Troubling shooting antenna problems can be made easier with the right equipment. In addition to the SWR meter and VOM or continuity tester, below are two items that have become almost a must for the average shop doing installs or the dedicated do-it-yourselfer.
Saving time on installations and trouble shooting systems means saving money. We have found a piece of test equipment for setting and adjusting antennas that does both. Depending on how many installs you do, it can pay for itself in a week or two - surly within the month. Manufactured by MFJ, there are five models available, with the MFJ-249 or MFJ-259 SWR Analyzers the most popular. Both models feature a digital frequency readout, an analog SWR meter, the ability to determine the band width of antenna and, on the MFJ-259, the impedance of the coax - all without using the radio. This not only makes it faster to install and tune an antenna, but also is very useful in trouble shooting the system. Using the MFJ will isolate the antenna from the radio as a possible source of a problem, and allow you to zero in on the problem much faster.
Both units are available either factory direct or from other outlets. Contact MFJ for a distributor, your nearest supplier, or purchasing the units direct.
Sometimes noise generated from within the motor compartment of a tractor or auto can be very difficult to trace down. One of the best ways is with the use of a RF Sniffer. A simple one can be made easily and quickly and is shown below.
1. Use an 18' length of either RG/58 or RG/8x stranded center coax. The 18' length should provide enough length to allow reaching all areas needed.
2. On one end, solder a PL-259 connector.
3. On the other end trim back 12" of the outer rubber covering and the shield. This will leave 12" of foam exposed.
4. Trim off 10"of the foam, leaving the center strand exposed.
5. Twist the strands of the wire together to form a tighter wire.
6. Using a broom handle or similar sized round tube, wrap the exposed wire around the handle forming two loops.
7. Tightly twist the excess wire back around the beginning of the wire, to prevent loops from undoing.
8. Remove the broom handle from the windings.
9. Spread apart the loops so that they are not touching each other.
10. Construction of RF Sniffer is now completed.
A more enhanced version that will eliminate most interference or radiation of coax line can be constructed by adding a 5 or 10 watt, 50 ohm resistor in line and soldering the other end to the coax shield as shown below.
USING THE SNIFFER:
1. Connect the end of the coax with the PL-259 connector to the radio.
2. Turn radio on, and the volume up enough to be able to hear the noise when you are outside of vehicle
3. Start and run vehicle engine.
4. Move the loop end of the coax around the motor compartment, over motors, etc., being careful not to touch any thing. CAUTION: Be careful around fan motors to avoid fan blades.
5. When the noise level increases, the item that the loop is over is the part causing the noise to be generated.
6. This part may be needed to be filtered, grounded, or shielded ( depending on the part ) to cure the noise problem. Additional information on filters can be found at Special Vehicle Notes
7. Repeat process to locate all the sources of noise.