Article by M0DGQ
This yagi antenna is particularly suitable for those with small plots ( myself included ) who wish to use a 2 element 20m yagi. The design is by AB4GX, the original article is listed below ( PDF ). Here different materials have been used for construction as these are easy to obtain here in the UK other than this the design is essentially the same. The original design uses aluminium tube for the elements, common 2.5mm pvc covered wire is used here, this will reduce the useable bandwidth slightly but approximately 150kHz of the band is still useable – you will have to decide which part of the band the antenna is to be adjusted for. CW is my main mode on HF, so the antenna was adjusted for this section of the band. Here, both solid 6mm wire and 6mm mains earth wire were tried for the elements, however 2.5mm pvc covered wire ( stripped from standard domestic 2.5mm twin and earth cable ) is used in the final build as it is easier to use and cheaper. Four 4m telescopic fishing poles are used for the element wire supports, the final thiner section of the pole is disgarded. The wire elements are soldered directly to the loading coils and sealed with self – amalgamating rubber tape. The diameter of the fishing pole is approximately 23mm at the loading coil.
The element support mounting brackets are made from angle aluminium and are secured to the main boom by U bolts, the pictures give a clear view how this done. The boom is made from aluminium tubing, mine is approximately 50mm diameter and a wall thickness of 2mm – 3mm. The coax feeder is soldered directly to the driven element and insulated with self amalgamating tape, a four turn loop in the coax acts as a choke balun. As per the original article, the antenna should be adjusted at the height it is to be used at, this is tedious but worth the effort. In use, mine resides at a height of approximately 13 metres and gives a 1:1 SWR at 14 Mhz to 14.1Mhz rising to roughly 2:1 at 14.2 Mhz so you will have to decide which part of the band you want to adjust it for.
The antenna has been in use for several months now and the results have been very good, JA VK VE stations have been worked with ease. One observation ; because the antenna has quite a high Q, when wet due to rain the antenna resonance and SWR does alter somewhat but is still useable. It will be interesting to see what happens when frozen.
Article by M0DGQ originally available at http://m0dgq.co.uk (now offline)