DK7ZB yagi for 144MHz

Introduction

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.

Construction

This is my initial sizing, all sizes in mm:

Element	Distance to reflector	Length	Diameter
Reflector	(0)	1029	6
Dipole	345	(?)	10
Director 1	615	958	6
Director 2	1175	930	6
Director 3	1870	915	6
Director 4	2675	904	6
Director 5	3505	897	6
Director 6	4345	913	6
Director 7	4980	895	6

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.

Progress

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…

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