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Effectiveness of decoupling radials
I have been playing with some NEC-2 models of a multiband vertical with radials. The vertical is an unloaded vertical of 13m height, and it is mounted on a 6m high grounded metal mast, and an ATU installed at the feedpoint (base of the radiator). I have fitted a pair of opposed nominal quarter wave radials for each of the 80, 40, 30 and 20m bands. To simulate ground loss, I have modelled a 20 ohm resistance in the bottom of the mast, and used a MININEC ground. An interesting observation is the sensitivity of this model to length of the radials. Properly adjusted, each pair of opposed radials near eliminate current on the mast (more than 20dB below the current into the radiator). The exception to this is the 30m radials which seem to suffer some interaction with the 80m radials (near third harmonic). Without the appropriate radials, current in the mast to ground is large, and losses can be 10dB or more. The ideas I take away from the modelling excercise is that: - inadequate decoupling exacerbates ground loss; - decoupling is very dependent on the length of the radials; - one pair of opposed radials is enough for a narrow band; - the radials for different bands have some interaction; and - the optimum length may be quite a deal longer than the expected length of legs of a half wave dipole in the same place. I am grappling with some other way to optimise such a system, other than measuring the mast current (which often isn't easy). These effects probably also apply to a trapped vertical with similar counterpoise, and the traditional wisdom of tuning either the length of the vertical or radial length to achieve low VSWR is probably less than optimal, there is an optimal length for each of them. The traditional wisdom that elevated radials are generally significantly lower loss than buried radials probably depends on careful "tuning" or isolation of feed point ground to minimise current flowing to the real ground. Comments, thoughts? Owen |
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