Thursday, March 25, 2010

Antenna Polarization Demo

One of the most interesting Ham Radio related demos I have seen was done by Mike Herr (WA6ARA). He presented a demo of antenna polarization. He setup a small dipole (partially assembled Arrow Beam) to transmit 5 watts on 2 meters, while he had someone assist him by holding a custom dipole. The custom dipole was a simple foil wrap on a stick, with a small incandescent light bulb connected between the two elements of the dipole. Mike had his assistant hold the custom dipole about half a foot away from his Arrow dipole while he transmitted from his radio. When close and vertical, the light surprisingly burned bright. When the custom dipole was rotated, without changing distance, it quickly went dim, to almost completely out around 45 degrees, and nowhere close to bright around 90 degrees. This was a perfect example to demonstrate one of the FCC Questions in the Tech class pool.

T9B08: What can happen if the antennas at opposite ends of a VHF or UHF line of sight radio link are not using the same polarization?

A. The modulation sidebands might become inverted
* B. Signals could be as much as 100 times weaker
C. Signals have an echo effect on voices
D. Nothing significant will happen

This also makes me think twice about standing near antennas.