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Author Topic: RF slug-tuned-coils and colors question?  (Read 5435 times)

Posts: 222


« on: October 20, 2011, 07:33:58 AM »

Hello all!

I am winding some coils for my new home-brew "Mighty Midget" QST receiver
that I'm building because I was unable to find the exact Miller coil required.

I have a few salvaged slug-tuned coil forms that I will be using.

I understand that the inductance of a slug-tuned coil can be increased
or decreased depending on the composition of the slug.

I also understand that high-frequencies demand a different core than low freqs because
the eddy currents and hysteresis losses become critical in high frequency applications.

And, the cores are color-coded, indicating their appropriate frequency range usage.

My question is:  What happens if I use a green core (or some other higher-frequency
core) for my relatively low frequency I.F./BFO of 455 kHz?

I guess I'll find out because I will use what I have, but I haven't heard much about
using a high-frequency core for a lower frequency application?

I am able to measure the inductance so, I'm not too worried about creating the wrong
inductance rage.

Would anyone have any thoughts on if this will work?




Posts: 16023

« Reply #1 on: October 20, 2011, 08:24:34 AM »

The most likely problem you would encounter if you use an HF coil form for a 455kHz
inductor is that you can't fit enough turns on it.

455kHz transformers (as well as the LO coil from an AM BC set) generally use ferrite cup
cores to get sufficient inductance.  HF cores (such as those used in 10.7 MHz IF stages)
often use powdered iron instead.

The 10.7 MHz transformers are often used on 80m and 40m by adding capacitance
across the tuned windings.  The 455kHz ones are often used on 80m by disassembling
them and rewinding them with fewer turns.  One receiver kit I have uses 20 turns on
a low frequency core for the VFO, though I don't remember whether it is operating
around 3 or 4 MHz.  (Many of the 455kHz coils are relatively easy to rewind, depending
on the mechanical construction.)

I have a lot of coils in my junkbox, and haven't found much standardization of
core colors.  Often the 455kHz transformer sets have color coded cores to
identify what stage they are designed for rather than the frequency range.
You used to be able to order coils from Amidon with slugs of a specific color
that matched their toroids, so you knew more about the material used, but
I haven't seen that in any other commercial coils.  When in doubt, you have
to read the data sheets carefully and hope they have the information you

I've run into a couple quirks using the ferrite material (from 455kHz IF coils) on
80m.  First, one receiver would drift out of band when left in the hot sun.  I suspect
this was more due to the varactor shifting with temperature, but I don't think
that the ferrite is as stable with temperature as the high frequency slugs are.
The second quirk, seen in hand-held 80m DF receivers, was a slight frequency shift
as the receiver was rotated.  This appears to be due to the Earth's magnetic field
affecting the inductance, and was resolved by reorienting the coil so the axis was
vertical in normal operation.

Here's an article on rewinding such coils, showing the two types:

though I'd say his "type A" coils are typical of 455kHz transformers and "type B"
of 10.7 MHz IF transformers.

Posts: 4314

« Reply #2 on: October 20, 2011, 10:10:37 AM »

Click this URL:

...then scroll down to "tuning core color codes". It looks about right, but note the words "commonly used". If you pop the lid on an AM / FM radio you can usually read the PC board in terms of signal flow and if it's a stereo rig you may see a BLUE or LIGHT BLUE slug. I think that's in the stereo matrix secret decoder ring circuit and could be tuned to 38 kHz (pilot tone) or be the 19 kHz whistle trap.......

Either way, if you put salt on a slug and it dies that's not an adjustable slug.

BTW: If you study schizmatics for the All American Five AC / DC receiver you will invariably find the 1st IF transformer has 100pf caps across both windings while the 2nd IF transformer uses 120pf caps on the primary and secondary. If you should have some old pieces you salvaged from long ago, that can be one way to figure out who's on first.........

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