basics

The 2-element moxon structure for the 2-m-band was created with the help of the small program "Moxgen" [1] by Dan Maguire, AC6LA, and subsequent processing with EZNEC +6. The interesting properties of the Moxon beam were discussed in detail in [2]. It has a gain of almost 4 dBd and an excellent f/b of 35 dB.

The 2-m-Moxon basic structure also has a 3/2-lambda resonance at 70 cm. However, there are three forward lobes that can be bundled into a single lobe when an open-sleeve element is skillfully positioned for 432 MHz. Figure 2 shows the currents when excited at 144.3 MHz, as expected, hardly anything is induced in the 70 cm elements. It looks different at 432 Mhz (picture 3). It can be seen here that the two outer current maxima on the 2 m radiator (wires 1, 2, 3) are suppressed quite well and the current maximum occurs on the open-sleeve element 8. The numbering of the individual sections is referred to in EZNEC as "Wires", this is also indicated in the dimensions.

First of all, I tried to move the 70 cm open-sleeve element inside, which functions as a virtual radiator. This would not have to prolong the boom. Unfortunately the result was not as good as the expectations, which is why this element was installed in the beam direction in front of the Moxon frame. An additional parasitic reflector brings more gain and a better f/b ratio.

As with the duo-band yagis developed by me for the 2 m and 70 cm bands, the length and distance of the 70 cm open-sleeve element also differ from the dimensions calculated with EZNEC, and the bandwidth is considerably larger than predicted by EZNEC. I have repeatedly stated that open-sleeve elements are usually less critical in practice than in calculations with NEC ll.

Mechanical realization

In order to make the antenna as light as possible, the elements were made with 4 mm round aluminum. Aluminum welding rods made of the alloy AlMg3 have proven to be the best. The bars are very stable, but they can still be bent at right angles. Other alloys can be too soft or so brittle that they may break when bending in a vice. The 2 m bars made of 4 mm solid aluminum material offered in hardware stores are also very useful. However, they are often anodized, which is of no importance to us electrically. However, the anodization must be removed at the contact points because it is not conductive.

It is important that the length and distance information according to Tables 1 and 2 refer to the middle of the elements. If you deviate from this, the antenna is quickly up to 1 Mhz away. With a 25 mm PVC installation pipe and the appropriate clamps, a very light and dismantable construction can be created.

Since the reflector length is longer than 1 m, I solved the problem by connecting two halves with a luster terminal. This method is shown in Figure 4. Luster terminals are also available for fastening the rods in the feeding point. For this purpose, the radiator pieces are bent inwards at right angles. The coaxial cable is attached directly without a socket and is wound up into a feeding choke as in Figure 5.

In order to give the frame the necessary rigidity, I pushed plastic tubes onto the ends of the elements. If the antenna is used for permanent outdoor installation, the feed point must be installed in a box or protected with hot glue.

Results

A possible adjustment is actually only limited to the distance of the 70 cm open sleeve element. If the resonance frequency is slightly too low at 2 m, it can be shifted upwards by a few 100 Khz by moving the Moxon reflector to a somewhat larger distance. This also increases the gap between the element ends somewhat.

The gain on 2 m is 3.9 dBd and thus just below that of the well-known HB9CV. The free space diagrams (Figure 6) show a very large horizontal 3 dB opening angle of 78 °with a SWR at 144.3 MHz of 1.2 and remains <1.4 throughout the band (Figure 7). At 70 cm, the result for the standing wave ratio is very good, without return in the middle of the band, as shown in Figure 8. The 70 cm azimuth diagram looks somewhat strange (Figure 9). The cause lies in the 3/2-lambda resonance of the Moxon frame, which in part also goes into the directional effect. The gain here is 5.3 dBd.

The extremely high f/b ratio of the Moxon antenna at 2 m is always astonishing. A beacon signal of S9 almost disappears when the antenna is turned 180 °. With the data shown, operation is not only possible in the SSB / CW areas of both bands. Vertical mounting for FM operation is also an option and can be used for this antenna. In this case, the opening angles are extremely large (141 ° and 114 °) and the bandwidth is fully sufficient for both frequency ranges.

I used EZNEC to study the properties of a stacking. A distance of 1.60 m is necessary for 2 m. This is far too large for 70 cm in itself, but the analysis nevertheless shows a gain of 3 dB. If you use 2x 3/4-lambda coax cables with 75 Ohm characteristic impedance, transform them with 9/4-lambda length to 70 cm, so that a stacked version can be used without problems for a two-band operation.

Table 1: Lengths of the Elements

Table 2: Positions of the Elements

Sources:

[1] Maguire, D. (AC6LA): Antenna modeling Software, download here www.qsl.net/ac6la/

[4] Steyer, M. (DK7ZB): Der Zweielement-Moxon-Beam, FUNKAMATEUR 58 (2009), Heft 3 S. 284