To Bob, ZL2CA and others from PA0SE
Bob wrote:
You did not comment on
the phase at the hot end of the coil compared to the cold end.
I cannot measure the phase difference between currents in and out of the
coil. I don't have a current probe with my scope but even if I had one the
high voltage at the hot end necessary to obtain a readable current on the
thermo couple ammeter in the aerial wire would make the applicatiion of a
current probe impossible.
I did look at the phase difference between the current at the bottom end and
the voltage end the top end. Holding the voltage probe of the scope near the
top of the coil was sufficient for this.
Not surprisingly the phase difference was exactly 90 degrees, as far as this
can be judged from a scope display.
Using the idea of I+Q current going into the cold end of the coil, I
suggest
there are two tuning conditions of interest:
- when the antenna is tuned for maximum current out the hot end (into the
antenna wire), for maximum radiation, then the condition at the cold end
is
not purely resistive
- when the loading coil is tuned so the current at the cold end is
resistive, the radiation current (hot end) may not be peaked.
I checked this with great care. In my earlier e-mail I reported that the
output waveform of the transmitter voltage was somewhat distorted due to
harmonics. I now reduced the drive to the final amplifier to a level where
the output voltage became a pure sinewave. The current into the bottom end
of the coil was displayed as the voltage over a 0.78 ohm resistor carrying
the current.
I very carefully tuned the system for maximum aerial current. Because
voltage at and current into the cold end of the coil were now both sinewaves
I could superimpose them on the scope by varying the amplification in one of
the two channels. The two sinewaves completely coincided, showing that
maximum aerial current occurs when the impedance at the cold end of the coil
is resistive.
I think I now have done the maximum that is within my measuring
capabilities.
But suggestions are still welcome.
73, Dick, PA0SE
| 