Topband: Magnetic Loops

[Tom W8JI]

> Expecting this and given the proximity of my transmission antennas to the 
> Wellbrook Loop (50-100'), I put 10 Type 31 slip-on beads over each end of 
> the RG8-X feed and rotator control cable, and insulated the loop from the 
> rotator with CPVC plumbing pipe.
>

Common mode shunt impedance is what largely determines the effectiveness of 
any CM choke.  The system really is like a low pass filter, pi network, or 
pi attenuator pad. The beads are just the series element.

Small antennas, like loops, have extremely high common mode impedance. This 
is why chokes are almost useless for decoupling or reducing common mode.

As a matter of fact the skewing I measured on loops was not only with large 
amounts of choking impedance, it was also with grounding rods! The antenna 
was isolated exceptionally well from the feedline system.

A small loop has very high negative gain because every area of the antenna 
fights another area. The only reason it responds at all is the spatial 
difference between opposing sides, and the phase delay of the signal through 
air. 160 meters is 1.5 ft per degree, so spatial phase shift is only 4 
degrees in a 6 ft diameter loop. That means the phase difference allowing 
the loop to respond at all is only 4 degrees, meaning opposing sides are 176 
degrees out of phase. This is why small loops either have terrible gain, or 
have astronomical currents for the conductor length. A short dipole, with 
the same conductor length, has much higher radiation resistance and much 
more gain for the same loss resistance.

That difference is the problem. A small loop would rather work in common 
mode, where the sides are in phase and not nearly 180-degrees out of phase, 
making any feed design errors virtually impossible to correct with beads.

You can choke the feedline until the cows come home, and it won't fix a 
poorly balanced feed. The most choking will do, if augmented with properly 
located grounds, is stop common mode noise from following the coax to the 
antenna. Beads by themselves without a low impedance shunt path at the 
antenna side, however, are a waste of ferrite.

This doesn't mean loops will never work, or loops with poor feed systems 
will never make people happy. It just means it is almost impossible to 
correct a bad feed design by stringing beads or winding conventional chokes, 
even if you get up to 5,000 ohms. It is always far better to make the 
antenna with a correct feed system, because a proper feed has far less 
common mode response without any common mode foolery.

73 Tom