Topband: 40 foot diameter shielded loop

[Tom Rauch]

Hi Ford,

This topic come up from time to time. There is a lot of 
misinformation circulated about loops. Newer editions of Reference 
Data for Radio Engineers has a somewhat plain text explanation of 
small loops. I know we try to restrict the bandwidth, but this is a 
common topic on 160 meters so I'll get a bit detailed. 

At times instruments like the Autek and AEA give the wrong sign, 
so be careful! Add a fixed component and be sure the sign is 
correct.  

> With that being said, I was expecting to see an inductive reactance at the
> loop.  Instead I see a very low R and capacitive.  What's up with that?

More is going on than meets the eye. I'll try to explain. 
 
> Can anyone explain to me what it is that is being "matched" on a shielded
> loop?
> Can anyone explain to me why 20 feet appears to be the maximum allowable
> loop size -- or so I've been told from several articles, none of which
> explained why.  Is the shielded loop a resonant or non-resonant antenna?
>The shield is the antenna, not the wire inside the shield. The shield 
>doesn't just "shield", it actually becomes the antenna.

The inside of the shield is *totally* isolated from the outside by the 
skin effect. Neither magnetic nor electric fields at the operating 
frequency pass through the walls of the shield when the shield is 
several skin depths thick. (Just like what they tell you about coax 
in transmission lines.)

At the gap in the shield, the current induced on the outside "spills 
over" the edge. The voltage difference across that gap excites an 
exactly equal and opposite direction current on the inside wall of 
the shield. (That's why if you short that gap the loop goes dead.)
We really have a path that involves the length of the shield TWICE, 
once all the way around on the outside and once all the way 
around on the inside. Only the outside is involved in responding to 
electromagnetic (not magnetic only!) fields. 

The turn inside the loop (the center conductor) couples directly 
ONLY to the inside of the shield. The center conductor's current 
flows the opposite direction of current inside the shield. Of course 
that makes the direction the same as the outside of the shield! 
There is distributed capacitance, extra loss resistance, and a lot of 
other effects involved in all of this.

It shouldn't be surprising the loop looks much "longer" than we 
would expect, since the inside and outside are effectively in series! 
There are also some distributed capacitance effects at work.

Within about 1/10 wl of a small loop, the field impedance is low. 
(Field impedance is the ratio of electric and magnetic effects, 
including the radiation field and induction fields that are both 
electric and magnetic.) That means the magnetic induction field 
dominates. 

At about 1/10th of a wavelength out, the field impedance increases. 
The total field response of a small "magnetic" loop is actually 
electric field dominant at that distance. (A very small vertical would 
be magnetic field dominant at the same distance!) 

At about 1/2 wl or so out the loop responds to fields just like any 
other antenna in the world. You couldn't tell if it was a loop or a 
small electric probe doing the radiating or receiving at that 
distance! 

The name "magnetic loop" is a misnomer, unless you are talking 
about what happens in the area immediately next to a small loop  
antenna (within about 50 feet or so on 160 meters). Even at that 
distance you have to remember the loop still responds to electric 
fields. If it didn't, it would be stone dead for hearing any signals!

To be called a "small loop" the antenna has to carry essentially 
equal current around the circumference. That's why you hear 
numbers like "20 feet" bantered about. Your loop will still receive, 
but it is becoming so large it is starting to move away from the 
characteristics of a true small loop. The field impedance 
surrounding your antenna is moving closer to that of a large dipole 
or monopole, not that it hurts anything. 

The shield does not make it a "magnetic only antenna", despite 
popular folklore that it does.

To be resonant, the loop has to have any reactance cancelled...no 
matter what sign the reactance is.

73, Tom W8JI
w8ji@contesting.com



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