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[TowerTalk] Force 12 Sigma 80

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Subject: [TowerTalk] Force 12 Sigma 80
From: (Tom Rauch)
Date: Tue, 22 Jan 2002 14:16:51 -0500
> Brad was considering the Sigma80, a center-fed vertical **dipole** for
> 80 meters, which does not need any form for counterpoise or radial
> system.   In order to match Force12's specs, I believe a shortened GP
> would hardly do the job. 

Not speaking of any vertical antenna in particular, but all vertical 
antennas in general we have to be careful!

We can feed power into a short vertical dipole mounted near earth 
and it will have a low SWR and radiate. We might even work some 
good distances, and it will certainly be much better than no 
antenna at all.

But we should remember any short vertical mounted near earth will 
not work nearly as well as a similar height monopole with even a 
modest ground system. (the exception is perhaps found over 

First of all, we have significantly lower radiation resistance (using 
the IRE method of quantifying radiation resistance) when a less-
than-quarter-wave-tall Marconi is fed as a dipole with no earth 
connection. Second we still have earth loss because the field 
density in earth remains very high with the end of the antenna near 
earth. ON4UN touches on this in his Low Band DXing book.

We often hear we can "make it a dipole and eliminate ground loss", 
but it just isn't so. Unless perhaps you move the lower end of the 
antenna 1/4 wl or more above earth!

Have a look at K7LXC's HF Vertical Test Methods and Results, and 
look at how center-fed antennas do on low bands (where they are 
close to earth) compared to high bands. You'll see they work very 
well at higher frequencies where the radiation center is higher 
above earth in terms of wavelength, and they really are very poor on 
lower bands.

If you can install more than ten or twenty radials 1/8th wave long, 
you will most likely do much better with a normal base fed vertical.
73, Tom W8JI 

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