Topband: First 160 Loop Antenna Disaster

[Tom Rauch]

> Well I made my first loop antenna:
>
> http://www.hard-core-dx.com/nordicdx/antenna/loop/coaxloop.html


Bob,

Did you remember to gap the shield at the top? Did you tune 
the loop with an analyzer?

Sometimes things that should not work or do not work 
according to plan wind up working through pure dumb luck. 
Multiple mistakes sometimes wind up correcting each other.

There are two flaws I see in that design.

1.) There are two independent parallel resonant circuits 
(each loop) in parallel. That's a very bad thing to do, 
because if it is done with short leads you wind up with ONE 
resonant frequency between the two independent resonant 
frequencies. Normally two independent parallel tuned 
circuits are SERIES connected. Series resonant circuits can 
be paralleled, not parallel circuits.

2.) There is no ground or return path on the side of the 
coax. You have a tuned center loop but one side simply 
floats!!

I think what is happening or what could happen is the two 
mistakes correct each other, if you are lucky. I'm just 
guessing, it could be it was originally intended you ground 
the center conductor on the left side of the loop and left 
that out of the drawing.

The stray capacitance from the center conductor to shield on 
the left side of the loop (looking at the diagram) could, 
through luck in values, make the loop a combination series 
and parallel resonant system.  This unplanned path (through 
center to shield capacitance) on the left might supply the 
ground path to deed the feedline shield. However, this is a 
badly unbalanced feed arrangement.

It also could be the author intended you ground the center 
conductor entering the box on the left side of the drawing, 
and simply left that out.

You can see a walk-through of feed arrangements at this 
link:

http://www.w8ji.com/magnetic_receiving_loops.htm


73 Tom