Not having a vector impedance meter, I wondered if there is a quick, cheap
and easy way to determine impedance of the rf choke at various frequencies.
There may be a way. Let us assume we have an Autek or MFJ rf bridge (or a
noise bridge). Let us then terminate this bridge with a 300 ohm
non-inductive resistor. If we place the rf choke in parallel with the 300
ohm resistor, the resulting impedance would tell us what the impedance of
the rf choke, even though it would normally be too high to measure with our
simple equipment. If the rf choke impedance is 10K, the resulting impedance
would be equivalent to 300 ohm in parallel with 10K ohm which is 291 ohms.
In this example, at any frequency the resulting impedance is greater than
291 ohms, the rf choke would be greater than 10K.
OK, that is the way it is supposed to work. In practice, the simple bridges
probably are not too accurate at other than 50 ohms. Still that may not
matter too much.
I wound a quick rf choke of 56 uh, which I figured would be adequate for 40
meters and above. I then grabbed my Autek rf bridge and located a 368 ohm
resistor in the junk box. Running the Autek bridge from 1.4 to 37 MHz, I
note the reading varies (somewhat like a zig-zag curve). So, this is the
limitation of the equipment. However, it is always between 250 ohm and
about 400 ohms. So, a simple sweep with and without the rf choke does not
work. But, I can take a reading at any frequency without the rf choke, then
place the rf choke in parallel and read the difference. If the change is
less than 3%, the impedance of the rf choke should be greater than 10K.
Using this crude method I discovered that there was less than a 1% change at
6 MHz and a 10% change at 3 MHz.
A similar approach (but with greater potential accuracy) would be to drive a
broadband 6:1 transformer, terminated with a 300 ohm resistor, with a signal
generator. Measure the rf voltage developed across this resistor. Then,
add the rf choke and measure the change in voltage. If the change is less
than 3%, the rf choke should be greater than 10K.
This would be a crude approach. But, maybe it will work and will not
require any outlay of money.
Now, ready for the darts.
73, Colin K7FM
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