After the somewhat disapointing results of the DJ9BV antenna, I decided to try another model. This time I selected a 9 element DK7ZB for my test. I already bought some square alu bars, each 250cm tall. Since the 9 element DK7ZB is 5 meters tall, this should fit perfectly.
Click here to visit DK7ZB’s homepage, including all his antenna designs and lots of references.
This is my initial sizing, all sizes in mm:
|Element||Distance to reflector||Length||Diameter|
The sizing corresponds to DK7ZB, except for the element length. I decided NOT to isolate the elements, so the length has to be extended with 5mm (when using a 20mm boom).
The picture above shows 3 bars, each 2500mm. The bars are bolted together according to the above picture. The middle section is just a small part of another bar. A hole is drilled through the three bars. A 70mm bolt is then put into the hole and closed with a wing nut. Apply wide washers on both ends of the bolt connection to get a more solid construction.
The picture above shows the way each element is mounted through the boom. A hole is drilled at both sides which will hold the element. A smaller second hole is drilled in the top side of the bar, to fit a self-drilling screw.
March 14th, 2009 At the yearly radiomarket in Rosmalen I bought a couple of prefab folded dipoles, including balun (1/2 wave coax) and N-connector. I decided to use these for the DK7ZB.
March 22nd, 2009 I started with the building activities. I drilled all the bars and mounted the reflector, dipole and 3 directors. Unfortunately I didn’t have enough aluminium to create the final 4 bars. I used 6mm alu for the reflector and directors. These are mounted through the boom (not isolated). Therefore I increased the length of each element with 5mm to compensate the shortening of the middle 2cm’s by the bar.
March 25th, 2009 I bought some stainless bolts and washers to connect the bars. Unfortunately stainless wing nuts where not available, so I bought “normal” wings.
March 29th, 2009 I finished the antenna and did some first measurements. The antenna resonates at 146MHz, which is too high. I would like to have it resonating at 144.3 MHz. The high resonance frequency is probably caused by the prefab dipole, so I might replace it in the future. However, the SWR on 144.3 is 1.5 which is not bad.
The gain seems to be ok, and the sidelobs are quite surpressed. F/B ratio is ok too. There is a very sharp dip on both sides (at 90 and 270 degrees). For more serious measurements I have to put it in my antenna post, but that one is still under construction…