Richard Clark wrote:
On Fri, 29 Dec 2006 09:04:01 -0500, chuck wrote:

The MFJ-259 has a resolution of one
ohm and the uncertainty of that LSD is probably more than an ohm. You
may not be able to reliably detect small improvements using the MFJ.

Hi Chuck,

This is rather focused at the wrong end of the scale. The antenna's
radiation resistance is going to be lost at the 1 Ohm resolution of
the scale. A 12 foot radiator in the 80M band presents us with about
5 Ohms (a 10:1 mismatch) resistance with so much reactance that it
would wrap the needle around the peg at the wrong end of the SWR
scale.

However, the "lucky" ones may in fact see a SWR that is closer to 2:1,
only to be satisfied that 20 of those Ohms comes from poor
connections, coax, heated coils, and chassis loss. We can all agree
that the MFJ is perfectly capable of measuring 20 Ohms without issues
of error (yes, there will be error, but it won't be focused in the
last digit).

Where will the error be focused?

You will also be able to see that 20 Ohms diminish
TOWARDS the 5 Ohm Rr when you struggle to IMPROVE the situation. Even
if that is everyone's goal, it is rarely achieved; and certainly not
because accuracy foiled them.

73's
Richard Clark, KB7QHC

Hi Richard,

It would surprise me to see a 15 ohm reduction due to incremental
grounding and bonding. I would expect changes in R to be in the range of
a few ohms: right up against the uncertainty of the instrument.

But if he can bond a fender to the hood and see a 15 ohm reduction in
loss resistance, I admit to raising a non-issue. ;-) You and I are
simply expecting different performance results from the incremental
grounding and bonding.

Absolute accuracy is unimportant in this application.

73,
Chuck

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On Fri, 29 Dec 2006 13:37:43 -0500, chuck wrote:

Absolute accuracy is unimportant in this application.

Hi Chuck,

Quite so, hence the last digit is merely resolution, and the
resolution of 20 Ohms of loss against an Rr of 5 Ohms is easily
resolvable, and its reduction is easily seen. Further, automatically
assigning an error of 1 to the last digit presumes the underlying
measurement hasn't been rounded up from a smaller range of error.
Given the product is MFJ, it is hard to credit it with that kind of
craftsmanship. However, even with a 1 digit error (which has been
presumed to be quantization error), it will always be inherent in any
Relative reading. If 20 Ohms reading is actually 19 to 21 Ohms of
loss (irrespective of absolute accuracy), then driving out 1 Ohm of
loss will actually be 18 to 20 Ohms exhibited as 19 Ohms. The only
problem here is that you may have to drive out slightly more than 1
Ohm to kick the reading down to 19.

If you cannot drive out more than 1 Ohm of loss out of 20, then
there's no point in trying, is there?

Dennis,

If you've followed the discussion thus far, you can well expect that
the SWR will climb as you drive out loss. This means you need to
anticipate matching to something like a 5 Ohm load (in all likelihood,
as described here, it will be higher). Drive out the reactance and
use a 9:1 UnUn. Beware that such an UnUn will require 16 Ohm
transmission line (if it was all that easy, everyone would have
stopped posting similar problems like this long ago).

73's
Richard Clark, KB7QHC

Richard Clark wrote:
Drive out the reactance and use a 9:1 UnUn.

Thanks Richard, yes I am following the replies. I do have a W2FMI
UNUN with 8 different options, 1 of which is a 9:1 (50ohm to 5.56ohm)
connection.

Many thanks guys...!!!

73, Dennis, K1DRW