Two Dipoles for VK7RTC

1 March, 2017 - Reading time: 4 minutes

On Wednesday 1st of March, a two dipole array was installed at VK7RTC. These were generously and professionally constructed by Dion VK7DB. The design picked is mainly an elliptical pattern. Dion built the dipoles with a long enough boom to be able to customize spacing. He also built the power divider with RG11 coax to suit. Below are MMANA plots.

You'll notice the plot has an elevation of 4 degrees. This is in fact downtilt incorporated into the array, the antenna is just modelled upside down to show this. The top dipole leads the bottom dipole by 25 degrees. Coax delay lines need to be inserted between the power divider and each dipole. (300/440) = 68.2cm (1 wavelength) x 25/360 = 47.3mm. This is then multiplied by the velocity factor of the delay line coax, in this case RG214 (66%). The result is the top dipole needs to lead the bottom dipole by 25 degrees or a physical length of 31.25mm. Below is a photo of the delay line(s). As both the dipoles had the same unknown length of cable from their feedpoints, a known length delay stub was made, and a second 31.25mm shorter. The shorter length was then inserted in the coax feedline to the top dipole.

There is a little room for error here. For instance 2 degrees of downtilt was 12.5mm physical length. So the tolerances are not really critical, well not so much as to worry me. Below is a return loss plot. I can only assume the two dips are from the two delay lines inserted.

Antenna in it's final mounting position on the mast