Monoband yagi  for 80 meters y 40 meters I recently designed an array of two yagis for 80m (2 el.) and 40m (3 el.) sharing one single 12 meters long boom for EA5FKX Andreu, who is building them for his superb QTH and contest station in Alcoy, eastern Spain. Definitive placement for these yagis will be on top of a 30 meters high self-supporting tower at EA5FKX’s QTH. Andreu is a well reputed professional telecom antennas installer and undoubtedly one of the most meticulous and careful technicians I’ve ever met. Design objectives for both anteras were to obtain maximum gain with at least 20 to 25 dB F/B at the center of each design frequency. On top of this, the 80m yagi should be able to work across the whole phone segment, something that is even tough for a dipole. Both antennas share the same boom with a total length of 12 meters (40 feet) and have independent feedlines. Elements dimensions and their placement along the boom have been optimized to eliminate interaction among both antennas. . Very high Q loading coils are used In order to shorten elements to a practical length of 23 meters. High Q coil are needed to keep losses at a minimum. Coil design also took into account the lack of any parasitic resonance on the HF spectrum. Loading coils for the 80m elements consist of 13 turns of 18 cm of diametre with a total coil length of 20 cm. The conductor is a copper (Cu) tube with an outer diameter of 6.5 mm. These dimensions and the fact that the coil is air wounded, lead to a Q of more than 1200. Element tubing is Al 6061 T6 for high yield.  Yagi dimensions Radiation diagram for 80m. Maximum gain is 6,6 dBi (free space) and F/B is 25 dB Radiation diagram for 40m. Maximum gain is 7,9 dBi (free space) and F/B is 23 dB The 80 m yagi keeps its performance through a very narrow bandwidth (around 45 kHz). In order to cover all the phone segment of the band, two switched arrays of loading coils were designed. These arrays are placed in the middle of each element and will split the band into four segments. Design tried to minimize the number of relays needed. EA5FKX 2º operator holds one of the 80m coils giving an idea of their actual size. (photo EA5FKX) 80m loading coil. Element spreader is a solid fiberglass rod which was mechanized to fit exactly inside the elements. Plates that hold the spacing or the turns in the coil are made of bakelite and are fixed to the elements through four aluminium Z‘s.  (photo EA5FKX) (foto EA5FKX) Detail of the 80m coils. (photo EA5FKX) Coils are painted with several layers of high quality liquid tape suitable for outdoors environment. Electrical conection between copper and aluminium  6061 T6 is performed through galvanically compatible fasteners and bolts. Additionally conductive paste was used to ensure durability of conections. Loading coil for 40m. It consists of 6.5 turns of 17 cm of diameter with a total length of 10 cm. Estimated Q is higher than 1400. Conductor is copper tube with an outer diameter  of 6.5 mm. (photo EA5FKX) Impedance on both antennas is 25 ohms. To adapt this value to the 50 ohms needed by the feedline, a beta- match was designed. Advantages of the beta-match are its simplicity, low losses and the fact that the feedline is DC shorted to reduce static discharges noise. To make the hairpin raise the impedance to exact 50 ohms, it is needed to slightly shorten the length of the driver element to create a capacitive reactance of Z=25-j25 on the feed point. The hairpin inductance in parallel to this value raises the impedance to 50 ohms. Afterwards a 1:1 balun is used. Driver element for 80m: The PVC box contains the relay switched loading coils, a hairpin to adapt impedances to the feedline and a 1:1 toroidal balun. (photo EA5FKX) Hairpin coil and 1:1 balun for the 40m yagi. SWR<2:1 bandwith is about 170 kHz, so there is no need to installa a switched array of loading coils. (photo EA5FKX) Hairpin coil and 1:1 balun for the 80m yagi . Using N connectors, losses are reduced. The hairpin coil can be stretched to adjust for minimum SWR in the adjustment phase. (photo EA5FKX) 40m boom to driven element plate Waterproof box contains the betamatch coil and the 1:1 balun on a ferrite toroid. (photo EA5FKX) Detail of the boom to element plate. Isolator is made of mechanised fiber glass. Its width will permit a low capacitive coupling between the element and the boom. This U shaped plate delivers higher mechanical strength than conventional plane plates. (photo EA5FKX) Placas de sujección de los elementos de 40 m al boom. El vástago vertical sobre la placa de unión al boom sujeta los tensores de cable Dyneema (no conductor) que contribuyen a reforzar los elementos.(foto EA5FKX) El mecanizado del boom de 12 metros se realiza en un torno de precisión. El boom consta de tres piezas, siendo el tramo central de una sola pieza de 6,5 metros para máxima resistencia. Su diámetro es de 11,5 cm. En la foto se muestra el mecanizado del boom de la long yagi de 20m que irá encima de las monobandas de 80m y 40m. (foto EA5FKX) Este rotor Prosistel será el encargado de girar el monstruito. (foto EA5FKX) Amortiguador de par de torsión, Para evitar roturas por el enorme momento de inercia de la antena se instala este amortiguador de par entre el rotor y el mástil. (foto EA5FKX) TO BE CONTINUED Hairpin N conector 1:1 toroidal balun RL4 Left arm loading coil RL3 RL6 RL5 Relay control line Right arm loading coil Click here to see a gallery with step by step images of the construction process Home        Shack       Antennas       Maritime Mobile       Propagation        Gallery          QSL info