I was forwarded a link to a Towertalk post archived at
http://lists.contesting.com/archives//html/Towertalk/2008-06/msg00022.html
<http://lists.contesting.com/archives/html/Towertalk/2008-06/msg00022.html>
from someone who, in light of his experience, found N6RK's comments
puzzling. When using the proper procedure, one obtains much different
results from those N6RK obtained. Using the procedure contained in the
documentation, the result is the ability to make measurements with no loss
of accuracy through an AM broadcast band filter. The procedure has been
available for months now. It is called "custom calibration." It is not the
result of a recent software update. Those wishing to explore this powerful
calibration tool can download a copy of the software and run it as a demo on
a PC. The help files and manual embedded in the help file are very
detailed.
It is specifically designed for cases of high BCB RF existing on a large
160m antenna, like N6RK's. It works extremely well, completely canceling
out the effect of a BCB filter, or any filter for that matter. In fact, the
filter used does not even have to be a symmetrical filter. The procedure
requires taking enough data points in a limited frequency range to
completely cancel out any filter transfer function no matter the loss of the
filter with the short, open, load technique used in VNA's. The calibration
function has been verified by the several broadcast engineers and is well
documented as very accurate. Since we have not heard from N6RK, we can
suspect he did not use the tool correctly or his analyzer was
malfunctioning. I invite him to call us, or send in the unit for repair if
he feels it is malfunctioning.
As an example of performance to expect when the correct procedure is
followed in conjunction with a suitable BCB filter, I will relate my
experience with my 160 meter vertical. I have strong AM BCB signals,
especially from KRLD (50 kW) on 1080 kHz, as well as a station in the area
with 25 kW on 1700 kHz! Running the calibration per the documentation
generated very accurate curves, confirmed by comparison to our PowerAim 120.
The PowerAim 120 has no problem blowing through antenna measurements on AM
broadcast stations that have more then 50 V peak-to-peak RF on them. The
correlation between the AIM 4170 and the PowerAim 120 was perfect.
Once a custom calibration is accomplished through a filter, one can verify
its accuracy by using a known load, either purely resistive, purely reactive
or complex. A 100 ohm resistor for instance should be shown as 100 R and X
of 0 (zero ohms) and plotted as a straight line in the AIM's frequency plot
over the range the custom calibration was done.
With as powerful a device as the AIM 4170, it is more important than with
simpler analyzers to delve into the documentation in order to use the device
to its potential. Array Solutions welcomes inquiries, especially when a user
is unable to get the desired results. We are also interested in test cases
to explore other strenuous measurements. We invite users to send in
suggestions for additional software enhancements. Please also join the AIM
user group - go to <http://aim4150.proboards24.com/index.cgi?board=analyzer>
http://aim4150.proboards24.com/index.cgi?board=analyzer
Finally, K7LXC mentioned in a post his practice of making antenna
measurements while sitting in the yard or up the tower, which he thinks can
be duplicated with an AIM only by hauling it and a laptop out to the yard or
up the tower. The former is not too hard, the latter is definitely too
hard, but neither is at all necessary. Once the feedline is "characterized"
using the AIM 4170, the measurements data is the same as if the analyzer
were connected at the antenna terminals. The data it generates is much more
detailed than that provided by simple analyzers. The graphs can be saved
and overlaid for comparison. This is helpful not only while tuning an
antenna but when later determining if there has been a change. The
distance-to-fault cable test for cable is also quite helpful when one
encounters antenna system problems. There are many more practical uses of
this instrument and those are set forth at the Array Solutions website.
Regards to the group,
Jay Terleski
Array Solutions
Jay Terleski
Array Solutions
972 203 2008
Military Communications Systems, Phased Arrays, RF Switches, Antennas &
Towers
<http://www.arraysolutions.com> http://www.arraysolutions.com
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