Whip length measurement for homebrew antenna

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W0BCQ

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Apr 8, 2014
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Walcott, IA
I am wanting to build a simple quarter wave antenna with 4 radials bent at 45deg for the 70cm band. Rather than soldering the whip (brass rod) to a panel mount SO-239 directly like many home-brew solutions I have found, I wanted to buy an SO-239 to 3/8" stud adapter then screw a ferrule onto that so I can easily change whip lengths if needed. It's easy enough to calculate the theoretical length needed, but I'm a little confused as to how the adapter and ferrule will affect the electrical length and impedance of the antenna?

73
W0BCQ
 

prcguy

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Jun 30, 2006
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So Cal - Richardson, TX - Tewksbury, MA
Most SO-239 to 3/8" adapters have excessive internal capacitance at UHF due to lots of thin internal insulating materials and lots of surface area that get squished together when you tighten the mount. They can work but you won't really know if its affecting the antenna performance.

If you end up using one, the antenna starts at the base of the mount at the insulator washer and the overall antenna length would include the stud portion of the mount. Since the stud is fairly fat it will shorten the antenna length very slightly compared to a small diameter whip for the entire length.
prcguy



I am wanting to build a simple quarter wave antenna with 4 radials bent at 45deg for the 70cm band. Rather than soldering the whip (brass rod) to a panel mount SO-239 directly like many home-brew solutions I have found, I wanted to buy an SO-239 to 3/8" stud adapter then screw a ferrule onto that so I can easily change whip lengths if needed. It's easy enough to calculate the theoretical length needed, but I'm a little confused as to how the adapter and ferrule will affect the electrical length and impedance of the antenna?

73
W0BCQ
 
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