"Wes Stewart" wrote in message
...
On Tue, 21 Jun 2005 11:37:57 -0400, "Walter Maxwell"
wrote:

[snip]

Walt,

Your's was a particularly heroic effort and I commend you for it.

I have taken the data you supplied in text form, converted it to comma
separated values (csv) and imported it into XLZIZL where I can use
your measured input values, add the two different transmission lines
that you used and compute the load resistance at the antenna.

I know that you supplied these results, but I find slightly different
answers and I believe the reason is as follows:

I'm guessing that you calibrated your two lines as one. If I'm wrong,
slap me upside the head.

Depending on the exact type, RG58(x) has slightly different Zo values.
RG141 is specified as 50.0. Your Zo of 54 and your phase constant
suggest a Vp of slightly less than .66, which is remarkably close to
specification, but slightly low, considering the ~5% (2', RG141) of
your composite line has Vp ~ 0.7.

This is really getting fussy, but I'm beginning to believe that to
possibly make a determination about ground characteristics based on
antenna Z measurements, the measurements are going to have to be very
precise.

Wes

You're right, Wes, I calibrated the composite line as one line. And I agree that
if you calibrate them separately a small difference in the results will be
obtained. But I believe the difference will be insignificant. Consider this: The
nominal Zo of RG58 is 55 ohms and that of RG141 is 50, as you point out. The
measured Zo of the composite line is 54 ohms. Now also consider this: The
nominal vf for RG58 is 0.659 and for RG141 is 0.695. Thus the difference in the
nominal Zo is 10% and the difference in vf is 5.5%, but the length of the short
portion is only 5% of the total length. From these small differences I presumed
the error would be significant. You didn't say exactly how much difference you
found, or the procedure you used to determine it.

So I made the following calculations from the data where the height is 10 ft at
14.55 MHz:
First transforming the measured input impedance through the 40 ft of RG58, and
then transforming the impedance found at the load end of the RG58 through the 2
ft of RG141. The results are as follows:

At the load end of the RG58 and the input to the RG141: 69.41 - j19.19,
At the load end of the RG141: ........................................79.23 -
j3.25.

At the load end of the RG141 using the composite data: 81.21 - j1.25.

The difference in the R values is only 1.98 ohms and in the X values is only 2
ohms. I consider this degree of error insignificant. Would you not agree?

Walt