My Antennas
A telescopic mast carries 2 times 2MXP20 H/V antennas from M2, mounted side by side in an X-pol configuration. With proper phase-shifting an H- or V-polarization is achieved.
The advantages of X-pol mounting are the following :
- less de-tuning of the antennas in wet weather (no "thicker" elements due to hanging water)
- less bird-droppings (which the xyl likes a lot) due to the 45 degree mounting of the elements
- less distortion of the vertical riser, even if made of fiber (also fiber will get wet and cause reflections)
- twice the power handling capability (3 dB), compared to H- or V mounting. 4 dipoles (with phase shift) must be used to produce an H- or V-pattern, while in a H/V mounted antenna system only the H or V dipoles are being used to produce the wanted polarization.
Fiberglass risers
It is difficult to find strong fiber poles. The local electrical tramway company helped me out. They use thick fiber poles to guide the high voltage overhead wiring. I could get some (left over) pieces of these poles at a construction site. Even with a wall thickness of 7 mm the poles needed some metal sleeves in order to prevent damage by the U-bolts. (see picture gallery
Installing the mast and antennas
Click HERE for a picture gallery of the installation.
Determining the phase length differences of the coaxes to the combiners
Coaxes from the dipoles to the combiners are all made of equal phase length. Also the coax runs to the shack are of equal phase. The actual combining/phase shifting of the 45 and 135 degree antennas is done inside the shack. At the combining /phase-shift point also the physical (phase) difference between the two antenna groups is compensated.
At first instance 4 coaxes of exact physical length were made (approx 7.5 meters). During phase measurements it was found out that 3 coaxes were within 1 degree of each other while one coax yielded a phase difference of 6 degrees. Although this coax was brand new (as the others) it came from a different cable drum, most probably with slightly different characteristics (although still within specification). So, in case you can not measure phase and can only rely on the same physical length: always make sure the phase line coaxes are coming from the same cable drum.