Thanks Paul,

My old Drake C line had a multiple ganged permeability tuned inductor
mechanically coupled to a variable cap.

One of the assemblies tuned the front end, one peaked the driver
stage, etc.

It covered from 1 Mhz to 30 Mhz with a half turn on the front panel
preselector control. Most likely the variable C and L tuned together
maintained a desirable Q across all the bands.

I was hoping for something similar, but have no idea where to get
permeability tuned inductors today.

Regards,

T


On Sat, 10 Sep 2005 12:16:44 +0300, Paul Keinanen
wrote:

On Fri, 09 Sep 2005 22:01:27 -0400, TRABEM wrote:

I need an LF to 30 Khz preselector (passive) for a homebrewed receiver
I'm building. I have most of the details worked out, but I am stuck on
the preselector design.

I want one band to cover 50 Khz to 500 Khz (or 1 Mhz).

That would be 1:10 (or 1:20) frequency range.

The second band would be 1 Mhz to 30 Mhz.

That would be 1:30 frequency range and would require a 1:900 reactance
range, if only one element (inductance or capacitance) is tuned, which
would be completely unrealistic. The alternative would be to tune both
the inductance _and_ capacitance in the 1:30 range. However, with a
300 pF variable capacitor, the minimum capacitor would be 10 pF
_including_ all stray capacitances, not very realistic. I have no idea
what a realistic tuning range would be for a permeability tuned
inductor. I have only seen those in FM receivers (87..108 MHz), which
only about 20 % tuning range.

50 ohm input with moderate Q (don't want to
have to retune it every 100 Khz. The LF unit can be very high Q,
peaking it every few Khz is ok as most of the signals in that range
are narrowband signals anyway. Input impedance is standard 50 ohms.

Maintaining the constant impedance if both inductance and capacitance
is tuned would be a challenge.

While maintaining the power match makes some sense in HF receivers
(and also if you are using some small magnetic loops on LF/MF), but
otherwise, I would question the idea to maintain the 50 ohm resistive
input impedance on LF (and possibly also MF).

On LF, any practical random wire antenna would have a high capacitive
reactance. On LF I have simply used a parallel tuned circuit and
connected the antenna to the hot end of the circuit with a small
(10-100 pF) capacitor. Of course, the tuning scale will not be
accurate if you change the antenna, since the antenna stray
capacitances will detune the resonant circuit. Use a secondary winding
or a source/emitter follower to get the impedance down to feed a 50
ohm receiver input or feed a coaxial cable (if the preselector is just
under the antenna).

If you intend to use magnetic loop antennas on LF/MF, which have a low
efficiency, I would suggest using a (remotely)tunable loop as the
preselector, with possibly a preamplifier at the antenna.

I would strongly suggest splitting the HF bands and switching in
various inductors for various bands and use an ordinary variable
capacitor.

The situation is quite problematic on LF, since you would need a
variable capacitor with a tuning range int several nF to keep the
resonant circuit impedances at manageable levels (otherwise, it would
be quite "touchy" due to stray capacitances). With practical variable
capacitors, you would have to add parallel padding capacitors, with
would further reduce the tuning range.

Permeability tuning might be an option. You could try making a coil
and slide a ferrite bar (from a medium wave antenna) into it, however,
I have no experience with this.

Paul OH3LWR