Top: DK7ZB 70 cm Antenna constructed by ZS1I
(Apologies for the quality of the outside photos as it was very foggy when these were taken.)
(Apologies for the quality of the outside photos as it was very foggy when these were taken.)
The antenna describe here is a 70 cm DK7ZB - Design antenna. Constructing this antenna is straight forward, to the point and you should not have any problems if you stick to the basic rules of antenna construction. This antenna is a 16 element closed-spaced 70 centimeter beam. It features high forward gain good pattern and acceptable bandwidth. It has a excellent front to back ratio and is especially suited for DX contacts where maximum gain and narrow beam width is required.
The feed point impedance for this yagi is 28 Ohm, the radiator is a simple split dipole with the DK7ZB-match. The yagi have insulated mounted elements above the boom. This is the best way for mounting, because the formulas for mounting through the boom and on the boom are not very trustworthy.
I prefer elements with 8mm or 10mm diameter, because they are much less critical than smaller diameter. Bandwidth and tolerance against rain, fog and stacking harnesses are much better with thicker elements. The elements are mounted insulated with insulators I made on the milling machine. The screw fixing the element to the boom has no influence to the length of the element as it is in the electrical "zero - point" and is connected to the boom.
Specs:
High-gain, high front to back ratio. Freq. 430 - 435Mhz
Boom 3.88m
Gain 15.5 dBd F/B > 40dB (!)
Top: Completed Antenna (Note: Some of the elements must be re-aligned and tightened after they were accidentally bumped while testing the antenna.)
Top: 16 element Yagi for 70cm
Top: Driven Element, Match and other elements.
Top: PVC holder for match and driven elements
Top: Driven elements fitted with insulator visible.
Top: DK7ZB Match
Top: Completed driven element.
Top: Closeup of the DK7ZB Match
(Click on images for larger view.)
(Click on images for larger view.)
OK time for and update:
Construction info can be found by clicking HERE
Many thanks to DK7ZB for making yagi construction information available to all radio amateurs!
ZS1I notes on the construction of the above antenna:
All the construction material were available at a local emporium. I had 99% of the material in my garage and decided to build a 16 element 70 cm DK77ZB Yagi. As the elements have to be isolated from the boom I had to mill my own insulators. The milling of the insulators took time and 15 isolators were milled over a period of two days. Construction is straight forward and to the point. The assembling of the driver element also took time as I wanted it to fit snugly into the PVC electrical box. The parallel 75 Ohm cables calculations for the DK7ZB Match came to 11.5 cm using RG59. I used a good quality RG59 which was very hard to bend and fit into the PVC box. I managed to fit it snugly with very little extra space left in the box.
Once every element was fitted it was time to test the SWR. I do not have the luxury of a MFJ antenna antenna analyzer so all SWR tests and measurements were conducted with a small but effective Nissei SWR Power Meter. I used this meter in many projects before and when tested afterwards with an MFJ antenna analyzer it confirmed all the readings as before. The fist test was 1m above ground and it reflected an SWR reading of 1.2 on 433.895 Mhz. I decided not to trim the driver elements and pushed the antenna up to about 9m above ground. SWR: 1.1 on 435 Mhz. Good enough for me.
Now for the real test. Willie ZS2CC obliged and we tested the antenna on air for nearly an hour. Different antennas for reception and different power levels. Distance 40km. Although the distance is very short, the antenna performed as expected. 5/9 + 20 on any external antenna at 1 watt power output level. ZS2CC even received the signal on a rubber duck antenna at his QTH. I am quite happy with the performance of the antenna and will in future test it over longer distances. Hope to use this antenna in future VHF/UHF contests.
That's it. A very effective antenna at a reasonable price tag. After all I experienced great satisfaction in homebrewing this antenna in my backyard. Now why don't you join in on the fun and construct this antenna?
