Topband: Recycled ferrites for 160m

[Dan Zimmerman N3OX]

>
> I have several doing service as chokes in RX antenna feeders. They seem to
> work but how well, I've no means of knowing?
>

If you have an antenna analyzer you can get a decent idea of how the thing
works by winding a few turns of wire onto the core and measuring the
impedance of that.   Shooting for a total impedance around 50 ohms is useful
for something like the MFJ-259B which is more accurate around 50 ohms than
way out in high and low impedance land.

Once you know the impedance of  the N turns you just wound on the core, you
can guess that the impedance of the finished choke of M turns will be
(M/N)^2 .

As a final check, I sweep the finished choke across HF and look for the
first low impedance series resonance, just to make sure it's not in my
frequency range of interest or close to the top end.   It's not useful to
measure the impedance of the choke directly since the impedance range of my
MFJ-259B is only up to 650 ohms, but it just lets me know that the choke's
not diving to nothing in the range I'm measuring in.

If you had a signal generator and an oscilloscope you could build a voltage
divider with a known resistor of 1k or so and the choke and measure the
voltage across the choke as a function of frequency with a known applied
voltage, and get a direct measure of the magnitude of the impedance of the
choke that way (and if you've got a couple high impedance scope probes
instead of just one you could technically get the phase, and therefore the
complete R+jX).  You do have to be a bit careful about stray capacitances,
etc, but since you're posting to Topband it's not so severe at 1.8MHz in the
way it would be at 30MHz or higher.



> Does anyone know what ferrite "mixes" all these are likely to be or how
> they
> could be simply  tested at home for suitability as rf xfmrs, baluns, chokes
> etc on Topband ?


I've never found one that is an often used ham mix since most of these
things are for ~100kHz applications, but I have found a few that are
reasonably useful chokes.

As far as baluns and other power handling applications go  (and even
receiving transformers), you need to be a little more careful because of
potential problems with core loss (which depend on the balun design and
application).

If you build a simple 4:1 balun to go from 50 to 200 ohms and intend to use
it that way, the easiest home test is to build TWO of them and connect them
back to back into a dummy load to transform from 50 ohms to 200 ohms back to
50 ohms.  Then you run power through and measure power in and power out.
You can't figure out that there's 0.05735dB loss this way, but since 1dB
total loss will give you 79W out for 100W applied, you can easily measure a
pair of random baluns/ununs and determine that they have <0.5dB loss
apiece.  You can also measure the effects of core heating if there is any
and you can do so at the power level you want to run.

Small receiving transformers could also be tested for loss the same way,
back to back,  but a signal generator and oscilloscope would probably be
needed (though maybe you could use a signal generator, a step attenuator,
and your receiver)

73
Dan