Friday, April 20, 2018

145Mhz Cross Yagi

2 x 10 el cross yagi 145 MHz. RHCP.
Feed system: adaptation of the dipole in T symmetrical match

The procedure is easy to take all elements and mount them at 90 degree angle on the same boom.
Traditionally circular polarization is generated by feeding two crossed dipoles 90 degrees out off phase. But you can also shift the dipoles 90 degrees in space and feeding then in-phase with the same effect. The advantage is that the two cables in the splitter have equal length and therefore gives identical transformation of antenna impedance
I prefer the mechanical method despite the electrical method (with different length of cable to obtain the 90 deg phase angle). So, I have shifted each V element exactly at ¼ Lambda forward on the boom. So the entire vertical antenna is shifted forward with reference to the horizontal antenna.
The forward shift (Offset) between the two dipoles is 517 mm to obtain 90 deg Phase angle.

Like this the two antennas are excited with a 90 degr. of phase shift. With this now a rotating field is radiated.
By shifting one section of the yagi forward by a quater wavelength the phasing cable to the two antennas becomes equal, impedances becomes equal and the antennas must share the power equally - regardless of the VSWR.
The two antennas must be feed with a splitter. An air line gives by the calculation an impedance of 35,35 ohms : ๐‘๐œ†4⁄∗1√๐œ€๐‘Ÿ =√๐‘๐‘∗๐‘0∗ 1√1 =√25∗50 ∗1= √1250=35,35๐›บ
But mechanical splitter is too big at this frequency.
So the splitter can be made from two 75 ohm ¼ lambda equal length of coax cable.
On 144 MHz a quater wavelength coax is 1/4 x 300/144 x 0,66 = 340 mm (the real velocity factor of the coax used must be applied . In this case + 0.66, solid polyethylene dielectric).  I use three quater wavelength 75 ohm cable, which gives the same transformation. The cable length of 102 cm allows the point where the three coax ends are joined together far from the dipoles.
Software: For Yagi antenna, I use the VK5DJ John Drew software ‘’Yagi calculator’’ of http://www.vk5dj.com/yagi.html