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Re: Explain the RG62 thing to me, please...



On Sat, Apr 07, 2001 at 10:53:02AM -0400, Bob Bruninga wrote:
> Hummh... I'm not sure that you can make a 100 Ohm dipole.  If anything, a
> practical dipole at VHF and UHF due to diameter of the elements is much
> lower than 70 ohms and goes lower still when over the ground plane.  If
> you inferred 100 ohms from SWR only, then it could just as easily have 
> been 25 ohms which would have the same SWR to a 50 ohm line?

I modelled it in AO. According to AO, the feedpoint impedance
is 96.6 + j7.0 ohms. With two of them at right angles, the
impedances are 96.6 + j8 and 95.5 + j6.3, so the 93 ohm cable
is a good match.

The model is:

2m dipole
Over ground
146.0 MHz
2 copper wire, mm
10   0 -497 820   0 497 820   12
10  -497 0 830  497   0 830   12
2 sources
Wire 1, center  1  0
Wire 2, center  1  90

The two elements are 994 mm long, and 820 mm above ground (0.4 wavelengths).

For an individual dipole the model is:

2m dipole
Over ground
146.0 MHz
1 copper wire, mm
10   0 -497 820   0 497 820   12
1 source
Wire 1, center


It seems to work that way in practice too.

Hamish
-- 
Hamish Moffatt VK3SB <hamish@debian.org> <hamish@cloud.net.au>
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