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Author Topic: permeability v.s. air-core IF xformer performace?  (Read 705 times)
KC9KEP
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« on: October 08, 2007, 07:44:37 AM »

Hello all  ..

I am currently building the following 1940’s ARRL
three-tube, superhet receiver;

http://www.bignick.net/temp/arrl_1941_2.pdf

I was lucky enough to purchase some NOS Millen 1600 KC IF
coils! :-)

Unfortunately, I thought that I was buying permeability tuned
coils, but they turned out to be air-core, and trim capacitor
tuned.  (At least, the trim caps are “bread-slicers” and
not the cheaper compression type.)

Now, I read that the permeability tuned coils have steeper
bandwidth and more gain that the air-core variety :-(

My radio works fine, but it seems to lack sensitivity on the
80m ham bands.

The “religious” stations come thru blaring loud!

Have anyone had experience with air v.s. permeability tuned
IF transformers?

Do they make a significant difference?  Should I tear out
my way-cool 1930’s vintage transformers and replace them with
smaller “modern” 1600kHz permeability ones?

Thanks!

--Tom Nickel AKA KC9KEP
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KE3WD
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Posts: 5694




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« Reply #1 on: October 08, 2007, 10:16:56 AM »

>>Now, I read that the permeability tuned coils have steeper bandwidth and more gain that the air-core variety<<

I'd go with narrower tuning ability, but not with the notion that the transformer is going to add any gain.  Likely a reference to the saturation point of the air core and not being able to drive it as hard more than anything else I think.  

Permeability tuning is going to be much more stabile and able to hold peaking over time (and environmental changes).  

The big gun broadacasters will indeed come right on in easily, but consider that they would also do so using a simple regenerative receiver circuit also.  Selectivity is what we want from the superheterodyne setup, better transformers of the permeability tuned type were indeed the big improvement that put the air coil cap-tuned xformers out o' bidness.  

Don't cry over it, think about the learning experience you now have, hands on.  You can always change them out for the modern design -- which would be a dynamite addition to the learning curve, the hard thing to find would be the ones you have now, not many will be able to experience that without breaking the bank, man.  


KE3WD
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KB1GMX
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Posts: 714




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« Reply #2 on: October 09, 2007, 07:06:51 AM »

Those transformers are quite good actually and prefered. The plate type caps used were very stable.   The only negative is they only tune a narrow segment but that intentional.

HOWEVER..

From experiements and experience IFs around 1600Khz
were always prone to "breakthrough" due to the strengh
and closeness of high power broadcast stations.

One solution is to move the IF, best direction is either down below 530khz or up to around 1750Khz. You may find a quiet spot near 1600Khz but no guarentees on that and it could be quiet only during day or night time.

The other solution is excellent shielding.  That translates to all of the 1600khz circuitry has to
be in a speerate box in side the radio box your using.
That can be mechanically more than desired and requires  bypasses on all signals going in and out so that AM BCB
interference will not be a problem.  FYI: this is why the old boat anchors of fame are so heavy, shielding
and boxing up circuits to prevent undesired signals from getting in weighs a ton.

Last tube RX I did I resorted to regen detector at 455Khz for that reason. The side effect was selectivity was better then expected.   This is typical as lower IFs made it easier to get selctivity at the expense of
image performance above around 7Mhz.

Hope this helps.

Allison
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KB1GMX
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« Reply #3 on: October 09, 2007, 07:10:52 AM »

I did forget one small item..

Back in the 1940s and through the 60s the broadcast band ended at around 1600khz. That made the top of BC a good place for an IF.  Since then the BCB was extended to 1750khz and the problem you experience is expected.


Allison
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KE3WD
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Posts: 5694




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« Reply #4 on: October 09, 2007, 09:43:21 AM »

Do some "blowby" experiments with em...
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KC9KEP
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Posts: 208


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« Reply #5 on: October 16, 2007, 04:20:20 PM »

Hello all & thanks for your replies!

Shortly after I had posted my first email, I was able to
hobble together a BFO coil & add it to the “second detector”
circuitry

What a remarkable difference!

I can understand how the BFO “replaces” the missing carrier
for SSB transmissions, and provides a beat frequency for CW
transmissions, but it also seems to improve receiver sensitivity.

I can clearly hear the background clutter come in amplitude
with the BFO turned on.

Now, can anyone explain this “perceived” improvement in sensitivity
as a result of the BFO circuit?

Thanks again,

--Tom Nickel AKA KC9KEP
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KE3WD
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Posts: 5694




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« Reply #6 on: October 16, 2007, 10:55:20 PM »

Welcome to the very beginnings of heterodyning.

The difference signal between the two freqs.  


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