measuring the dipole

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measuring the dipole

Postby KF5CSW » Mon Nov 15, 2010 12:33 am

Hi everyone,
I'm almost ready to get on the air; I got my General privileges last week, and I'm going to pick up a SWR meter shortly. I've got a few things I need to do with the antenna. One question: When measuring for 14 mhz, do I include in the dipole length the wire I wrapped up around the insulators? Or do I just measure the unwrapped part?'

John Vogel
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KF5CSW
Bentonville, AR
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Postby NT9K » Mon Nov 15, 2010 2:05 am

Hi John, you measure it from the middle of the center insulator to the end of the wire, not the part you wrapped. Look in this graphic:
Image

At the top of the picture, it says insulator eye. That is the end of the wire and your mark. Just under that in the picture, it shows and arrow pointing at the middle part of the center insulator, that is the other end you want.

Another thing to remember is if you use insulated wire. The rule-of-thumb for insulated wire is to make it 3% shorter than bare wire. Good luck with your antenna John. Take care.
73, Bill - NT9K
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Postby k8jd » Sat Mar 12, 2011 2:20 am

If you use the formula; L=468/F. the L is the entire length from one loop at the end insulator to the other end loop around the other end insulator.
Each wire from the coax connection to one end insulator is 1/4 wave.
The entire dipole length is 1/2 wavelength.
This formula is an aproximation. The exact length is arrived by using your SWR meter and trimming the wire for a fine adjustment .to get the minimum reflected power.
You may want to start a foot longer than the formula length and trim equal amounts off each end after measuring the reflected power and repeat the trimming a few inches at at a time until a minimum SWR reading is attained..
Raising and lowering the antenna for trimming sounds tedious but the result is quite rewarding.
The length will vary according to; the height above ground, the ground conductivity in your location and surrounding objects.
Have Fun
73 from K8JD, SKCC 1395, Centurion 18, Tribune 12, Band Endorsements; 160, 80, 60, 40, 30M, 20, 17, and 10M

Ham radio is real Radio, CW is real Hamming!
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