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1/2 Wave Dipole Question


Paul F
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I want to make my own half-wave dipole for 216Mhz using a couple of SMA whips. What is not clear to me from literature:

 

- What constitutes L in the diagram? Is it the total length, including the case (as implied by all the diagrams I've seen) or is it just the length of the whips?

- If it is the latter, is case length crucial?

- Can the case be metal or does it need to be an insulator?

- A variable length dipole I bought has a resistor between the poles (200k). Is this necessary? The diagrams I looked at don't have one.

 

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L is 1/2 wavelength or 26" at 216 MHz. The individual arms would be about 13". The upper and lower connectors'  outside shells are grounded to the case, though this has little to do with operation. The 200k resistor is to discharge static electricity and can be almost any value greater than 1k. What is not shown is a balun to convert the balanced arms of the dipole to the unbalanced shielded cable. Also, the way this is drawn, the case would have to be plastic or the lower arm is shorted to the case by the output connector. Look for an example on the web that uses a folded cable or a ferrite transmission line transformer at the output to form a balun.

Best Regards,

Larry Fisher

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It's the middle connector that is the offender. Note the lower arm is going to the outside of the middle output connector which is connected mechanically to the box. There should still be a balun in the circuit.

Best Regards,

Larry Fisher

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