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Troubleshooting Multiband Antennas


Multiband Antenna Troubleshooting

 Cushcraft, hy-gain, and MFJ Antennas


This is a simplified explanation of current flow in antennas.  There will be no technical jargon, which tends to cloud the issue.  It should be fairly helpful in finding the antenna problem.  It is not meant to be strictly accurate; only a guideline.


First, consider current flow in a single band dipole or monopole antenna.  The current flows in at the feedpoint and moves out to the ends of the elements in a series fashion, like water flow.  I like to think of it as sap flowing in tree.  If any break is encountered, the flow stops, and reflects back, creating a bad swr situation.  If the break is not entirely fatal to current flow, the path distance is changed, therefore changing the resonant frequency.


Second, consider a multiband dipole or monopole(vertical).  Use a standard three band trap antenna covering 10, 15, and 20 meters.  Current flows from the feed point and out to the end of the conductor.  The end of the conductor is set by the first break that applies at the operating frequency.  For 10 meters, the end of the conductor is located at the 10 meter trap.  It can go no further, as the trap electrically chops off the current at the 10 meter physical location.  The length sets the frequency.  


A special case occurs if the trap is electrically open, and defective.  From a current view from the transmitter in this case, the current flow is the same, and stops at the same point, or very close physically.


To a certain extent, the resonant frequency of the trap will affect how much of the current is stopped.  If the trap is off frequency, or the operating frequency is far from the trap frequency, the amount of trapped current will start to reduce.  Beyond a certain frequency, the trap effectively disappears.  This is what you want when operating other bands.


At a transmitting frequency of 15 meters, the current flows through the 10 meter trap like it is not there and flows out to the next current blockage.  This now occurs at the 15 meter trap.

 For 20 meters, the current flow continues on the the next blockage, which now occurs at the end of the element.  Now, neither the 10 or 15 meter trap shows much resistance.



To be continued...



Article details
Article ID: 6
Company: Knowledgebase
Date added: 2013-11-26 06:08:41
Views: 749

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