Topband: Making my tower disappear - results

Tom Rauch
Thu, 10 Oct 2002 06:30:38 -0400

Good work Tree. Your results parallel what I have seen over the years 
in BC systems. 

> It turns out the minimum current in the tower was pretty close
> to the maximum current in the wire.  However, I found the 
> setting for minimum tower current was slightly lower in 
> capacitance than maximum drop wire current.

The reason you see the slight difference is the equivalent series 
reactance you are creating between the tower and earth is most 
effective when it is skewed towards the SAME sign as the series 
reactance of the tower. Also the drop wire diameter is very different 
than the tower diameter, remember you really want exactly equal 
currents in the tower and drop wire of exactly opposite phases.

If you cut the tower in the half, insulated it, and measured the 
impedance in the area of the drop wire...and if the tower were a j0 
resistance at that point... you would want exact resonance. Obviously 
the tower is not perfectly resonant, so there will be some skewing of 
the optimum setting. Plus the drop wire system is not symmetrical.

As a general rule maximum current in the drop wire will be about 
optimum, but exact optimum can skew either side of that point.

> Another interesting find was that the current probe on the drop 
> wire had an effect on the setting as well - along with a pretty
> big impact on the depth of the null.  The null was much better
> without the current probe on the drop wire.  I think that making
> the measurement on the tower is a better place to do it.

That is because of the construction of your current probe. The 
construction of a current probe is critical, we should be sure the 
probe does not perturb the system. That generally means using a low 
resistance termination across the sampling loop, a compact but thick 
core of modest to low permeability at the operating frequency, and 
sufficient turns. You want a winding many times the impedance of the 
termination resistor with very high coupling throughout the core, and 
a low termination resistance.

It is exactly like designing a line sampling transformer in a 
directional coupler.73, Tom W8JI