Re: Variable Capacitance and Inductance

["Tesla list" <tesla-at-pupman-dot-com>]

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

Tesla list wrote:
> 
> Original poster: "David Thomson by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <dave-at-volantis-dot-org>
> 
> Hi Matt,
> 
> >The vast majority of  receivers in the world don't use crystals for the
> local oscillators. The commercial transmitters that use them and the tiny
> number of military receivers that do, keep them in temperature-controlled
> environments, because even a crystal's frequency IS temperature sensitive
> (drift).
> 
> Every military radio I have sold (about 100 in the past year) had crystals
> in them.  I haven't heard of a military radio without crystals.  

	I have, and own a operate a few.  The Navy ARC-5 and its USAAF
counterparts were excellent gear and plenty stable enough for general
use.  The infamous BC-375 was an antiquated transmitter used in almost
all US bombers during the war, although as one guy put it "it was made
mainly to be shot down in bombers".  Some equipments, like the Collins
line, were exceptionally stable and probably still in use; certainly
their use persisted long after the war and hams such as I still have
them as part of their operating equipment.  Crystal-controlled receivers
and the accompanying transmitters came into military use during WW2, as
they could be used by inexperienced or incompetent operators.  The vast
majority of HF transmitters and receivers were not crystal controlled,
and the use in radios and TV's began only in the late 70's and early
80's, when inexpensive synthesizers came into use.

	"VFO's" for frequency control are still used in ham radio gear and
inexpensive short-wave receivers, although crystal-controlled
synthesizers are becoming so cheap that they are used almost exclusively
in top-end equipment.

	
> 
> As I mentioned to Dave P., the inductance used in the oscillators of radios
> is far smaller than the inductance used in Tesla's coils.  And most
> inductances in radio antennas that I know of use ferrite cores, which help
> to stabilize the inductance.  In older radio oscillators most of the
> oscillator inductance is in the wires between the vacuum tubes, and the
> oscillations are maintained mostly with RC circuits.

	Incorrect.  For example, I have a Navy RBL receiver built during WW2
(the design was older) which uses inductors with inductance as high as
as 50 millihenries and more, which is well within the range of some of
the larger coils.  The RF input coil to almost all broadcast receivers
had an inductance of about 330 microhenries, etc. etc.  Only very high
frequency receivers depended on the inductance "in the wires" for
tuning, and only very shoddy ones.  RC circuits were never used, to my
knowledge; if they were they certainly aren't mentioned in any of the
radio text or hand books.
> 
> In radios using only air core inductors in the antenna circuit, constant
> tuning is necessary throughout the day.  Dave P. mentioned he turned on the
> radio to one station, and the next day the same station was there with no
> drift.  This is not surprising.  To check for radio drift according to the
> diurnal effect Tesla mentioned one would have to monitor the same radio
> station throughout an entire 24 hour period to notice any drift.
> 
> I have 4 old AM tube radios.  The stations come in fine during the day but
> just after the sun sets and just before it rises the radio stations drift
> considerably.

	As has been pointed out already, that is due to changes in ionospheric
propagation, not frequency drift.  The effect was observed as early as
Marconi's 1904 transatlantic effect.  Long-distance sky wave propagation
is the primary mode for short-wave communication and broadcasting, with
the appropriate frequencies changing with time of day and year (affects
the angle with which the sun's radiation strikes the ionosphere) and
solar conditions which can vary the particle flux striking the earth. 
There was a major disturbance yesterday morning, for example. 

> I know stranger things have happened.  Ham operators tell me strange stories
> all the time.  

	I've been a ham for well over half a century and hams operators have
told me many strange stories.  A few of them have even been true, but
readily explained by known laws of nature.

> But there is no doubt that the earth's position with respect
> to the sun can alter circuitry such that there are frequency changes.

	You and Tesla appear to be the ONLY ones who have ever observed such
alteration in the circuitry due to solar influences other than direct
heating!

> And
> if frequency changes, that can only mean the inductance and or capacitance
> have changed (in a LC resonant circuit.)
> 
> Dave

	If the frequency did indeed change, that would be the reason.  No
evidence whatever that it changes at all!

Ed