In late 2015 I’ve ordered a TelePost Inc. LP-100A Digital Vector Wattmeter at Difona. Because I wanted the dual coupler option, the delivery took quite some time, as it was not in stock. End of January the LP-100A finally arrived. I really like the decent look, ease of use and versatility.

Working as an RF and EMC engineer, I was of course curious about the power measurement accuracy. So I did some measurements in the RF lab I’m working in with a signal generator, a 75 watts EMC amplifier, a dummy load and a Rohde & Schwarz NRVS power meter including a fresh calibrated URV5-Z4 power probe. Unfortunately we have no high power equipment here in the lab, but the measurements give a good indication about the overall accuracy of the LP-100A.

LP-100A Test Setup
LP-100A Test Setup

LP-100A (serial #4537) with coupler LPC1 (serial #4537-1)

Power R&S   Power LP-100A   Deviation
Watt        Watt            %
-------------------------------------
5.0          4.9            2.0
10.1         9.8            3.0
20.3        19.7            3.0
30.3        29.3            3.3
40.0        39.0            2.5
50.3        48.7            3.2
60.1        58.2            3.2
70.0        68.6            2.0
80.5        79.0            1.9
LP-100A (serial #4537) with coupler LPC1 (serial #4537-2)

Power R&S   Power LP-100A   Deviation
Watt        Watt            %
-------------------------------------
 5.0         5.0            0.0
10.0         9.8            2.0
20.2        19.7            2.5
30.2        29.5            2.3
40.7        40.0            1.7
50.2        48.8            2.8
60.0        58.4            2.7
70.1        69.0            1.6
80.5        79.7            1.0

This measurements have been done at 14.15 MHz in CW. I also checked the deviation across the classic ham radio bands at 75 watts and there have been only minor variations:

LP-100A (serial #4537) with coupler LPC1 (serial #4537-1)

Frequency   Power R&S   Power LPC1   Deviation
MHz         Watt        Watt         %
----------------------------------------------
 1.8        75.3        71.8         4.6
 3.5        75.9        73.8         2.8
 7.0        75.0        73.3         2.3
14.0        75.3        73.7         2.1
21.0        75.5        73.1         3.2
28.0        75.8        73.1         3.6
LP-100A (serial #4537) with coupler LPC1 (serial #4537-2)

Frequency   Power R&S   Power LPC1   Deviation
MHz         Watt        Watt         %
----------------------------------------------
 1.8        75.2        72.0         4.3
 3.5        75.8        74.0         2.4
 7.0        75.9        74.6         1.7
14.0        75.0        73.8         1.6
21.0        75.1        72.4         3.6
28.0        75.3        73.1         2.9

Larry N8LP states on the TelePost Inc. website:

Power accuracy is better than 5% at any frequency from ~1W to maximum, 3% typical with NIST traceable factory calibration

The LP-100A has a per-band power calibration, which is user adjustable. Also the build-in frequency counter is user adjustable. Maybe I’ll adjust that at some point, but for now I’m pretty satisfied with the results.