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InnovAntennas LFA explanation

InnovAntennas sells antennas that have a driven element in the form of a co-planar loop. (InnovAntennas 2013) describe their Loop Fed Array (LFA):

The LFA (Loop Fed Array) Low-Noise Yagi is very different from the traditional dipole fed Yagi in many ways with its primary benefit being unwanted noise rejection. The LFA has a rectangular shaped, full wave loop driven element that is laid flat on the boom between and in-line with the parasitic elements . Then there is the way in which the loop functions. The smaller end sections which run parallel to the boom, are engineered to be 180 degrees out -of-phase with each other. This provides the same effect as is seen within ladder-line feeder; each side cancels the other out and therefore, minimum radiation occurs. In practice this translates to highly suppressed side lobes and side-on signal rejection. This feature also plays a role in reducing F/B (Front to Back ratio), F/R (Front to Rear ratio) and broad-banding of the antenna too. It is these attributes which help give the LFA class-leading all-round performance at almost any boom length and for any given band.

Of particular interest is their description of how the co-planar loop works and the benefits to which it contributes.

The contentious statement is [t]he smaller end sections which run parallel to the boom, are engineered to be 180 degrees out -of-phase with each other. This provides the same effect as is seen within ladder-line feeder; each side cancels the other out and therefore, minimum radiation occurs.

Fundamental principles

If two parallel conductors carry currents equal in magnitude and opposite in phase (ie 180° out of phase), then in some directions the magnetising force of those currents will cancel completely, but in other directions where the difference in distance to both wires is not some integral number of wavelengths (including zero), they will not cancel completely, indeed in some directions they will reinforce.

Fig 1:

Fig 1 shows the relative magnetising force (in dB) in the far field at different directions normal to two conductors carrying currents of equal magnitude and opposite phase for a range of conductor separations in terms of wavelength. Zero degrees is in the plane of the conductors.

It can be seen at that at 0° and 180° there is complete cancellation of the magnetising force of one conductor by the other for all separations.

At a separation of a half wavelength, at 90° there is a relative magnetising force of 6dB, or double that of a single conductor, total reinforcement of once conductor's current by the other due to the phase and distance from the far observer to both conductors.

Application

InnovAntennas liken their driven element loop sides to a ladder line feeder which they describe as radiation cancelling without qualification.

Fig 1 shows that when the separation of the two conductors of a ladder line feed is extremely small wrt wavelength, the cancellation is quite good in most directions, for example the worst case total magnetising force is -45dB for a separation of 0.001λ (20mm at 15MHz).

However, in the case of the LFA sides which are probably around 90% of a half wave or 0.45λ the total magnetising force at 90° is 5.9dB. There is not cancellation in general, rather there is almost total reinforcement in some directions.

Conclusion

InnovAntennas claim that [t]he smaller end sections which run parallel to the boom, are engineered to be 180 degrees out -of-phase with each other. This provides the same effect as is seen within ladder-line feeder; each side cancels the other out and therefore, minimum radiation occurs is plainly incorrect.

Whilst it is common place for antenna designers, manufacturers and sellers to make extravagant claims for their antennas, to offer explanations that are inconsistent with conventional engineering and science, it damages the credibility of all of their claims.

Simply put, if it doesn't work the way they describe, then does it deliver the benefits that they claim as a result?

Nothing in this article is to suggest that the InnovAntennas LFA might not be a good antenna, merely that the explanation that they give for some of its properties is unsound.

Links / References

Changes

Version Date Description
1.01 29/09/2013 Initial.
1.02    
1.03    

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