On Fri, 31 Aug 2007 11:13:41 -0400, "Rick (W-A-one-R-K-T)"
wrote:

I also have no antenna analyzer and no means to acquire one any time
soon... best I can do is my transmitter (cranked way back in power) and an
SWR meter.

Hi Rick,

Then using what you have, replace the antenna analyzer with the SWR
meter. That is, pass a short length of wire through the core you
intend to invest in (which begs the question: Why don't you simply go
to the right source and buy the right product?) and pour enough power
to this "load" to see what it measures in SWR.

If it is the right material (Amidon 73, 75, or 77 material). and the
right size (101 size bead) and the right frequency (60M or 12M); then
you should read something less than 2:1. Make it two beads, and it
would still read something less than 2:1 (but closer to 1:1).

You should note there are three variables above: material, bulk, and
frequency. Bulk adds isolation. Material and frequency go
hand-in-hand. As you have ferrite of unknown parentage you are adrift
on two counts and must accept what this ******* will have to offer.
You might be lucky, but it will undoubtedly cost more than doing it
the right way.

Anyway, returning to the practical investigation.... A 101 size bead
will support perhaps a Watt or two at 100% duty cycle and you can
raise that by the inverse proportion of DC. If you pour 100W into
this load to read the SWR, I doubt your meter will have a 10mS peak
reading capture time (unless you have a Daiwa). If you pour 1W into
it, you probably are not going to get a reading at all (that lends any
confidence at being accurate). 10W excitation should be adequate and
lest anyone complain that the SWR is indistinguishable from that
arising due to X (we are seeking R) - I will anticipate that as I have
in the past and ask you to grab onto the bead immediately after the
reading.

A blister on your finger and thumb will stand in telling corroboration
to the reading of nearly 1:1 that the bead exhibits an R of 50 Ohms. A
cool bead will be silent testimony to high X. It is sometimes a
wonder here how commonsense has to be spelled out.

Now, as you have none of the "right" variables, you need only judge
what you DO HAVE in relation to them. Merely repeat this with the
Radio Shack item. You need to balance the consideration that
bulk=Resistance and that your SWR reading may rise because it is
either:
1. Doing TOO good;
or
2. Doing TOO poorly.

The Radio Shack bulk may lend itself to presenting 100 Ohms (too good)
or is just sputtering along with 10 Ohms (too poor). As with
resistors, you can judge this through series/parallel combinations to
seek improved indications.

If two loads in parallel bring the SWR reading into 1:1, then you have
your baseline to build out to the proper sized Isolator. If two loads
in series bring the SWR reading into 1:1, then you have your baseline
to build out to the proper sized Isolator. (Amazing what knowledge
can do when you have just a little of it.)

This, of course, returns us to just how many Radio Shack clip-ons do
you need? My guess is they could handle 4W to 8W at 100% D.C. and if
one of them exhibits 50 Ohms you can do the math - depending on your
choking requirements.

I have no way to define "expected performance" any way other than that.

Then we once again enter into the fascinating world of Amateur
experimentation, an arena that comes with no guarantees and the
prospects of blistered fingers. As you have no way of quantifying
your goal, you have to judge it subjectively (no one else can do this
for you, as you are the subject in subjectivity).

Not knowing what Z is present at the distal end of the 20 Foot
transmission line drop can demand isolation from 500 Ohms to 50,000
Ohms. It is unlikely that the isolation will absorb all the power
your rig has to offer (but if it does, it will make a nice SWR match,
one of those glowing rewards for focusing on the wrong goal). So, by
these metrics, you can get along fine with 10 to 1000 of the mythical
50 Ohm Radio Shack clip-ons.

Start with 10 (adjusted in proportion to their actual R) and add more
until you have met your expected performance. Simple, n'est pas?

73's
Richard Clark, KB7QHC