This is the simplest phasing system I have ever used. It uses one part, a potentiometer. What it does is couple a common loop antenna and a whip. In this particular case I use an untuned 24" large ferrite rod antenna and a 7' balanced, loaded vertical. Both antenna types are described here. Coax goes from the ferrite rod to the receiver with a "T" connector at a convenient spot in the middle. There is a 350 ohm potentiometer in a box with two connectors. The pot wiper connects to the center conductor of the coax from the vertical. The top (full CW) connection goes to the "T" connector and to the center of the coax from the ferrite rod to the receiver. It's the summing point. The bottom connection (full CCW) is grounded, as are all the coax shields.
To use it, first turn the pot all the way down. Then the loop is turned to null the undesired station. Then the pot is slowly turned up. If the signal increases, turn the rod 180 degrees and re-null. This inverts the phase. Turn the pot up until the signal minimum is found. Then slightly turn the rod. Repeat until the best null is found. There are only two adjustments, the pot and the position of the rod.
Nulls have been deep enough to run any of my locals into the noise so other stations could be heard. Unlike some phaser adjustments, I have not found any situation where all signal levels have dropped. It's always useable. It's also quite useful in nulling noise indoors.
The ferrite rod was simple to make. Six turns of wire around the ferrite rod and soldered to a coax connector. The vertical, equally easy. Cut two lengths of wire anywhere from 25' to maybe 50' or more. Wind two segments from the center out to either end of a PVC pipe used as the frame. A balun is needed for the connection to a coax, though there are ways to do that by just winding a coil of coax. The combiner is a single small box with the pot and two coax connectors. I did make one with just a couple of clip leads and it worked fine. The pot value can be anywhere from maybe 100 ohms to as high as 1000 ohms. I've used a 350 ohm and a 500 ohm pot in two of these built so far. Both worked well.
A preamp such as the DX Engineering RPA-1 here was used in the two test setups. That is a very good preamp.
A comment on potentiometers... Many web sites recommend Allen Bradley Type J pots. While these are OK, pots using a conductive plastic rather than carbon are better. In broadcasting there are 24/7 uses such as the headphone volume control on a mixer console. I have used Type J pots but they get noisy fairly soon. Conductive plastic pots can last years. I bought Clarostat conductive plastic pots and have never had one fail yet. YMMV, but this is my experience over nearly 40 years in radio.
To summarize... This setup works in a scenario where a loop antenna can be turned to null a station. A simple whip can be tucked behind a door and combined with the loop output to significantly deepen the nulls and create a more cardioid pattern. It's cheap to buy the parts and very easy to build and use. I've used it in both my indoor radio room and in my truck. It is significantly better than the dual loop setup in my truck and has permanently replaced that.
|Last update: 13 December 2008|