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The question is often asked in online fora, and the answers offered are almost always one of:
The first answer is the most popular.
Are they both be correct? Well no, appealing as short answers are, neither is correct as written.
A pair of conductors in proximity of some other conductors or conducting surface (such as the natural ground) can operate in two modes simultaneously, differential mode and common mode.
Differential mode is where energy is transferred due to fields between the two conductors forming the pair, and common mode is where energy is transferred due to fields between the two conductors forming the pair together and another conductor or conducting surface.
The currents flowing in the two conductors at any point can be decomposed into the differential and common mode currents.
Differential current Id is the component that is equal but opposite in direction, it is half the difference in the two complex line currents I1 and I2.
Common mode current Ic is the component of the line currents common to both conductors, it is half the sum of I1 and I2.
So, for example, if I1=2A and I2=1A, Id=(21)/2=1.5A, Ic=(21)/2=0.5A.
A line that is operating with perfect balance has only differential current, ie common mode current is zero. It is unlikely that a feed line in a practical antenna system is perfectly balanced, but with due care, it can have very low common mode current, 20dB or more less than the differential component.
In the case of differential current:
Fig 1 shows the relative net current moment at a great distance normal to a short line in the plane of the conductors for a range of line spacing in wavelengths. It can be seen that the net current moment (and therefore radiation) is very low at small spacing but is much greater at spacing above 0.01λ. So, for practical purposes for HF, a line with spacing of 150mm will have worse case net current moment of less than 20dB in the plane of the conductors, and even less in other directions.
Twisting a two wire line will reduce the net current moment due to differential current if the twist rate relative to wavelength (twists / wavelength) is high. With sufficiently high twist rate and even geometry, even for larger line spacing, the relative current moment can be very small.
Twisting a uniform two wire line reduces radiation due to differential mode current. On receive, Twisting a uniform two wire line reduces the differential current induced directly in the feed line due to external noise and signals.
In the case of common mode current:
Fig 2 shows the relative net current moment at a great distance normal to a short line in the plane of the conductors for a range of line spacing in wavelengths. It can be seen that the net current moment (and therefore radiation) is highest with small spacing, and reduces only slightly for larger spacing. It in not uncommon that the common mode component of current is greater than 20dB relative to the differential component, and in such cases can be a greater contribution to feed line radiation than that described in the previous section.
Twisting a two wire line will not reduce the total net current moment due to common mode current. The net current moment averaged in all directions will be exactly the same.
Twisting a uniform two wire line does not reduce average radiation due to common mode current. On receive, Twisting a uniform two wire line has little effect on the common mode current induced directly in the feed line due to external noise and signals. Common mode current on the feed line is quite likely to inject differential current at the end of the feed line.
Smaller spacing reduce feed line radiation and direct feed line pickup due to differential current component.
Twisting a uniform two wire line reduces radiation due to differential mode current. On receive, Twisting a uniform two wire line reduces the differential current induced directly in the feed line due to external noise and signals.
Spacing has only a small effect on radiation or pickup due to common mode current component.
Twisting a uniform two wire line does not reduce average radiation due to common mode current. On receive, Twisting a uniform two wire line has little effect on the common mode current induced directly in the feed line due to external noise and signals. Common mode current on the feed line is quite likely to inject differential current at the end of the feed line.
If you have a feed line that is very well balanced, twisting the line can help to reduce feed line radiation, or noise pickup directly by the feed line. If there is substantial common mode current, any benefit to differential induction from twisting the feed line is likely to be masked by the common mode current. On the other hand, if common mode current is high, there will be little benefit from twisting the line.
To be fully effective, the feed line twist rate needs to be high.
Version  Date  Description 
1.01  23/09/2011  Initial. 
1.02  
1.03  
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1.05 
© Copyright: Owen Duffy 1995, 2017. All rights reserved. Disclaimer.
© Copyright: Owen Duffy 1995, 2017. All rights reserved. Disclaimer.