[AMRadio] Antenna/Coax


Larry Will lhwill at verizon.net
Sun Dec 31 09:22:21 EST 2006


Jack,

A dipole is only 73 ohms in free space away from ground etc.  A 
typical low antenna such as we hams might use often is 50 ohms or 
even lower.  The various antenna handbooks have a lot of info on this.


A 1/4 wave vertical series fed monopole over perfect ground is also 
about 37 ohms.  Typically the ones I have measured at broadcast freqs 
usually have some reactance when put over a 120 radial ground screen.

Larry  W3LW

At 11:45 PM 12/30/2006, you wrote:
>The more I read, the confusder I get.
>
>If a half wave dipole has a center feedpoint impedance of 73 ohms, 
>why am I using 50 ohm coax.
>
>I just put my new EZ hang to use today and discovered that my coax 
>is not long enough to take advantage of the much higher ends and is 
>pulling the center down.
>
>I was going to buy a roll of RG8X but then learned that RG58U is 
>good to over 430W so I might as well stick with this as weight is an issue.
>
>In the process of hunting up a source for coax I stumbled into this 
>quandary.  Every source I look at says 73 ohms for a dipole.
>
>What am I missing?
>
>JACK K9ACT
>
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