Topband: Why bother with the coax shield loop RX antennas?

[k3bu at optonline.net]

> After the the Stew Perry event I need to get some RX improvement on
> topband. A small-space candidate is what many refer to as a 
> "magnetic loop"
> or "shielded loop" antenna. After reading lots of contradictory 
> material on
> the subject, I think W8JI has it right. If I understand 
> correctly this
> antenna...
> 
> - does NOT work on the principle of favoring magnetic field over 
> electricalfield
> - does NOT use the outer conductor of the coax for "shielding" 
> parts of the
> electromagnetic energy.
> 
> The antenna consists of basically three components:
> 
> a) The outer surface of the coax shield, which acts solely as 
> the antenna.
> The antenna currents on the outer shield result in RF voltage at 
> the shield
> edges at the gap.
> b) The inner surface of the coax shield. The RF voltage excites 
> current that runs along the inner shield.
> c) The center conductor fo the coax. The current on the inner shield
> induces current along the center conductor, which resonates with a
> capacitor.
> 
> 73 - Kristinn, TF3KX


One of few things that W8JI has wrong on his web site.
 
If you like to build effective small loop antenna here is my proven design.
Get copper tubing bend it in a circle about 4 ft diameter, cut in half, two identical half circles.
Use metal box at the bottom  and mount ends of the tubing in its sides, leave open gap at the top.
>From the connection box thread three turns of insulated wire and connect capacitor trimmer (about 1k) between the wire ends. This will tune the turns to resonance at choice 160m frequency.
Thread another single turn insulated wire through the tubing. Connect shield of your coax to one end of it and have trimmer capacitor between center conductor of coax and the other end of the loop. Do not connect to tubing or metal box. Ground metal box and tubing to mast supporting it and ground it with ground rod. You can use split plastic tubing at the top gap and wrap it with tape.
You can use small power and SWR bridge to tune the three turns to resonance and one turn to minimum or 1:1 SWR, or use antenna bridge.
You are ending up with nicely balanced 3 turn loop antenna tunable within the band, with one turn coupling and match for coax. 
Grounded tubing provides electrostatic shield and nulling capability for locally generated electrical noise.
Anyone can see that antenna are 3 turns, coupling is one loop and shielding is the tubing, which is grounded and symmetrical.
Max signal is in the plane of the loop, max rejection off the sides.

73 HNY  Yuri, K3BU