RFX Receiver Basics:

How to use the RFX Receiver in Null Mode:

The RFX is a Null Mode type receiver as apposed to a Peak Mode type receiver. A Peak Mode receiver detects the strongest signal directly over the wire, whereas a Null Mode receiver detects the weakest signal over the wire.

Why use Null Mode?

Using Null Mode, you can locate a hidden wire, say behind a wall, to within the width of the antenna. This is an accuracy of about 1-inch. You could never do this in Peak Mode because the receiver will swamp when you are near the hidden wire. Then you are left trying to figure out which spot is loudest.

The ANTENNA does not RECEIVE at the tip. The tip of an RFX antenna is a dead zone. This is what makes the RFX a Null Mode receiver.

The BLACK control knob is a RF GAIN control, not a volume control. Though the effect to the user will often seem the same. The distinction is important as you will often want the gain turned down so as not to swamp the receiver.

The RFX receiver has extremely HIGH GAIN.

The RFX is a RADIO receiver, and is tuned to detect a carrier at 455khz, just below the AM radio band. It demodulates an audio signal taken off the RF carrier at 600 hz or 800 hz depending upon which tone is selected on the RFX transmitter (A/B switch).

NULL MODE Detection Basics:

 The image on the right shows the signal detected over a transmitting wire. This signal is the AUDIO heard coming from the receiver. The audio level heard is in direct proportion to the RF signal received by the receiver.

Note the deep NULL in the middle directly over the buried cable. This is followed by loud signal PEAKs on either side of the NULL.

Between the deep NULL and a signal PEAK is a rising signal EDGE. This EDGE is very important for locating faults on buried wire and cable. It allows you to visualize the width of the NULL, as you will see below in the section on Cable and Wire Fault Locating.

Cable and Wire Locating:

 Turn the GAIN control on the RFX way up. You want to follow the NULL of the hidden wire or cable so swamping of the receiver is not a problem.

Sweep the RFX back and forth, quickly over the wire path, as shown in the animation on the right. This will allow you to find the NULL, which will be directly over the wire path. Follow the NULL down the wire path.

Other wires and cables nearby could pickup and radiate the RFX signal, but these false signals will not have a sharp NULL associated with them./p>

You will either detect a strong signal with no NULL. Or a signal with a spreading NULL where the rising EDGE is apparent.

If you encounter coupling to another conductor, turn the GAIN control DOWN, so you can detect the rising EDGE. Your wire will have a sharp NULL, with no real rising EDGE detectable. This will allow you to distinguish between your wire and another wire running close by.

Cable and Wire Fault Locating:

The first thing you must do is turn the GAIN of the RFX way down. You do not want the receiver to swamp. With the GAIN turned down, you will then be able to detect the rising EDGE between the NULL and the PEAK.

Notice how the PEAKs and NULLs look the same on both signals on the right. But the rising EDGE is completely different. The clean section has a very fast rising edge. So fast that you would have trouble hearing it. You would, instead hear the NULL and then the PEAK and not even notice the rising EDGE.

This type of signal is typical of a good wire. If you walked down this cable and only heard this type of sharp NULL, then the trouble is somewhere else.

Unless you had the GAIN turned up too high. In which case you just walked past the fault. Turn the GAIN down, and follow the wire again. You will detect more of the rising EDGE on this pass. If the width of the NULL stays the same all the way down the wire, then the wire is clean.

On the faulted section of wire, however, there is a distinct rising EDGE. You will be able to hear signal change when you move the antenna from the NULL to the PEAK. The effect of this is that the NULL will appear wider, and the Peaks will seen lower in intensity, even though they may actually have more signal volume then they do on a good section of cable.

Locating a fault on a buried wire involves detecting this sudden change in the width of the NULL. As you walk down the wire path, you are listening for the rising EDGE. When you come up on a fault this rising EDGE will become more apparent, as the NULL widens. This will also make the PEAKs seem wider apart. The fault will usually be within a foot or two of this effect.

 To illustrate the spreading NULL effect more clearly, look at the drawing above. As you are walking down the cable/wire path, sweeping the RFX receiver back and forth, you will notice the PEAKs will spread apart as you come up on the fault. You noticed this because; you are visualizing the position of the NULLs.

After the fault, if there are no other faults close by, the PEAKs will come back in. The NULL will not be as sharp as before the fault, as there will be some noticeable rising EDGE between the NULL and the PEAKs.