To All from PA0SE
Andy G4JNT wrote:
> I also suggest that the different current measured at each end of a
> loading coil, as mentioned in an earlier posting, could quite easily be
> due to the slight mistuning when the extra hardware associated with the
> ammeter is connected in circuit on the top side.
Steve, GW4ALG, responded:
As mentioned in my earlier posting, I was careful to check for
system resonance before each measurement. How about you, Dick?
The effect Andy mentions is indeed present. Apart from the thermo couple
ammeter that I moved from top to bottom of the coil there is another
instrument permanent in place at the top end. Inserting the second meter in
series caused the aerial current to drop from 2A to 1.85A. But a slight
retune restored it to 2A again.
Also when measuring the current of 2.1A at the bottom end, the instrument
at the top still indicated the same 2A.
In my former e-mail I said the difference was 10%; it is 5% of course.
Modelling showed that a difference of 1.55% is caused by the "normal"
capacitive current that leaves the coil as part of the near field around the
aerial as the coil is part of the radiating system. The remaining part must
be extra capacitive current flowing to earthed objects surrounding the coil.
To check this I moved the coil upwards and sideways in the shack and
expected this would at least cause a small change in the current difference
between top and bottom of he coil. But no, there was no detectable change at
all. So I'm afraid I still don't quite understand the situation.
73, Dick, PA0SE