[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Extra coil



Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-qwest-dot-net>" <evp-at-pacbell-dot-net>

Tesla list wrote:
> 
> Original poster: "Malcolm Watts by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>
> 
> Hi Robert,
> 
> On 9 Feb 2002, at 9:31, Tesla list wrote:
> 
> > Original poster: "rheidlebaugh by way of Terry Fritz
> <twftesla-at-qwest-dot-net>" <rheidlebaugh-at-zialink-dot-com>
> >
> > NOT correct:  the crystal set used the natural resonant frequency of the
> > coil with no capacitor. The tap slide on the coil was the only tuning for
> > phase 1 crystal sets. Capacitor tuning came later and was much more
> > convinent than the coil tuning. Coil tuning was a trial and error play
every
> > time you wanted to change and every day you started to recieve.1/2 to 1
hr..
> >    Robert  H
> 
> It is then difficult to see how tapping could have changed anything
> other than loading and antenna matching to a degree. Tapping the
> aerial at various points would have been a somewhat ineffective way
> of tuning because of coupling between the tapped and untapped
> portions of the coil. I can only attribute the low Q of the circuit
> to the loading that it was subjected to. An extra tuned circuit
> coupled to it would certainly have made a difference. I still can't
> relate this to Tesla's extra coil arrangement.
> 
> Regards,
> Malcolm
> <snip>

	I have built a couple of "high-performance" crystal sets here using
what appears to be a similar configuration.  There is a tapped coil in
series with the antenna, a variable capacitor in series with that.  The
"bottom" end of the capacitor is connected to the rotating primary of
the main tuning coil.  The tuning coil itself is parallel-tuned with a
capacitor, and there is a tap switch to vary the amount of the coil
across which the detector is connected.  For any given station the first
tuning step is to set the coupling to minimum, the loading coil to the
highest inductance which can be tuned, and then fine tuning with the
capacitor.  The coupling is then increased slightly and the main tuner
adjusted for maximum output.  By playing with all of these adjustments a
point is found where the output of the detector is maximum.  The net
result is a double-tuned setup with variable impedance matching, which
can be highly selective if the coupling isn't too tight and the detector
is tapped toward the "bottom" of the tuning coil.  Maximum detector
output comes with increased coupling, the detector tapped higher on the
coil, and the minimum loading coil inductance that will tune with the
maximum series capacitance.  On a local 1430 kHz, 10 kW station a couple
of miles away I can get as much as 30 volts DC out of the detector when
high-impedance headphones are used.

	I suspect that the adjustment of an extra coil configuration is
similar, except that the "antenna" capacitance is fixed at a value set
by the top loading and distributed capacitance of the extra coil. By
adjusting the primary circuit and the coupling a setting can be found
which results in maximum energy transfer, and hence longest arcs.

	By the way, I can see no reason while a configuration in which the
loading coil is connected directly somewhere along the main tank circuit
wouldn't work just as well as the inductively-coupled configuration. 
Are any of the "extra coil" setups guys have built connected like this?

Ed