A 1.6 l-yagi for the IARU-Region 1 2m-band (by DK7ZB)

While looking for a good design for a portable, not too big in size, all-purpose-yagi for the 2m-band here in IARU Region 1, I found this antenna in the german magazine 'funk' issue 11/98. The designer DK7ZB meanwhile did a lot of antenna work and published some very interesting antenna designs on HF, VHF and UHF. I choosed this specific antenna because of its allround properties:

• extremly clear pattern

• covering the whole 2m band

• good gain

The first version I built was a 10mm version (driven element 12mm) with the original element holders for keeping it portable (dismantable).

The general datas of the yagi are:

The goal was to build an antenna that doesn't need a support structure, provides good datas and is easily handable at portable operations.

The boom is a aluminium square tube of 20 x 20 x 1,5 mm. All elements are aluminium tubes 10 x 1 mm except the driven element, that's a 12 x 1mm aluminium tube.

As common use the elements are mounted insulated. I used the original suggested commercially available polyamid holders obtainable from [1]. For a detailed view look at a holder picture.

These holders are made from polyamid and are UV resistant. I am using only the bottom part. With a 3mm stainless steel screw the elements are hold in place under normal conditions. Although grounding the elements should detune them, the designer DK7ZB said with 15 x15 or 20 x 20 mm boom on 2m/70cm you can use free space data without disadvantages.

The mechanicals datas were given for parasitic element diameters of 4, 6, 8, 10 and 12mm (driven element always 12mm ! ). I only realized the 10 mm version ( and soon maybe a light weighted version with 4mm isolated mounted elements through the boom. This antenna doesn't allow dismantle the elements, provides however very low weight). Maybe a compromise in terms of weight and portability/dismanteling is the use of 6mm aluminium tube elements.

Mechanical datas:

Note: The driven element is a straight split dipole with a lenght of 980 mm built from 12mm x 1mm aluminium tube

Inside the split dipole is a acryl glass rod to hold the dipole legs in place. Other materials may also be used like polyamid, PVC even bamboo was used (not recommended !).

Don't forget to ground the RF-socket at the boom to avoid currents on the braid of the coax cable !

A close look at the feed (early stage)

A close look at the element holder

New version of the 1.6l yagi antenna de DK7ZB

Ok, time goes by and so do antennas:

Until I realised the above antenna I got access to a new version providing even better datas:

Improved items are:

• more constant impedance, therefore

• more broadbanded, SWR of max.1:1.2 at 144/146

• better F/R over a wider range of frequency

Take a look of an overall comparision of the two antenna versions (10 mm elements version) done with YO 6.58 de K6STI.

Mechanical datas:

The mechanical dimension just changed slightly:

Note: here is the DE always 10 mm !

More detailed pictures:
Take a look at the 8mm version of DC6GF, a local ham starting VHF weak signal activity
Further, very detailed pictures of the feed and the element fixing you can see at the latest 2m DX yagi de Martin, a 8ele-2wl, high gain yagi-uda antenna.

You may want to compare the 4mm and the 10mm version.

The antenna was modelled with 4, 6, 8 and 10mm parasitic elements. If different element diameters are needed please e-mail me.

You may take a look at the horizontal radiation plot at 144/145/146MHz and the vertical radiation pattern on 144/145/146 MHz done with AO 6.59 de K6STI (10mm element version).

A completely dismantable portable version

The last version of this antenna I built was done with aluminium tube at hand, so there are some compromises in weight and windload. For real portable operation the antenna should be lightweigthed and collapsible.
At the moment some TV antennas are not longer requested and sold very cheap. Among them a channel 10 (~215 MHz) yagi the fits right to the yagi of Martin, DK7ZB. He has used several to rebuilt amateur yagis out of them. They provide some nice features like a dismantable boom, a heavy boom-to-mast-plate and a nice leight aluminium alloy. They originally use insulated on the boom mounted elements. The holders could be reused for a 2m or 70cm yagi.
I chose the mounting method insulated through the boom. And for all you non-metric guys this time 5mm (3/16")elements are used. The driven element is again a straight split dipole of 10mm (3/8") aluminium tube.

See the radiation pattern at variuos frequencies. Note the little deviation in the vertical symmetry due to the missalignement of the driven element. The dipole is placed ~ 20mm above the element plane to fit the mechanical needs.

The original design of DK7ZB was further optimized for a very flat SWR curve across the whole band (144 - 146 MHz).

A picture of on-the-air-use will follow.

Cable balun

To match the 28 Ohm intrinsic resistance of the yagi to 50 Ohm coax impedence a l/4 coax balun is used. Two l/4 long pieces of 75 Ohm TV/Sat cable (pay attention to velocity factor) in parallel resulting in a 37.5 Ohm balun.

Before you connect the balun first measure the VSWR connecting a dummy load of metaloxid resistors (low inductance !) of 2 x 56 Ohm/2W or 8 x 220 Ohm/2W in parallel for a perfect load of your TX. You should get a VSWR of 1:1 (almost). The velocity factor maybe isn't given reliably so you have to test it out.

See different 75 Ohm cables (left to right): sat cable (thin braid); RG-59; sat cable (good quality); Belden H125CH (good quality, dense braid); very good sat cable (dense braid, choosen for the above antenna); RG-179 3mm teflon cable, very good quality but quite expensive (up to ~300W on 2m and ~200W on 70cm only ! ).

Final tuning

If you measured the the cable balun exactly the only factor for correcting the resonant frequency is the lenght of the straight, split dipole. It is likely to be too long so maybe you need to shorten it due to additional inductance of the ring lugs and the cable connections.

[1] Fa. Konni Antennentechnik, D- 97839 Esselbach, www.konni-antennen.de