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Last article was on matching the Radiation resistance of the antenna to 50 ohms. Observing with a Noise Bridge and using the earth ground to affect match.


What if the antenna is way up in the air out of reach?


This is where a little forethought will help. No matter where you put your antenna, give attention to being able to bring it down easily.


Many operators use pulleys to do this. They mount it in the trees, on poles, towers or where ever you plan to install your antenna. With a rope system, the feed point of the antenna can be lowered for access and maintenance.


The best place to check the antenna is where it will be fixed. Not always the best place to make measurements.


Using the Noise Bridge, you can connect the little instrument to the feed point of the antenna with a double male connector. Set the X control on 0 and the R control to 50. Then send it up into the permanent position. Go to your receiver and observe the noise generated from the bridge. Now you can scan the band to find the point where the noise is minimum. This will be your resonant point and should be addressed first. If you need to lower or raise the frequency of the antenna, this is the time to do it.


If you have found that the minimum noise is not completely gone, then you can drop the antenna again and move the R control slightly in either direction. If you still have your phone handset to hear the receiver, then you can make this final assessment at the antenna site.


If you are doing a fixed antenna on a tower that needs to be climbed, take the Noise Bridge and phone up with you. You can do all your work at one time while up there.


If you are going to use a metal pole to mount your antennas, give it some thought about being able to tilt the pole over easily for tuning and maintenance. There is a variety of pulley arrangements that can make raising and lowering the mast quite easy.


Using the Noise Bridge takes a little pre-planning. Are there analyzers that can test an antenna from the convenience of the radio room? There are. Next article will mention what to look for.



Ralph WD0EJA

February 2014

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