Topband: Steering null angle with K9AY loop

[Tom McDermott]

It the request of some friends, I have been modelling a K9AY loop using 4NEC2.
One interesting thing that has come up is that it is possible to steer the vertical angle
of the null by changing the resistor value. Higher values of R increased the vertical
angle of the null, while lower values of R lower that angle.  Values of R away from
the optimum value distort the pattern badly, and result in less-deep notches.

However, if I added a reactance in series with the resistor, it would clean up the
pattern for the non-optimum values of R -- resulting in steering the null up and
down with again a clean pattern. But this requires two 'knobs': the resistor value
and the value of reactance.

Using Norton/Sommerfeld ground "average" (which is probably not the best
model to use - but one readily available here) I got the following results at 1.8 MHz:

350 ohms + 1000 pF    Null angle is 35 degrees up off the horizon
420 ohms + no reactiance  Null angle is 50 degrees up off the horizon
500 ohms + 9 uHY     Null angle is 60 degrees up off the horizon.

I tried this model with other geometries (somewhat larger than the K9AY) and that
NEC model achieved lower null angles at the expense of a larger sized antenna.

Has anyone tried this with a real K9AY antenna (I'm antenna challenged at the moment).

    -- Tom, N5EG


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