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2m Yagi : 2 element

Receiver Integrated and Collapsible YAGI

Overview

YAGI

A 3 element yagi is generally excepted as the best compromise between gain, beam width and handling size, but is still relatively large, so the attraction of a smaller option remains.

The 2 element HB9CV is a popular alternative but has mechanical complexity which can make it difficult for the home constructor to build. The HB9CV is commercially available ready built, but built with the stiff aluminium tubing, so not ideal for carrying through dense forest undergrowth.

The 2 element yagi presented here is a scaled down version of a D/DE Yagi for 10m, by L. B. Cebik, W4RNL.
The claimed performance of ’ a modest gain (about 6.5 dBi free space) and a very good (greater than 20 dB) front-to-back ratio with a feedpoint impedance of about 20 ohms. A hairpin match is used to increase the feedpoint impedance. The authors practical experience using this yagi has been positive.

Dimensions

  • Driven Element 980mm

  • Director 920mm

  • Spacing 165mm

  • Hairpin 130mm wire length, bent to U 25mm wide

Construction

The construction principles described here, of course, can be equally applied to other yagi configurations.

YAGI-BOX above: showing boom mount and antenna element sockets

The yagi boom is made from 20mm plastic electrical conduit, alternatively 20mm diameter aluminium can be used. A plastic endcap is bolted to the front of the receiver enclosure, using a 6mm bolt. An 6mm aluminium bolt and nut intended for greenhouse construction is ideal, but equally, a short 6mm roofing bolt which has a large head, can be used. A pad of double sided foam between the end-cap and enclosure ensures a solid fix. If you are lucky, the boom will be a tight friction fit into the end cap, without requiring additional packing.

Enclosure/Element Connection

The elements ‘connect’ to enclosure using 4mm banana plugs. These are a specific low-cost, open type, readily available on-line. The socket is re-assembled ‘backwards’ with the fixing nut outside the enclosure, and the solder tag positioned behind the top flange for connection to the receiver input.

ELE SOCKETS

Boom/Element Connection

Using the same banana sockets. Shape the plastic sleeves (glue parts first?) to fit the boom tubing. Re-assemble the parts as shown in the photo below. The socket solder tags can be soldered together to connect the director element halves. However, the authors preferred alternative is to use a thin strip of aluminium, shaped to fit the inside or outside of the boom. Of course, if an aluminium boom is used, the sockets will be automatically connected.

BOOM SOCKET
shaped socket sleeve

BOOM END VIEW
aluminium strap, connecting sockets

Director and Driven Elements

The elements are made from 12mm measuring tape. A 3m tape will provide enough length for the 4-half elements (460mm) and 4 shorter stiffeners (290mm).
This provides double thickness at the plug ends. If a longer length of tape is available, the author recommends triple thickness for a more robust antenna.

Element Plug

ELE PLUG

A low cost type with a screw supplied, as shown in the photo, also please note, the flange arrowed. The flange provides essential extra support to the plug when fully engaged in the socket. Some plugs do not have this flange and results in excessive movement between the plug and socket.

Making the elements is a lot easier if you have a solder pot, a hot-air gun, clean all contact faces and use lots of solder flux.

Scrape the paint off the ends and both sides of each tape length and tin.
Tape element and stiffener together and re-heat to fuse the ends together.
Punch a hole in the element end to align with the grub screw in the plug. Screw the plug to the tape-element, check the alignment, and solder together.

BOOM SOCKET
plug soldered to the element end

Trim all the elements to the required yagi dimensions.
Cover the plug end with shrink tubing, and all other cut ends with electrical tape. (see photo top)

LINKS