Permeability-tuned Crystal Radio

DickCappels

Joined Aug 21, 2008
5,884
Where does permeability come into the picture? From the title, I expected something to vary the permeability of the tuning inductor.
 

MrAl

Joined Jun 17, 2014
6,489
Hi,

I also expected to see something else as i think Dick C. did, such as a DC current being used to vary the actual permeability of the entire core. This is just sliding the core in and out of the coil form, which changes the permeability of the entire construction, but that is not new at all and has been used for as long as transistor radios have been around. So you change the permeability of the construction but you also change the physical construction, which is not just changing the permeability. If you changed the permeability by changing the DC current thought the coil, that would be purely electrical and so would be more interesting i think. You could also try using a separate electromagnet close to the core end to try to change the slope of the BH curve and thus change the operating permeability.
 

MrAl

Joined Jun 17, 2014
6,489
Hi MrAl,

Many thanks for your tips for my next project!

Regards,

Nandu.
Hi again,

You are very welcome, and i look forward to your next experiment with this too.
I also like your original idea because of the screw type mechanism that actually moves the core.
 

DickCappels

Joined Aug 21, 2008
5,884
Few are old enough to remember these. The silver tip is connected to the tuning slug. One would slide the rod to which the tip was connected to tune the receiver.





These coexisted with pocket transistor radios for a few years then disappeared from the market.
 

alfacliff

Joined Dec 13, 2013
2,449
another example of the permeability tuned radio was the "Rocket Radio" yo pulled and pushed a rod in and out of the end of the plastic "rocket" a fairly simple crystal radio, still available.
 

MrAl

Joined Jun 17, 2014
6,489
another example of the permeability tuned radio was the "Rocket Radio" yo pulled and pushed a rod in and out of the end of the plastic "rocket" a fairly simple crystal radio, still available.
Hi,

Isnt that what Dick C. posted in post #12 ?
 

KL7AJ

Joined Nov 4, 2008
2,208
Few are old enough to remember these. The silver tip is connected to the tuning slug. One would slide the rod to which the tip was connected to tune the receiver.





These coexisted with pocket transistor radios for a few years then disappeared from the market.
Didn't EVERYONE have one of these once?
 

DickCappels

Joined Aug 21, 2008
5,884
On this topic, sometime in the late 1970's Stan Horton at Prem Magnetics designed an adjustable linearity coil for CRT displays. A linearity coil is a a saturable inductor with a given inductance vs current slope and was usually adjusted with a permanent magnet to provide magnetic bias to the core. Stan placed a solenoid winding with its field perpendicular to the field from the inductor part of the linearity coil. Varying the current in the bias winding was analogous to mechanicallly adjusting the permanent magnet. This was true adjustment of the permeability of the core, and was ideal for adjustment with a microcontroller. Unfortunately, it was way ahead of its time as microcontrollers were still several years in the future.
 
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