A lightweight Yagi for 2 m

SOTA: HF or elsewhere?

40 m activity dominated the first 18 months of SOTA in VK (and continues to be the dominant band). But what about other bands? This is a 2-edged sword:

• The Activator needs Chasers to gain points
• Chasers have been hard to find on bands other than 40 m.
• 40 m gives common ground to all VK licence classes.

Is this the Chicken or the Egg? 2 m has worked for several activators, especially for summits closer to major city centres.

Why not 2 m?

Well, why not? There are relatively few Chasers on 2 m FM. Even when close to a major city, it can be hard to find someone to answer an FM CQ call. Of course, most major centres have some regular 2 m SSB operators, but are they interested in SOTA? Well, there are some active in SOTA, but more would be nice. If you can find SSB operators, then you might be able to stir up some of those listening in the background. Once contact has been made on 2 m, why not try higher bands? But remember that the Activator needs to carry their equipment to the operating site without motorised assistance. Therefore, what should the Activator take?

Basic Activator gear

The Activator will have his/her own gear preferences, but a typical set up might include a 7 m squid pole, a 40/20 link dipole (or EFHW or other HF antenna), and perhaps a 2 m vertical. For the radio, an FT-817 is a good flexible choice, plus a battery, coax and perhaps some other RF related gear. Add the usual bushwalking safety equipment, food, water, etc. So if we want to add 2 m SSB, we need to add a Yagi for 2 m. There are many options, but remember that the Activator will want to keep the mass as low as possible, as it all must be carried onto the hill.

Which Yagi?

I looked at many published designs before settling on the Lightweight Yagi by Martin Steyer DK7ZB (http://www.qsl.net/dk7zb/start1.htm). It is a 6 element OWA 2 m Yagi, with a 200 cm boom. Practicalities meant that I modified the design a little.

DK7ZB claims: 9.2 dBd gain, 18 dB F/B.

VK3PF variations

I wanted to minimise any extras to the existing SOTA gear. I was using a 7 m heavy duty squid pole to support the HF link dipole, so I choose to use 2 sections of squid pole: Yes – I drilled holes through the pole sections!

Materials used:

3.2 mm Aluminium rod for elements
3.96 mm Aluminium tube for driven element
Poly box, screws, coax, BNC connector
2 O rings + poly T piece

The Driven element is offset from other elements.

From the modelling, it looks as if the Yagi should work well, even if at only 4 m above the ground. Of course, there will usually be lots of mountain underneath the Yagi!

The Yagi packed in a modified fishing rod holder

The Driven element, mast and mast-boom bracket

The mast, boom and other components

 

 

 

The Yagi: The first image is the Yagi packed in a modified fishing rod holder. The next image shows the squid pole with the driven and reflector elements, plus the poly T piece – the boom to mast bracket. In the third photo, you can see that the four directors are stored inside the top section of the squid pole. Also visible are the two O rings used as packers to remove “slop” between the T piece and the boom of the Yagi.

The components ready to assemble

After unpacking all the elements (see the image at left), the top section of the squid pole is not used when using the Yagi. The next 2 sections down from the top have been marked out and drilled for the reflector and the 4 directors. Care is needed when drilling – you want to drill through the boom perpendicular to the long axis of the boom. As the boom tapers, you need to do some calculations to set up the boom correctly! The boom is fibreglass, so appropriate safety precautions should be followed. I used a 3.3 mm drill that had not been sharpened for a long time and drilled with care and high drill speed. After drilling the holes, I carefully sanded the edges of the holes to remove any fibreglass splinters.

The reflector is slightly asymmetric, with the section of chock-block setting the position of the element against the edge of the boom – more arithmetic! For the directors, I used some tape to mark the position against the boom, plus small sections of heatshrink tubing to label the director: 1 piece of heatshrink means Dir1, 4 pieces means Dir4.

The Yagi is assembled by placing the driven element roughly in position, then mounting the reflector, followed by the first director. Then position an O ring close to the expected centre of balance, then the T piece, followed by the second O ring. Then place the remaining directors in position, aligning the front section of the boom so that the front directors align with the elements on the rear half of the boom. Finally, position and align the driven element with the other elements and tighten the hose clamps which hold the driven element assembly on the boom. Assemble the bottom four sections of the squid pole to form the mast, placing the T piece over the top of the mast and secure with some electrical tape. Attach the coax and secure the coax to the boom and mast (more electrical tape). Finally, raise the mast, making sure that the Yagi is balanced and in the correct polarisation.

The Yagi assembled and ready to raise to vertical

The Yagi assembled and on the mast, ready to be raised – in this case, vertically polarised. Coax not yet connected.

 

Yagi Feed

The inside of the driven element box, showing the simple choke balun and direct feed.

The T piece over the boom, with electrical tape to fasten to the mast. Just visible at right is one of the O rings which stabilise the T-piece on the boom.

DK7ZB also claims that the Yagi performs reasonably well on 70 cm, but I have not yet tested this claim. Based on the results of modelling in EZNEC, it looks as if will work nicely, even if it has 3 forward lobes.

Yagi & mast at Mt Emu

Summary

The design goals were met: a lightweight Yagi and mast combination was constructed. The total mass is 1260 g, with the squid pole alone having a mass of 910 g. The whole system can be rapidly assembled or disassembled. If not wanting to use the Yagi, the squid pole can be used as a 7 m (or shorter) mast to support a HF wire. One can also mount the HF wire just below the Yagi, but then the HF wire is very low. This may not be an issue, as most HF wires supported by such masts are at low heights with respect to wavelength and are probably acting best in NVIS propagation.