Topband: [time-nuts] Minicircuits 10% discount in December

[Tom W8JI]

Anyone with proper test equipment can measure and verify what I say below is 
absolutely true.

I'd hate to see anyone accept this information as factual or accurate:

> The single turn resonance of this core is around 10 MHz, with a Z at 
> resonance of about 120 ohms. Like any other ferrite core, winding turns 
> will increase L as N squared, increase C as N, thus moving the resonance 
> down in frequency.  I'd guess that 8 turns would move the resonance fairly 
> close to 160M with Z in the range of 4-5K ohms. The catch is that the i.d. 
> is pretty small, so the choke would need to be wound with something like 
> one pair out of CAT5 cable.

10 MHz is the core "resonance", not the combination of winding inductance 
and winding capacitance. Somewhere around 10 MHz the core, no matter how 
many turns are wound, crosses from having any inductive effects to 
capacitive. This is because the core becomes diamagnetic, not because of the 
winding.

The dominant impedance anywhere above 2-3 MHz is resistive.

If you wind one pass around the center (out and back to start through both 
holes) you'll find the reactive sign of core impedance crosses over to 
capacitance at around 10 MHz. If you wind five turns, it remains about the 
same. The capacitance effect does not matter much because core resistance 
dominates. As turns are added, the resistance shunting the winding increases 
with only a slight shifting of apparent "resonance".

It is the resistance that dominates and parallels the windings. The loading 
effect can be minimized by proper winding techniques.

This core (or any 73 material) reaches X = R, or Q = 1, at around 2.5 MHz. 
In other words, at around 2.5 MHz, one pass (through the hole and back to 
start) is about 75 +j75 ohms, where inductance and resistance are equal. You 
want at least a two-pass (out and around and back two times) 50-75 ohm 
winding for 160 meters, and it will be good well beyond 30 MHz.

> Fair-Rite considers this a suppression part, not an inductive part, 
> although it is widely used for winding transformers for MF RX antennas. 
> The laws of physics don't change with what we call something, so this will 
> be a fairly lossy transformer.

The last sentence is incorrect. A typical primary-secondary modest impedance 
broadband transformer using that core, with minimal attention to winding 
style, has about 1 dB loss at 50 MHz. Loss decreases with a reduction in 
frequency, and is a fraction of a dB on 2 MHz.
Without special care, this transformer material is easily much less than .5 
dB loss across HF.

With higher power you might have to move to a lower loss core, or with very 
high impedances you may want to choose a core that allows the winding to 
become resonant, but characterizing this core as "fairly lossy" (whatever 
that really means) is not correct unless we consider 1/2 dB "fairly lossy". 
Generally, 1/2 dB (10% power loss) core loss becomes worrisome with a core 
this size at about 10-20 watts. At 20 watts the core will be dissipating 
about 2 watts on HF, less at the low end of HF or on 160 meters.

At higher power, the core loss over the operating frequency range has to 
decrease or the size increase. The ALS1306, for example, uses a stack of 43 
mix cores just like this style for the input transformer. That transformer 
has less than .2 dB loss from 1.8 to 100 MHz, and safely handles well over 
100 watts.

For receive, and if extreme impedances are not required, the 73 mix core is 
good to 60 MHz or so.

> For RX transformers, it may not matter (and the low Q may even help), but 
> don't be surprised when you see the added resistance beyond what the turns 
> ratio predicts. :)

You will see a loading effect in high impedance applications, because even 
several thousand ohms core resistance shunting a winding will "load" a 400 
ohm or higher resistance load.

Broadband transformers almost always use a core material that is well beyond 
magnetic effects at the top end of the frequency range. That is what makes 
them broadband.

73 Tom