SADeployment

A probe antenna is categorically different from a matched antenna in that no power is extracted from an incident wave. Although no power is extracted, the presence of such a probe element does deform the nearby electric field of an incident wave which is to be received. Put another way, the presence of conductive material causes scattering. For both matched and probe antenna types there is a region of influence for such an element generally called an aperture. For best operation it is important to keep this region clear of conductor.

For the SAS use case, induced current in the CAT5 can produce re-radiation that counters the impinging electric field near it and cast a shadow that can overlap the probe's aperture. Rerouting the CAT5 or reducing current within it can reduce this interaction. Empirical measurements have shown that a few percent degradation in WSPR spots is produced when the CAT5 simply exits vertically, parallel and close to the dipole (CAT5 90) but not far enough away to escape the shadowing as compared to spots produced when the CAT5 comes away at an angle (CAT5 45). Compare the e-field magnitudes at the dipole tips, graphically represented in the z-axis plots above. Sight rerouting seems to be enough to cause the interaction to become negligible. For this reason, a revised mounting technique different from a closely parallel one is being followed whenever possible.

The sketch on the left below shows rather the reverse of the SAS use case. In it a scattering dipole is shadowing a CAT5 90 'monopole' rather than the other way around. Though the areas are to scale, the shape and field intensity distribution depictions are not intended to provide precise information but only a general impression of the situation when a sensing antenna is shadowed by a nearby scatterer. In this sketch, possible routes for the CAT5 'antenna' that might be used to keep it sufficiently far away from the probe "shadow" are shown. These are ways the interaction between the CAT5 and the probe may be minimized.

NEC2 (4NEC2) visualizations of the resulting vertical e-fields produced by an incident electric field are shown below to the right. These are with a uniform linearly polarized incident field and above a MiniNEC ground. In each case the test frequency is that where the CAT5is a quarter wave length, where it has highest Q and produces the largest fields. The bottom of the CAT5 is directly connected to the 'ground', which produces higher current and more scattering than an actual use case where a continuation of the CAT5 cable continuing a considerable distance to the ShackBoard location will have non-zero impedance.

An idea of the degree of scattering may be obtained by comparing the e-field intensity near the tips of the dipole for the CAT5 90, CAT5 45 cases and the"bell-bottom" case where the lower portion of the lower mono-pole element is simply moved away from the CAT5 at an angle. These strategies are suggested by SWTL theory which indicates (see A New Antenna Model)

no significant TEM coupling for these geometries between the CAT5 and monopole element far from the tips. This is maintained if the smallest conductor of a parallel conductor line is kept 100 diameters away from the other so that the line is TM only.
the aperture associated with radiation is mostly around and beyond the tip of the element

The fourth plot at the bottom right shows the mast moved 4.9m away which also seems "far enough".

Though of use only over a limited frequency range it may be that ferrite chokes or lossy beads placed on the CAT5 line can also be used to further reduce the shadowing effect. This has not yet been investigated.

CAT5 cable routing, Scattering & Shadowing