LP100A Vector RF Watt Meter

I finished building my second LP-100 wattmeter - this time it is the LP-100A. LP-100A is the newer version, equipped with nice graphic VFD and it is ready to accept the "Dual channel" option (not released yet). Larry N8LP is doing great job with this watt meter and I cant wait to add the second channel coupler! Hopefully it will be out soon - the enclosure also has extra holes on the back for the BNC connectors of the second channel. Another small change is the addition of a front panel power button. On LP-100 there was none and I had to add it as modification. The PCB is also re-designed. Second processor takes care of the graphic display, extra connector is provided for the dual-channel option, some components values are changed a little bit. LP-100A is most certainly an improvement over LP-100. I like a lot the high contrast and the nice green color of the PLED in LP-100, but the graphic display in LP-100A is also great and provides higher resolution. Luckily I have one of each now! The firmware is also a bit different for each version but the differences are minor and they have more to do with the menus and the calibration.


This is the content of the box. Everything is well packed. All high-quality components and powder-coated aluminum enclosure! Very good kit indeed!


The PCB comes with all of the SMD components factory preinstalled and tested and they are not that many. Here is a picture during the building process with all of the IC sockets installed. I actually replaced the supplied sockets with machined socket type as they are more reliable and provide better contact. The values of all of the discreet components were double-checked!


Picture of the attenuator board located in the coupler! These SMD resistors are pretty big in size and very easy to install! Larry provides some spares should something goes bad during soldering!

One of the unique features of LP-100 is that the power detection is not done in the coupler (as is the case in all other digital wattmeters). Instead, the coupler samples the voltage and the current and the main unit detects the power, which allows for more complex calculations and more data like Z, phase angle, etc. This is a picture of the voltage (left) and the current (right) sample transformers. The transformers are wound in a very specific manner (they are mirror image on each other) on large ferrite toroids. Winding those transformers is probably the most difficult part and requires great precision to achieve for highest possible accuracy! The little piece of silver-teflon coax is the primary winding of the current transformer. Larry provides in the manual exact measurements for cutting and stripping the coaxial and detailed description of how to wind the transformers. Pieces of special self-adhesive tape on the nylon bushings are helping to keep the windings in place and to have proper coverage of the ferrite cores. The windings need to be evenly spread.

The aluminum enclosure of the coupler with connectors and attenuator already installed. The coupler must be build with precision for good accuracy! The instruction in the manual are pretty good and easy to follow. Optional N connectors are sold by TelePost but any standard chassis N connector will work should such connectors are desired.

Picture of the coupler with both transformers in place! The wiring inside the enclosure can be a little tricky. One should not rush thru the building process - this is lab equipment and accuracy of the measurements is essential.

The RF coupler is ready. It has very professional construction and look! Before soldering the primary of the current transformer, the walls of the enclosure have to be pre-tensioned to a specific size, so when the cover is screwed on there is no stress on the connectors and coax.

This is the finished unit. I added another small heat sink (nested inside the main heat sink) to the voltage regulator (on the right). The original heat sink is small and the regulator runs hot. Second heat sink improved the cooling a bit! There is a jumper inside to disable the buzzer of the SWR alarm, leaving the relay fully functional.