On Mon, 06 Dec 2004 01:05:54 GMT, Richard Clark
wrote:
On Sun, 05 Dec 2004 22:51:03 GMT, (Robert Lay
W9DMK) wrote:
New Measurements -
I created a terminating load consisting of 4 composition resistors in
parallel. That measured 4.3 + j0.65 AT 20 MHz.
I threw together two Allen Bradley 10 Ohm 5% 1/4 Watt resistors and
came up with 5.1 -j0.5 in a quick test at 20 MHz.
I then measured the
input impedance of the 5.33 meter length of RG-8/U Foam coax
terminated with the 4.3 +j0.65 ohm load at 20 MHz, and that was 5 -
j7.1 ohms. The SWR at the load is 11.63 and the SWR at the input is
9.88.
Using a velocity factor of 0.745 and an attenuation value of .77, I
calculated the theoretical input impedance of the coax with the above
terminator. That gave a result of 5.17 - j7.3 ohms (theoretical). The
SWR at the load is 11.63, and the SWR at the input to the line is 9.88
(theoretical).
In setting up the simulation, it is necessary to pick an attenuation
and a velocity factor that are not only within the normal distribution
for that particular coax but also give a reasonably good match with
the measured values. In my opinion, the values that I used in the
simulation are well within the normal distribution of values for this
type of line, which has published values of VF=.8 and attenuation =
74 at 20 MHz.
Hi Bob,
I would say that your data shows a very good correlation to the models
and certainly the presumptions you made are well within the production
variables.
The simulation also predicts the losses, and I used two different
models for that calculation. Both loss models predict a total loss of
0.723 dB, which is 0.589 above the matched line losses based on the
normal attenuation. The two math models used were as follows:
1) ITT Reference Data for Radio Engineers, 5th Edition, pages 22-8 and
22-9.
2) The ARRL Antenna Book, 17th Edition, page 24-9.
Based on the limited tests that I have made so far, the two models
seem to give the same results. However, I am hoping to be able to
conduct measurements on configurations that involve much higher SWR
values. The immediate problem to be overcome is the measurement of
such impedance values as will be encountered.
Measure Q by the BW of the Half Power points.
73's
Richard Clark, KB7QHC
Dear Richard,
I finally created a test load that gives me the higher SWR that I
wanted.
It measures 7.0 - j2008 at 1.8 MHz. I placed that test load at the end
of a 150 foot piece of RG-59/U and measured the input impedance as
38.5 + j 151.6 at 1.8 MHz. The load SWR is 7901 and the input SWR is
10.18.
Solution of the transmission line equations for this particular load
and with coax characteristics of 73 ohms, VF = .646 and an attenuation
of 0.57 dB per 100 feet gives an input impedance of 38.67 + j 149,
which is a very good match to the measured value.
Losses are calculated using the same two methods as reported in my
previous posting, as follows:
1) ITT Reference Data for Radio Engineers, 5th Edition, pages 22-8 and
22-9.
2) The ARRL Antenna Book, 17 th Edition, page 24-9.
Matched line losses = 0.855 dB
Additional losses = 28.087 dB
Total losses = 28.942 dB
I am satisfied that the methods of calculating losses as described in
the two references are in agreement and are valid.
I am also reasonably satisfied that the 1 dB steps that are printed on
Smith Charts as the means of determining matched line losses are
valid, as are the nomograms provided in the ITT Handbook, pages 22-7
and 22-8, above.
73,
Bob, W9DMK, Dahlgren, VA
http://www.qsl.net/w9dmk