RF inductor question

Thread Starter

mbohuntr

Joined Apr 6, 2009
430
Good morning,
I am still working on the Rf signal generator, and have a inductor theory question. The print shows a tapped coil with a slug. The "Repair " was a 5 turn tapped coil no slug as shown in the pic, I have located a slugged coil that is not tapped. Does this make it operate differently?, or should I buy a second higher value coil and put it in series so it becomes "Tapped" and follows the print??

https://www.ebay.com/itm/17-25-variable-RF-inductor-vintage-ferrite-core-slug-radio-tuning-coil-NOS/302686276594?hash=item4679820bf2:g:q2kAAOSwol5Y2HZB&_sacat=0&_nkw=slug+inductor&_from=R40&rt=nc&LH_TitleDesc=0|0 eico inductor.jpg
 

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MrChips

Joined Oct 2, 2009
19,277
The purpose of the tap or the slug is to have an adjustable inductance.
Changing the position of the tap or the slug allows one to be able to tune the circuit.
 

Thread Starter

mbohuntr

Joined Apr 6, 2009
430
Thanks, So if I just use the Ebay slugged inductor from one switch, to the other, instead of the air coil, will it resonate with the 290pF tuning cap? (I am still planning on replacing the 1-8pF cap as well)

(Will the circuit operate correctly without the extra unused turns going to ground?)
 
Last edited:

ebp

Joined Feb 8, 2018
2,332
In a doubly tuned circuit like that, typically one of the tunable parts is for "calibration" so that the other tunable part gives the intended range. Since inductor tuning almost always involves multiple turns, whereas capacitors can be single turn and made to have the appropriate angle versus capacitance characteristic by making the plates the appropriate shape, the inductor is usually the calibration part and the cap the panel part.

A tapped inductor is also a transformer (auto-transformer). In that type of circuit is is often thought of as an impedance transforming device rather than a voltage or current transforming device. Of course voltage, current and impedance go together, it is just more useful to think in terms of impedance for that circuit.

There are slugs and there are slugs. Many different materials are used for the slug in RF inductors, all with different properties in terms of magnetic permeability and characteristics versus frequency. A core material that is suitablel at 300 kHz may be useless at 50 MHz, just as steel laminations are useful for an AC mains transformer and nearly useless at 50 kHz.
 

Thread Starter

mbohuntr

Joined Apr 6, 2009
430
That agrees with an old post I read in calibrating it, The inductor was tuned to the low end, the 1-8pF cap was for the upper end to "fine" tune it. Do you think it's worth it to buy the inductor and "bench build it first'? I have an unused 300pf air cap laying around.
 
Last edited:

Ylli

Joined Nov 13, 2015
776
That coil is part of a common collector Hartley Oscillator circuit. The tap on the coils is needed, and it must be in the right place. The full inductance and the associated caps determine the resonant frequency, and the position of the tap determines the amount of feedback.
 

Thread Starter

mbohuntr

Joined Apr 6, 2009
430
Thank You everyone, I guess the best way to proceed is to build a replica on the bench, and see how it behaves, or doesn't.
Thanks again!
 
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