I used to run several polar-plot tests to evaluate modifications to my vertical Waller Flag (WF). My reference signal was a 1700 kHz AM broadcast station about 20 miles south of me. Its ground wave is always vertically polarized, and during the day there is almost no QSB, which makes it a stable test source. Even so, I noticed big distortions in the pattern that I wasn’t expecting. After a few months, the reason became clear: cold fronts can reflect the AM signal. My plots showed one strong signal coming directly from the south (the station itself) and a second, reflected signal arriving from the northwest or northeast, depending on which way the front was moving across South Florida. Listen to the noise The best way to evaluate a receiving antenna is to listen to its noise. Like a musical instrument, every antenna has its own sound. A vertical is a good reference for local man-made noise because it is omnidirectional. A good directional receiving antenna will sound different. The intensity matters less than the character of the noise. Common-mode noise Common-mode noise is what happens when the vertical’s ground wave couples into any parallel wire, cable, or metal object that reacts to the magnetic field. If your Beverage, EWE, loop, or any other receiving antenna has the same noise signature as your vertical, you are receiving on the feedline, like a BOG. To test this:
1. Disconnect the antenna at the end of the feedline. 2. Install a shielded 50-ohm resistor in its place. 3. Compare the noise with a resistor connected directly at the receiver input. The two should sound the same. If you can still hear the AM broadcast station with the resistor at the end of the feedline, you are only receiving common-mode noise coupled to the feedline, not signals from your loop, EWE, or Beverage. Remember: 100 ft of LMR400 running from your 2m Yagi makes a good vertical for 160m, too. So does 100 ft of rotor cable, or any other wire or cable without a good isolation choke. 73's JC N4IS _________________ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
