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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 23rd Oct 2019, 12:17 pm   #1
Join Date: Oct 2019
Location: Pesaro, Italy
Posts: 1
Default Twin ferrite rods in radios from the '50s

Hi, i post some photos of a ferrite rod antenna that i have found; i would like to understand why it is composed of two ferrite rods that look identical and the LW and MW coils are connected in parallel (see the hand sketch i drawed).
These twin rods look quite common in old tube radios of big dimensions.
Why two rods? what is the advantage and why isn't this configuration used anymore?
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m_a_x_i_m is offline  
Old 23rd Oct 2019, 2:48 pm   #2
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 5,568
Default Re: Twin ferrite rods in radios from the '50s

I guess the LW coil by itself for LW, and the MW coil switched in parallel for MW? Means the switching can be done with a single-pole single-throw switch.

Usually, the MW coil is by itself for MW, and the LW coil is switched in series for LW. It does mean that the LW coil (lots of turns, so high inductance and high self-capacitance) is left open on MW and if its self-resonance is anywhere the MW band, there can be spots of weird sensitivity when tuning across the band.

Putting the coils in parallel avoids this effect (and the open MW coil will self-resonate well out of both bands), but the coupling between the coils needs to be very small (inductance of coils in parallel flows product-over-sum rule, as for resistors, but only if there is zero coupling between them. If there is magnetic interaction, effective inductance drops to zero due to circulating currents). So, two rods would be needed.

That's my thinking - and why is it not done more? Probably because benefits in practice do not justify the extra expense.
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