TopBand: RE: loops and preamps

Fri, 10 Oct 1997 09:41:29 +0000

> From:          K3BU@aol.com

> In a message dated 97-10-08 20:09:51 EDT, W8JI writes:
> 
> << The real effect of the shield is the shield itself becomes the 
>  antenna, and the coil inside simple extracts energy from the 
>  shield (the "real" antenna)! >>
> 
> Huh????
> So why is the max gain in the plain of the loop/shield? (Off the ends of
> "bent dipole" at that small dimension?)

Let me answer that question with another question. Why would the 
pattern change directions when the shield and the internal wires are 
all in the same plane? Do you think a one turn loop loop has a 
different pattern than a five turn loop of the same overall 
dimension?

> So the shield that is very short, grounded in the middle becomes "real"
> antenna, rather than resonated loop of wires?

Absolutely. That's why balance is improved, it is easier to balance a 
single turn to the outside world rather than balance several turns. 
The voltage and impedance is also less across the single turn's 
gap than the opposite ends of the multiple turn winding, reducing 
direct capacitive energy transfer if something does get closer to 
one end of the winding.

>So if there is no shield, the
> coil would have nothing to extract energy from?

No, the coil would simply become the antenna instead of the shield.

> I don't think that Mr. Maxwell and Faraday would agree with this explanation.

They exactly agreed, because it was in their formulas. It is 
impossible, according to Maxwells equation's, to have a time varying 
magnetic field without a time varying electric field. Take one to 
zero, and you take the other to zero. No exceptions are allowed 
according to Maxwell.

> YES, shield cuts down on static vs. "naked" wire loops. (Found out in
> practice, not from antenna simulation programs.)

Only by improving balance.

> Pardon my intrusion to top band waves, but this is too much to swallow.

I wouldn't know why. It's basic science, and one of the first things 
taught in electromagnetics.

73, Tom W8JI
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