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Multiband Antenna Common Problems

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Multiband Antenna Common Problems

 

1. Current Flow Stops - This can happen for a large number of reasons, and affects both swr and resonance.  For most multiband antennas, the current flows from the feedpoint, to the highest frequency first (10m), and out to the end point for the lowest frequency capable (80m).  Trouble spots develop from the balun/feedpoint, through traps or resonator stubs, tubing joints, and capacity hats.  Also, read the article Troubleshooting Multiband Antennas, located on the MFJ Knowledgebase.

 

2.  Balun Problems - Baluns usually kill all bands when they fail, but not always.  Sometimes the lower band will fail first, and look like a bad trap.  This goes against most rf solutions, which indicate that higher frequencies fail first.  Most baluns are sealed, and can only be replaced.  If you can open it, check for opens and shorts, burn spots, and loose hardware such as a wire connection.  Confirmation of failure can occur if the balun is bypassed with a direct hookup cable.  Do not run high power, as rfi can occur.  Use choke or current type baluns, located at the feedpoint.  Reheat solder connections when a cold solder joint is suspected.

 

3.  Feeder Problems - Cold solder joints are the most likely failure here.  Pigtail wires from the balun, beta or gamma straps, and tubes can have bad connections.  Removing the driven element connections and checking with a dc meter can show a shorted driven element insulator if continuity is still there.

 

4.  Trap Problems - Trap problems may be visible, but usually are not. Lightning and high static is the most common cause of failure.  With a multiband antenna, the outside trap on the front or the rear elements is the most likely to get damaged.  The heat from lightning can be extreme to the point of melting the trap, and the damage can also be hidden inside.  The MFJ series of inline traps (Cushcraft/Hy-gain) are made with a single screw which secures the outer capacitance tube to the inner smaller tube, with a tab connection from the larger tube.   Lightning can cause crystallization of the metals which can cause rf current problems.  It may check fine with a dc meter. After removing the outer screw, the outer tube will slide off for inspection.  There are dimples on some models to prevent the trap spacers from sliding around.  You may have to drill out the dimples to allow passage.  The inductor inside is a simple coil wound around a formed plastic bobbin.  Look for anything discolored, swelled, or otherwise abnormal.  Remove the screws one at a time on each end of the bobbin and clean the area where the screw attaches to the wire, and replace the screw.  If the resonant frequency is slightly high, there may be a shorted turn.

 

5.  Resonator Coil Problems - If the coil support is not burned or malformed, look for loose or corroded connections at each end of the coil.  Clean up the connections as necessary.  Many will have a solder lug attached, which can have a cold/hot solder joint which may read good with dc, but act poorly at ac(rf).  If bare wire is used for the coil, inspect with a good light and magnifying glass.  Look for thin bridges between the turns which can appear during stress to the wire.  For stiff wire, cracks can appear.  The resonant frequency may shift outside the band, and have a good swr at that frequency.

 

6.  Capacitance Hat Problems – Spokes should not be bent, or touching anything else, except at the tie point.  Check for clean connections at all metal joins.  Reheat and solder connecting lugs and any other connection points.  Trimming the hat usually raises the frequency, so check the swr curve for the resonant frequency.

 

7.  Stub Problems - Stubs are usually easy to get going again.  Look for corrosion on any of the metal contact points, including the mast/driven element.  Clean any suspect joints. Clean any screw threads involved.  Look for anything that's not normal.  No other metal should touch the stub or brackets.

 

8.  Measurement Problems - With a swr analyzer, be aware that you can get false readings from nearby rf sources, on some models. If in doubt, try another antenna, or check with a dummy load to make sure the analyzer basically works.  Use another antenna if possible.  Double check your reading with an rf swr/power meter with at least a couple of watts applied.  If there is a difference, believe the power meter over the analyzer.

 

9. SWR Indications - Draw the swr curve.  Find the resonant frequency if you can.  If the swr is good at resonance, then the antenna is working; it just needs tuning.  

 

If the swr cannot be found, or is over 10:1, then the driven element, or the balun, is the problem.   If the swr is around 5:1, it is not the driven element, but either the reflector or director(s). When the driven element is working, it can resonate, and have a bad load from the reflector/director array.  If it’s not resonant, no swr curve can be found.

 

With a swr analyzer, be aware that you can get false readings from nearby rf sources, on some models. If in doubt, try another antenna, or check with a dummy load to make sure the analyzer basically works.  Use another antenna if possible.  Double check your reading with an rf swr/power meter with at least a couple of watts applied, for comparison.

 

10. Lightning damage – Any number of problems can occur during a hit.  Look for melted tubing, crystallized solder joints, burned and melted traps, missing wires, bad baluns and coax.  Damage may not even be visible.  Wire on traps can flake and curl up and short out turns.  Plastic parts may be gone or damaged.

 

AA5MT

 
Article details
Article ID: 14
Company: Knowledgebase
Date added: 2014-09-23 16:15:10
Views: 1083

 
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