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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 21:44:49 -0500
I enjoyed your ground loss comments, and mostly agree with 

> The Sigma eliminates 1) by just not using the ground as 1/2 the
> circuit. Feeds the top half against the bottom half.

I'm not speaking of any antenna brand in particular, but only of 
system in general. there is no magic bullet. But here are some 
interesting facts:

Making a dipole out of a Marconi-feed structure of given dimensions 
quadruples the amount of loading reactance required, all things 
equal. That quadruples loss in a loading system if that system has 
equal Q! An additional shortfall is bandwidth becomes narrower 
while efficiency of the loading system decreases for a given system 
efficiency! Bandwidth can actually become more restricted while 
efficiency decreases!

For example, a 30 foot tall one inch diameter vertical has a loop 
radiation resistance of 5.2 ohms (zero ground resistance 
considered for this example) and a feedpoint reactance of 400 
ohms at 3.6 MHz.

The same structure center-fed and isolated from ground has a loop 
radiation resistance of 4.78 ohms and a feedpoint reactance of 
about 1600 ohms. 

Since we need four times the reactance, for a given Q we would 
have four times the loss resistance. We would also have about 1/4 
the bandwidth even though loss increased four times!

We must be VERY careful to keep loading system Q high in a 
ground-independent vertical of the same height as a Marconi-fed 
system. Linear loading would be a poor bet for doing that, because 
Q's are normally in the double-digit numbers even with large 

A few west coast amateurs learned that playing with linear loaded 
yagi's on 75 meters.   

> If you also want to run high power on such a solution, some vertical
> solutions have ohmic losses in the antenna (due to design and/or
> aging) and have been known to literally melt down (been there, done
> that...). The Sigma is efficient, and won't melt down. It's not the
> only one.

"Melting down" is a poor example of loss, unless you confine the 
heat to a small area where you can measure it. One way to do this 
would be to enclose the whole antenna in a thick styrofoam box 
and measure temperature rise, or we could measure field strength 
and input power over perfect ground. 

Distributed loading systems (such as linear loading) CAN be very 
inefficient and not show the slightest sign of heating. Loss is 
distributed over a large air-cooled area with good thermal 

A small inductor such as a traditional 2" #12 gauge miniductor can 
"cook" from as little as 30 watts of heat. A physically-long air-
exposed aluminum tube could easily dissipate many hundreds of 
watts of heat without any noticeable effects.  

The only way we know if efficiency is actually high is to measure 
the FS in a A-B test. We won't know that from reading an 
advertisement, guessing, or watching for melt-downs.
73, Tom W8JI 

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