LP-100A Integration

Since I started using the LP-100 watt meter some time ago, I have not touched my old Bird 43. The LP-100A is one of the best on the market and it has some very unique features not seen in other meters!
The firmware is user-upgradeable and I am proud to be the originator of a few ideas about the ergonomics and the "SWR Alarm snooze" feature which Larry implemented in the firmware.
There is a great Yahoo Group for technical support and discussions!

Both of my units - LP-100 and LP-100A integrated in my shack. LP-100 (with the green PLED display) is showing the transceiver output (input to the linear amplifier). LP-100A (blue VFD, I actually used gel filter to get the nice blue color out the display) is showing the output of the amplifier.

Looks like the amplifier does a pretty good job. The T-mode is very helpful during tuning!
In vector mode, the meter displays a lot of very useful information about the antenna!

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.

N8LP SteppIR Tuning Relay

I wanted to protect my SteppIR BigIR antenna from accidental Tx while the antenna is tuning. SteppIR antennas can be damaged by high-power TX (>100W) while the antenna element is moving - arcing between the moving antenna tape and the static brush will cause damage to both. The 80m coil for the BigIR Vertical (which I have) is even more vulnerable to damage from high power Tx during tuning. The coil taps are switched by a special Roto-Switch, utilizing copper PCB pads as contact points for the switch(see my BigIR article at http://www.kotarak.net/). Arcing during high-power transmit can damage the copper foil of these contact points! The easiest way to protect the antenna is to inhibit the Tx Key signal going from the transceiver to the amplifier, while the antenna is tuning. It can be done simply by interrupting the Key line signal with a simple switch. Larry, N8LP is selling kit which does exactly that, but it is automatic. It is a very simple device actually, using an Op Amp as basic comparator. The comparator measures the voltage across a shunt resistor (so it measures the current draw by the antenna motor) and compares it to a pre-set value (adjustable via potentiometer). The Antenna controller is powered through the shunt resistor. Normally the stepper motors are always powered with the so called "holding" (idle) voltage, intended to prevent the antenna element from unintentional movement. When the antenna element is tuned by the controller, the current drawn by the stepper motors increases . SteppIR Tuning Relay detects the difference (increase) in the current draw and triggers relay which on the other hand interrupts the KEY signal to the amp. Very simple and easy to do project. The box is just connected in-line with the antenna's power supply and it doesn't need a separate power source.

This is the kit version. It took me about one hour to assemble, test and install it. This project can be done on a universal PCB as well and parts can be substituted. Lately, I just don't have the time to order and collect components and play with universal boards and wires so I went with the kit.

This is the completed SteppIR Tuning Relay. The manual (including schematics) is available at - http://www.telepostinc.com/.
On the front panel there are two LEDs - green, indicating Power to the Tuning Relay (and antenna controller) and red LED indicating that the antenna element is moving. It is a much more visible indication than the blinking asterisk on controller's LCD screen. The blue potentiometer is used to set Sensitivity or in other words the current draw threshold. On the back panel there are two RCA jacks for the PTT line. The pigtail with the power connector goes to the antenna controller's power jack and the power supply for the antenna controller is plugged straight in the Tuning Relay box. The integration in the shack was very quick and easy process and the Tuning Relay works perfectly! Another level of protection (again with the Key signal but this time for the amplifier) is provided by "High-SWR Alarm relay" in LP-100A Vector wattmeter.

Finally - 240V line in the shack

In general, HF power amplifiers (especially those in the kilowatt range) have very high current demand when supplied with 120V AC. They work much better and are more efficient when powered with 220-240V. An added benefit is that the AC current draw drops in half giving less chance for the standard 15A household breaker to trip.
Finally, I found some time to install a 240V line in my shack. It was a bit challenging because I had no available breaker positions in my electrical panel (load center). The electrical panel was really crowded, all breakers were in use and and even separate circuits were sharing common breakers. There was no way that I can free 2 breaker positions for the 240V line so I decided to install a sub-panel. It was time to put on the electrician's hat ;-)

My existing electrical panel - power is supplied to the house by 2 hot wires (single phase) and a neutral wire (which is also grounded in the panel as per NEC) . These are the 3 black heavy wires in the middle of the picture. They are connected straight to the outside service (the electric pole transformer on the street). There are 120V between each hot wire and the neutral (common) wire and 240V across both hot wires. These lines are actually connected to the center-tapped 240V secondary winding of the power transformer outside. Each end of the winding is a "hot" wire and the center-tap is the common.

To install a sub-panel I needed first to install a double-pole breaker in the main panel to serve as a feeder (main) breaker for the sub-panel and bring this way the outside service to the sub-panel thru the double-pole breaker in the main panel.

Piece of plywood was installed next to the existing panel. I used Tapcon screws to attach it to the concrete wall. BTW I did not install the main panel - judging by the angle, whoever did - was obviously drinking at the time :-)

Here is the new sub-panel, attached to the plywood. I installed 8 (16) position 125 Amps sub-panel. This should cover all my future needs. The double-pole 100 Amp breaker is installed at top-right in the main panel. Three AWG #4 stranded copper wires serve as feeder for the sub-panel. Wire size #4 should be able to handle up to 125 Amps (something very unlikely to occur). The green wire (AWG #6) is used to connect sub-panel's Ground to the main panel.

This picture shows the finished sub-panel. I moved some circuits from the main panel including those which I had to disconnect to make room for the sub-panel feeder breaker. I installed also a 20A utility receptacle under the sub-panel. The yellow wire (3- AWG#12) is the 240V line. It is connected to a double-pole 20A breaker at top-left. Safety reasons require the use of a double-pole instead of two single-pole breakers. When using a double-pole breakers, both hot wires will be disconnected even only one is overloaded. (Same reason to use double-pole breakers at the sub-panel feeder line)

This is the 30A in-line RFI filter - Corcom 30VB6. This AC filter is rated at 25A/240V or 30A/120V and provides pretty good attenuation throughout the HF range. The insertion loss is over 50db at 30 MHz. I have similar filters on the line powering the rest of the shack and on the line powering all of my computers. This means that the insertion loss is over 100 db between devices from each group.

To accommodate the filter I used electrical box from a "pull-to-disconnect" device sold for Air Conditioners. It accommodates perfectly the EMI filter and provides the necessary high-voltage safety.

My new 240V/20A AC line is finally operational! As it turns out - I can use it as AC voltage reference :-))