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

On Tue, 25 Jan 2005 21:39:50 -0800, Richard Clark
wrote:

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

That's an interesting dissertation Richard, but I'm not sure that it
readily suits my needs.

Last time I used a calorimeter was nearly fifty years ago in a Physics
I lab - I can't remember what the experiment was.

Thanks
MikeN, ZL1BNB

On Fri, 28 Jan 2005 12:13:51 +1300, MikeN
wrote:

That's an interesting dissertation Richard, but I'm not sure that it
readily suits my needs.


Hi Mike,

Well, you are going to have to make up your mind. In one post you
want a simple instrument, in another post you are fishing for answers
with an antenna analyzer (but you don't have a FSM?). That seems to
me to be at least a 10dB variation in $$$. Now if the spec is that
loose, I can up the ante another 10dB to give you a direct reading
answer.

How much are you willing to spend?

73's
Richard Clark, KB7QHC

On Thu, 27 Jan 2005 16:12:27 -0800, Richard Clark
wrote:

On Fri, 28 Jan 2005 12:13:51 +1300, MikeN
wrote:

That's an interesting dissertation Richard, but I'm not sure that it
readily suits my needs.


Hi Mike,

Well, you are going to have to make up your mind. In one post you
want a simple instrument, in another post you are fishing for answers
with an antenna analyzer (but you don't have a FSM?). That seems to
me to be at least a 10dB variation in $$$. Now if the spec is that
loose, I can up the ante another 10dB to give you a direct reading
answer.

How much are you willing to spend?

73's
Richard Clark, KB7QHC

Hi Richard

Well that calorimeter is a simple instrument, but how can I apply my
simple knowledge to give a meaningful result.

Would it go something like this?

1. Take a polystyrene cup packed in a piece of polystyrene foam,
and with a polystyrene lid.

2. Pass a piece of thin - say RG174 - coax through the lid.
Run the centre conductor through the bead and wrap back around bead
and solder to to braid.

3. Immerse the bead in say 150 cc of H20 - initially at freezing.
Use an in-glass thermometer to measure temperature. Apply some 70 cm
rf from my handheld (on low power).

4. Measure the rf voltage across to the coax at the ferrite bead
using a simple diode detector - calibrated to give RMS applied
voltage E.

5. Measure the temperature rise with time until a steady state
above ambient temperature is reached.

6. Replace the ferrite bead with a resistor, and apply DC voltage
same as E to get the same temperature rise in the same time period.
Change the resistance value as necessary to get the same temperature
rise over same time.

7. Dummy load is now dissipating the same power as the ferrite
core did.

8. Calculate the impedance from Z=E^2 / W.

9. Repeat to see if results are reproducable.

10. Substantial inaccuracy will result from differing thermal mass
of bead and resistor. Bead is 29mm long, with 16mm OD, and 8mm ID.

11. So what should I do to refine this procedure?. What am I
missing here?

I'd like your comments.

Thanks

MikeN, ZL1BNB

On Fri, 28 Jan 2005 16:55:33 +1300, MikeN
wrote:

6. Replace the ferrite bead with a resistor, and apply DC voltage
same as E to get the same temperature rise in the same time period.

Per your earlier use of an RF detector, you have to first consider the
conversion factor (was the meter peak reading or average?). Why
bother, you already have a power source (the HT) you already have a
detector (your same simple detector). No conversion necessary.

Change the resistance value as necessary to get the same temperature
rise over same time.

7. Dummy load is now dissipating the same power as the ferrite
core did.

8. Calculate the impedance from Z=E^2 / W.

Hi Mike,

Step 8. is unnecessary given step 7. You only want to know the
ferrite R which is directly obtainable from the Resistor setting (or
Resistor choice). You don't even need to compute power anymore as
that has fallen out of the equation in the method you describe -
which, by the way, is a good example of crafting a solution. It shows
you simplifying my stark description of caloric measurement to instead
engage in bolometrics (comparison of heating). This method is
probably superior in simplicity and results would easily be within 20%
(which is not shabby for UHF).

Your introduction of a resistor is also an example of what
Metrologists call a "transfer standard." It is an example of using
the "substitution method." Your only concern is that the resistor
present a true resistance and not some complex Z. In other words it
should match the feed, and not offer much stray X. With that said, it
becomes a tougher problem (but then you needed to do the same thing
with the actual Z of the ferrite) that is, matching. It is arguable
that they would both mismatch equally (and given the Power term is
canceled, match is not particularly necessary). Everything here
depends on your re-obtaining identical indications.

This discussion reveals the cost of absolute determinations.

To increase the success it behooves you to up the power to cut down on
environmental temperature biasing the experiment (also cuts down on
other subtle influences like the difference in mass of heated
samples).

There are also indirect methods which can tolerate far more
imprecision. Ferrites are composed with bulk properties that have
frequency dependencies. These properties, however, vary quite
smoothly and slowly across great ranges of frequency. They also
exhibit distinctive family properties. The different grades of
Ferrites react with peak Resistances in different bands, but for our
purposes one family of Ferrites can be quite useful across a
significant percentage of bandwidth. Consult:
http://bytemark.com/products/ferrmat.htm
Unfortunately, Bytemark.com has fallen short of complete
documentation. They offer a link to illustrate Z over F, but it is a
dead link (and has been for years).

However, by this one page alone you can discern the family
characteristics I speak of. Your best hope is that your beads are
composed of something like type 43 or 64 instead of type 61 or 73.

An indirect method would be to measure the bead in series with a good
resistor - at two or three frequencies. HF, VHF and UHF would be
eminently suitable. If the bead shows a higher R at UHF, this trend
would tend to support an assumption you are have a suitable material
type. Both types 43 and 64 should exhibit useful resistances in both
bands. The slope frequency characteristics of these materials easily
span both higher bands.

Let's put some useful context to this. For any bead that snuggly fits
over the jacket of an RG-58 cable the following Rs should be seen for:

Material HF VHF UHF
75 ~20 ~10 ~5
73 | 77 25-30 ~20 ~15
43 ~20 ~32 ~30
64 ~5 ~30 ~35

This should put material identification within reach.

73's
Richard Clark, KB7QHC