On Wed, 26 Jan 2005 17:48:26 +1300, MikeN
wrote:
How could one measure the effectiveness of a ferrite bead used to
decouple a feedline from a driven element at 70cms, with simple
equipment which could be built by a homebrewer, and what would that
simple equipment be.

Hi Mike,

One of the "best of class" questions.

A test that would work at the most fundamental level would measure the
conversion of RF to heat.

Let's take one bead whose characteristic Resistance is 25 Ohms,
substantially the same as the Z of a quarter wave antenna. Instead of
adding this bead surrounding the transmission line of the antenna (its
usual, purposeful application) we place it on the radiator at the
base. This adds 25 Ohms to the 37 Ohm native Radiation Resistance,
and thus the bead "should" absorb roughly half the power (what ever
you throw at the antenna typically).

Of course this is all hip-shot math, but the details contain so many
variables that this post would become encyclopedic if I attempted to
go to that granularity. Suffice it to say that the more important
discussion lies in the how and what, not the how much.

How? Measuring the heat of a bead may not be straightforward if you
were to attempt this directly with a surface temperature measurement.
So better, measure it in a bath of water. The experimental
sophistication goes up, but conceptually remains rather simple. You
now have to brush up on your understanding of the caloric bomb.
Others may head for the exit to buy a "better" solution, but the
caloric bomb remains one of the most accurate methods available to
even the guys with money to "burn" on figuring this out.

Going further, you may discover that there's just not enough heat to
gear up to with your thermometer. Well, now it becomes time with the
What. Boost the ante and use a fever thermometer (after first
elevating the bath temperature) but now you have to calculate how to
build a bath with a small enough time constant but high enough
isolation from the otherwise "cooler" environment. If this is getting
a bit too much, try boosting the heat generation (wrap the radiator
through the bead to raise its loss resistance by the square of turns).

This caloric method will lead to an absolute evaluation of the bead R.
Finding the relative evaluation of the bead R will probably provide
more resolution. Comparisons are easier to make too. However,
barring having a known sample bead to compare to, you are left to ask
yourself "yes this is better, but is better good enough?"

How could you do a relative test? Take the same scenario above (the
bead surrounding the radiator of a quarterwave vertical) and take
Field Strength readings as you change beads. The best bead is
evidenced by the poorest Field Strength. Is best enough? Well....

Is it sensitive enough? Try the same turns ratio boost described
above (but it will still not resolve if that particular bead is good
enough).

The list could go on, but the two above evidence: Power,
Current/Voltage/Resistance. There's not much left which is not a
variation on a theme (which returns us to How). I hope you note that
this requires only a thermometer or FSM which qualify as simple
equipment. Give me a bigger budget, and it will only increase cost to
no measurable increase in accuracy. You might have the advantage of
seeing the answer on a digital display - but who's to say that it is
actually right, much less close?

73's
Richard Clark, KB7QHC