Dear Mal, Gary, LF Group,
G3KEV wrote:
The Collins equipment used this method in their VFO'S...
...and in their tunable RF amplifiers, etc., but as far as I am aware they
did not make permeability tuned ferrite rod antennas... perhaps there is a
Collins equipment expert who knows different?
There seems to be some confusion about Variometer. A Variometer consists
of
two coils in series with one rotated within the other to vary Inductance
This seems a rather restricted definition - I think other devices where you
achieve variable inductance by altering the mutual inductance between
different parts of the winding are called a variometers too, for instance a
smaller diameter coil telescoping inside a larger coil. The "shorted turn"
tuning method, where you have a movable metal slug, ring, etc, inside a coil
is a variation on the same principle also. It is also worth pointing out
that there can be more than two coils, and the coils can be in parallel as
well as series. This is actually quite useful - if you have a
series-connected variometer for 136k, you can connect the coils in parallel
to get roughly 1/4 the maximum inductance for 500k use.
G4WGT's extended range variometer reminded me of the device in the picture -
it is a rotating coil type of variometer, but the moving coil is actually
wound on a nearly-spherical iron dust core about 100mm in diameter. By
utilising the various taps, and changing from series to parallel connection,
the inductance can be varied in various ranges from a few hundred uH to over
3mH.
Cheers, Jim Moritz
73 de M0BMU
whereas a ferrite moveable rod inserted inside a coil is referred to as
Slug
Tuning.
mal/g3kev
----- Original Message -----
From: "James Moritz" <[email protected]>
To: <[email protected]>
Sent: Tuesday, August 16, 2011 7:17 PM
Subject: LF: Re: ferrite coil
Dear Mal, LF Group,
>Instead of winding a tight coil on to a ferrite rod to get 270 uH and
>tuning with a 250 p variable capacitor...
The required tuning capacitance would be about 5000pF.
>...instead wind an air core coil and move the ferrite rod into/out of
coil
>for the required inductance necessary for frequency >of interest. This
>would further reduce losses.
You could tune inductance downwards in this way, but unfortunately any
reduction would be accompanied by reduced magnetic flux due to the
received
signal in the antenna winding. So the signal amplitude would be reduced.
Also, I doubt if losses actually would be lower - certainly, the coil
without the ferrite rod will have a much lower Q than with the rod in
place,
so a point must come as the rod is withdrawn where Q decreases rapidly.
Cheers, Jim Moritz
73 de M0BMU
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