G4DHF 144MHz

Please feel free to build or distribute the information on these antennas. I retain the copyright to the designs and ask that they are not manufactured for commercial gain. 

It is part of my life as a radio amateur to experiment. These antennas were developed between 2004 and 2005 in response for a need to build an ultra-lightweight antenna array for CW EME that suited my purpose and location. The traditional commercial antenna route can be an expensive way of giving predictable performance and usually requires a significant supporting structure to keep the metalwork in the air. I wanted to design my own antenna system and spent many weeks modelling the antennas using YO and AO software developed by K6STI. Designs for 9, 7 and 5 element yagis are described below. More images are available in the Photo Gallery elsewhere on this site.

  4x9ele Tropo / MS array on the 2nd Tower

• Lightweight. They flex gently in the wind and return to their original shape.
• They have good gain and G/T characteristics.
• Low visual impact.
• Easy to construct. Several antennas can be constructed quickly.
• No brace supports are required for antenna lengths up to 2.5WL
• Low cost. For example, each 2.2WL costs around £15 to build.
• Easy to match. Each antenna is designed around a direct 50 ohm feed.
• The necessity for metal boom to pole clamps is removed, which greatly reduces weight, particularly when erecting groups of antennas.
• They have a proven track record of over 200 CW EME QSO's and several hundred MS/Tropo DX QSO's and have enabled me to erect and maintain a system that I otherwise could not have managed. 

These antennas are not suitable for everyone or all locations.

• They are not "plug and play" and require individual alignment.
• They are highly susceptible to rain, ice and fog.
• As the 50 ohm match is at a specific frequency the operating bandwidth is very narrow, typically some 400KHz. The LF side is shallow but the SWR increases, the gain decreases and the F/B falls away quickly on the HF side.
• The affects of mutual coupling can cause minor variations in the feed impedance when stacked, which becomes more pronounced when the Driven Element diameter is reduced to 3.2mm.

The Booms of these antennas use telescopic fibre fishing poles (Roach Pole), which are available in a variety of sizes and boom diameters. I was fortunate in having a good supply at a local market, which had a fishing tackle supplier, but these are now readily available on auction web sites such as eBay. The larger size is 6M, which is used for the 9ele 2.2WL design. It has x6 sections and measures 115cm closed and tapers from 30mm to 7mm. The thinnest section is not used. The next size is 4M, which also measures 115cm closed. It is x4 section and tapers from 25mm to 7mm when the last section is removed. This makes an excellent support for either the 5ele or 7ele designs. The smallest pole is just 48cm long and has x3 usable sections giving a total boom length of 150cm. They only cost £1 (E1) and are available from many "One Euro" outlets. This is an ideal size to construct an ultra-portable 5ele yagi (see separate page listing). I have constructed several of these and carried them in a suitcase when travelling on airlines on several of my European VHF trips.

As mentioned earlier these antennas are so lightweight they do not require heavy boom to mast fittings. The elements fittings described below are perfectly suited to secure the boom to the mast. The larger designs use a square sheet of acrylic with the plastic element clips mounted on one side and x2 1.5" metal hose clamp fittings on the other. Two small lengths of aluminium trim provide additional support. When each antenna is correctly aligned it is secured using miniature pop rivets, which pass through the element clip into the fibre boom. Initially, I was concerned that the prolonged affects of sunlight would eventually weaken the acrylic. These antennas have been in continuous use for over three years now and show no visible signs of deterioration. The trade-off in weight appears to be well worth the effort.

The portable designs use all plastic fittings between the antenna and support. Small strips of anti-slip matting are placed between the mounting clips to increase friction. Small holes are also drilled above and below each clip. These are used to thread cable ties, which tighten around the open end of the clip thereby increasing the grip on each section. At the end of each portable session these are simply cut away and taken home for disposal.

All Director elements use 3.2mm aluminium welding rods, which are available in lengths up to 1 metre. These have a small Nickel content and appear quite resilient to weathering. The longer Reflector can be composed of two rods secured in the centre or by using a single length of 3mm - 4mm aluminium rod, which is available from a range of suppliers. Size and construction of the Driven Element is described below. All of the designs described are modelled using these sizes. 

Elements are mounted on the fibre boom using a range of plastic pipe clips, which are readily available from DIY stores. The most common types are 28mm, 22mm and 15mm. These are reasonably flexible and can be mounted anywhere along the tapering boom. The really small yagis employ a reducer in the form of a piece or 15mm plastic water pipe with the centre modified to accommodate the 7mm taper. Each clip is drilled at 3.2mm and the element pushed into place prior to mounting. Friction is sufficient to hold the element in place. Make sure that you select pipe clips that will allow drilling without damaging the internal webbed supports. Once the element has been mounted on the boom a miniature pop rivet secures the element in place. Do not secure Director D1 to the boom until alignment has been completed.

The portable antennas employ a modified version were the clip is drilled and then cut at the top to allow the element to snap into place.

The picture on the right shows how the elements are stored inside the boom for easy transportation. The loop of coax is actually a balanced to unbalanced balun and simply serves to isolate the feed.

The photographs show how the dipole is located in a suitably sized miniature plastic box. Note that the same types of element clips are used to fix the dipole assembly to the boom. As the feed is designed around 50 ohm no impedance transformation is required. The coaxial lead that passes in and out of the box is a quarter wave stub, which provides an unbalanced to balanced match at the feed. Omitting this will almost certainly produce standing waves on the outer of the coax, which could lead to erroneous readings during alignment. Ranges of element thickness have been used successfully for the Driven Element. The 9ele design uses 12mm tubing with 10mm inserts that telescope in and out thereby increasing or decreasing the overall length. This should be around 1015mm from tip to tip. The gap in the centre is around 6mm and is created when a small length of plastic tubing passes inside the dipole. Do not use wood, as after time moisture ingress will rapidly deteriorate the insulating properties, which will affect the feed impedance. Miniature solder tags are pop riveted to the outer and inner sections allowing attachment of the balun. The portable 7ele and 5ele yagis use detachable 6mm elements, which locate onto small sections of 8mm tubing that are attached to the dipole box in the same way as the fixed 9ele design. When alignment is complete these boxes are sealed in fibre resin. Several types are available from car body repair shops. The resin uses a hardener to start a chemical reaction, which can get quite hot and possibly distort the box so the dipole lid is fitted before it sets. This makes for a completely waterproof enclosure. The purists among you may be concerned about the suitabity of using resin in high RF environments. Tests suggest that the resin used is impervious to RF at 144MHz provided that it is used in small quantities and just covers the space between the bottom of the box and the top of the solder tags. I've also tested a small quantity of hardened resin in a microwave oven to confirm its suitability and have used RF Powers up to 400W into a single antenna. If you are in any doubt I suggest that you find an alternative and take adequate safety precautions if performing your own tests.

This balun serves to provide an unbalanced to balanced 50 ohm match, which helps reduce RF currents on the outer of the feed. For RG-58U or RG-213 coax with a velocity factor of 0.66, cut a 34cm length and trim back the braid 5mm at each end. Note that the two lengths run parallel to each other and that the braid and inner are isolated at opposite ends. In earlier designs I had successfully used ferrite beads threaded on the outer of the coax to force a match. This solution, which I obtained from the excellent VHF/UHF DX Book by GM3SEK is rather more elegant and has the additional advantage of eliminating the need for the much heavier ferrite. All these antennas were constructed using military specification RG-58U coax, which is flexible, light weight and capable of handling medium power levels.

During construction I recommended that Director D1 should be mounted on the boom but not secured. This is because it's length and position from the Driven Element has a marked effect on impedance. So too does the length of the Driven Element. You will need an antenna analyser in order to determine exactly what is happening. Mount the antenna about two metres above ground in a clear environment well away from cars and buildings. Connect a length of 50 ohm coax and secure it along the boom using cable ties taking care to pass it over the centre of elements. Move the position of Director D1 by a few millimetres and observe the effect on matching. The gain of the antenna is not unduly affected. Do not move the position of any of the other elements. If a match is not achieved after moving D1 by up to 5mm reduce the length of the driven element to around 1010mm tip to tip and repeat the process. It should be possible to achieve a near perfect 50 ohm match. D1 can then be secured in position.