TelePost LP-100A, Operation Manual

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Appendix A
Powering the LP-100A:
How should I power the LP-100A? This is up
to you, but the most common methods are…
Wall wart power supply capable of delivering 11-16 VDC @ 320 mA
A RigRunner type power manifold powered by the main or accessory station power supply
A battery pack capable of the required power
I recommend a linear power supply, although there are some good switching supplies available. In my case, I power my entire station from a deep cycle
battery and charger so that it will operate uninterrupted in the case of a power failure. If you use a wall wart, it is a good idea to select one which will
provide the required current and voltage, without soaring above 16 VDC with no load.
Placement of LP-100A in the transmission line:
Where should I place the LP-100A in the transmission line between the rig and antenna?
The best place for the LP-100A coupler to be inserted is between the rig (including any amplifier) and the antenna tuner or antenna. The tuner should be
considered part of the antenna system. Use of an internal tuner in the rig will result in inaccurate power and SWR readings on the LP-100A (or any other
external wattmeter). The LP-100A is designed to work with a 50 ohm source impedance. When an internal antenna tuner is used, the output impedance
of the rig will no longer be 50 ohms. You will also experience a power loss in the tuner of up to 20% or so, which will be seen on the LP-100A. To
measure an antenna’s actual impedance requires that any internal tuner be bypassed, as well as any external tuner which follows the LP-100A. With an
external tuner following the LP-100A, you can adjust the tuner while monitoring SWR or Return Loss on the LP-100A until a match is found. Switching
the external tuner between operate and bypass will show the effect of the tuner.
Power Measurement Accuracy:
This is a subj
ect that’s almost as controversial as antenna gain measurement. It seems like it should be much simpler, since it’s a measurement that can
be done in a controlled laboratory environment, but it is an insidiously complicated measurement to make. The process used by TelePost, and the ARRL
lab is basically this…
Connect the wattmeter being calibrated to a lab quality digital wattmeter such as the HP438A that we use, which has a calibrated precision
thermocouple sensor. These sensors are generally designed to operate in the 0.1 to 1W range. This necessitates the need for a precision power
attenuator between the meter under test and the sensor. A convenient value for a 0.1W sensor is 30 dB, and for a 1W sensor it
’s 20 dB. This allows
direct reading of the power in watts by viewing the milliwatt scale of the meter.
There are a number of error sources which need to be managed in this scenario…
Actual attenuation vs. frequency for the attenuator
Input and output Return Loss of the attenuator
Input Return Loss of the sensor
Sensor response vs. frequency
Non-linearity of the sensor or meter with varying power levels
Output impedance of the transmitter vs. frequency
Calibration lab errors for the sensor vs. NIST standard
Some of these factors are small, but some can be significant. The sensor frequency response is specified by HP when the sensor is sent in for
calibration, and a table of Cal Factors is supplied with the calibrated sensor. The actual attenuation of the attenuator vs. frequency can be characterized
using a Vector Network Analyzer. This is what we do. We use a HP 8284A power sensor coupled with a JFW 50FH-030-100 attenuator. The attenuator
is measured with our HP87510A VNA. The total measured error vs. frequency of this setup, including N type adapters, is just under 0.1 dB (~2%) before
correction, but after applying the measured corrections, the residual error is about 0.02 dB (~0.5%). There other smaller errors, like temperature related
ones, return loss related ones, etc. The absolute power accuracy of the HP meter/sensor, compared to NIST, is about 2%. To be conservative, we
specify the overall accuracy of the LP-100A(A) as better than 5% from 1W to 3KW, with a band-to-band peak error of 1% from 160 to 6 meters. We
know of no other meter that can match this level of accuracy.
As you can see, it is very difficult to specify the accuracy relative to NIST as any better than this for any given band and power level, even with this level
of test equipment. I have had a number of discussions with the lab staff at ARRL, who confirm this. They specify their measurement error for power as
+/- 5%, and for PEP power, +/- 8%. Mike Tracy, KC1SX, of the ARRL lab wrote an interesting sidebar in the QST review of the Alpha 4510 power meter
which discusses these issues. The sidebar appears in the July 2006 issue of QST.