Coaxial Switches

I've been looking for a good coaxial switch for quite some time! I went through the "usual suspects" - MFJ, Daiwa, Diamond and Alpha-Delta. The quality of execution varies from pure junk (MFJ) to very good (Diamond and Alpha-Delta).

Alpha-Delta 2 and Diamond CX-210.

When it comes to internal design and construction, all of these switches share common design concept (especially the A/B (SPDT) type) - a moveable contact "finger" engaged by the lever and a couple of contact points in the same cavity. They are not "true coaxial switches" and in my book they are far from "great"! This design is just a notch better than an "open frame relay" type A/B switch. There are problems with poor port-to-port isolation, insertion loss, SWR, frequency response, reliability and power handling, etc - all common problems for many "ham grade" (read "inexpensive") coax switches.
I was on a quest for a great coax switch and I finally found it - the Transco M1460 series switches.

Transco M1460-30 double pole manual transfer switch.

Transco was a US company, manufacturing high-end RF switches and relays many years ago. Their line of switches was acquired by DowKey and the manual type of the M1460 series was discontinued.

It looks to me that when it comes to RF switches it is the classic case of "they don't make them like they used to"! Truth be told - the Transco switches are not your average "ham grade" type - these are very high quality commercial / military RF switches with the price tag of hundreds of dollars in their time. The good news is that now they can be found sometimes at hamfests or eBay for anywhere from $10 to $50. These unassuming, old switches have incredibly clever yet simple internal mechanics and were manufactured with extreme precision and tight tolerances for the US Army, Navy, aviation, etc. (some of them are labeled as "US Property"). There are a couple of types - manual and motorized type, each in different configurations, port number (and voltages for the electric type). At 10 MHz, typical isolation is >80 dB (the actual is close to 100dB), VSWR 1.05 and Insertion Loss 0.05 dB. Some switches are rated for use at up to 10 GHz with maximum power of 300W CW at 3 GHz! On the HF frequencies, this translates to more than 1.5 kW (in practice it is only limited by the N type connector power ratings).
I managed to acquire a couple of M1460-30 (Double-pole transfer switch) and M1460-4 (4 position switch) for the price of a single Diamond or Alpha-Delta STDP switch!
M1460-4 (or 5-6) is a straight-forward 4(5,6) position switch designed to connect single port to 1-4(6) ports.

M1460-30 transfer switch can be used as an A/B switch as well but it is really handy as a transfer switch - I am using it to connect my two amplifiers to one of the antennas and a dummy load. With this switch I am able to swap the amplifiers to different ports and decide which one is connected to the antenna while the other is connected to the dummy load. It can be used also to swap 2 antennas to 2 transceivers or as by-pass/in-line switching of amplifier, preamp, pre-selector, filter or antenna tuner. It is very versatile type of switch around the ham-shack.

These Transco switches are good up to the GHz range and are using silver-plated N-type female connectors with teflon dielectric.

At the back side (internal, pictured on the right) of the N connectors there are (4) contact points with spring-loaded pins. The internal switching assembly (picture, left) contains two contact "bridges" (true coaxial lines) with 2 silver plated contact points (each) in constant-impedance channels (teflon dielectric). When the switch is rotated, two ball-bearings located at 180 degrees from each other (left, center) are pressing against a special action disc with 4 (at 90 degrees) radial groves (right, center). In the center of this disk there is a sleeve bearing for the shaft. The groves are with beveled edge to facilitate easy sliding in/out of the ball bearings. Normally, the ball bearings are resting in the radial groves, but during rotation of the switch they come out of the groves and press against the raised part of the disc, pushing back the whole switching assembly on its axis and away from the static contacts. In other words, the radial movement is converted into axial movement.

There are 6 springs in the coax bridge assembly creating an opposing force in the axial direction (located on the front side of the bridge assembly around the switch shaft and pressing onto a special graphite covered friction plate). The axial play causes the whole assembly to retract away from the contact points plane and allows for a clean rotation. When the rotation is completed and it comes to a stop at the next switch position, the steel balls are pushed back into the next set radial groves. The "bridge" assembly is pushed on its axis by the springs, flush against the contact surface (back plate) and the contact bridges in the assembly push against the N-connectors contact pins (also spring-loaded) and close the circuits. The back face of the switch assembly is pressed against the back-plate, grounding the assembly. As a result from this design feature, both contact pairs are completely sealed/shielded from each other creating very high port isolation. The SWR is extremely low and there is no impedance bump because of the constant 50 ohm impedance cavity of the contact channels. The actual contact points are silver plated and with large contact areas allowing for high power use. Very good electrical contact is established by the large, polished, flat faces of the contact points and internal springs in the N-connector portion, pressing the pins against the corresponding contacts in the bridge assembly.
There is a loud "clank" when the switch is completed and in the next position, but the rotation is nice and very smooth. The switching knob slightly moves out on its axis (about a 1/8 inch) at the beginning of the switch cycle and retracts back at the end of the cycle. Its all automatic, not like in Bird 74 where the operator must manually pull the lever out, rotate and then push it back at the end of the rotation.
The only "drawback" of this design (if I can even call it "drawback") is the relatively high force need to operate the switch - it feels a bit "heavy" at the very beginning of the rotation. A large (easy to handle) knob or lever is needed to actuate the switch with less effort.
There are 4 screws on the front of the switch used to attach a "face plate" with labels for the positions. Two screws on the side of the switch are used for mounting.

M1460-4 is a more complex design but very similar. Instead of a sleeve bearing in the center of the static contact back-plate with the N-connectors, there is a special silver-plated coaxial two-part contact arrangement. Also, the grooved plate and the steel ball-bearings are reversed in place - the plate is located on the switch rotor and the ball bearings are on the static back-plate. There are more springs (8) and the pressure plate (friction surface) located on the back, around the shaft is larger.

The motorized version is comprised of two parts - RF Head and Actuator. The Actuator is driven by an AC or DC motor, and includes a "gear box" speed reducer and a rotary switch which disconnects the motor once the switch is in the next position. The RF Head could be separated from the actuator and used as a manual switch if needed. The separation is not that difficult but a proper lever needs to be used for manual actuation since the shaft of the RF head is shorter than the standard manual version.

For maintenance, I cleaned and lubricated the contacts with Deoxit D5 solution and treated them with Deoxit ProGold. I removed the old grease and lubricated the pressure plate, both sleeve bearings and the action ball bearings with white lithium grease. I also removed the tarnish from the silver plated N-connectors with a product called Tarn-X.

Transco switches are clearly the best RF switches I've seen so far and IMHO are better than the famous Bird 74 switch. I replaced all of the RF switches in my shack with Transco getting excellent results!

M3 Electronix LCR Meter

The LCR Meter by M-Cubed Electronics is another very nice kit - IMHO - a must for every workbench. The kit is using very high quality components and PCBs. Included is a powder-coated aluminum case which gives the LCR Meter very professional look. This LCR Meter by itself has an enormous amount of features (all listed on their web site). There are also a few optional test fixtures available. The accuracy is impressive for a kit - basic accuracy of 0.2% using the supplied calibration resistors. The best part is the use of the "4-wire test method" which eliminates errors caused by the test leads and fixtures - a feature normally seen only in high-end test equipment. Upgrades of the firmware are done by sending the unit back to M-Cubed. There is a JTAG connector on the main PCB which allows for re-programing of the PIC but the firmware code is not made public. All SMD components (incl. the PIC) were pre-installed on the PCB along side with some other components, needed for the initial programing of the PIC (crystal oscilator, a few caps and the JTAG connector). The rest of the components were sorted by type in plastic bags. The assembly instructions are very clear and logical. The only gripe that I have is about the capacitors - some of them are small and on the top of this, some of the value markings were partially erased. Even with the help of my Zeiss-Winkel microscope I was unable to identify some of the caps just by looking at them. I had to use another LC meter to actually measure their values - this is not a problem if you have a LC meter handy but the irony is still there - you need a LC meter to build LC meter. Another small issue is the power connector - a male 5.5mm x 2.1mm barrel connector is not provided with the kit and you have to find one - I rather pay a little extra and get it with the kit instead of wasting time to look for the proper connector. Other than that, the assembly went very smooth over a couple of evenings.

This is the content of the kit right out the box. Calibration resistors are also provided with the kit. The basic kit comes with a probe made out of 4 Pomona-style clips - everything need to construct the probe is supplied with the kit.

These are the 4 boards - Input board, Keyboard board, Main PCB and LCD. Most of the resistors supplied with the kit are of the 1% tolerance type but there are also some 5%. I had plenty of 1% resistors in stock so I replaced pretty much all of the 5% resistors. This might not improve the accuracy a lot but at least the 1% (blue body) are metal-film type resistor with more stable values than 5% carbon-film type. The specified accuracy for the meter is between 0.2% and 0.8% when calibrated with the supplied 0.1% calibration resistors.

The PCBs are "sandwiched" together (using connectors and stand-offs) in the aluminum enclosure. 5-pin DIN connector (on the right) is used to connect the test probe or fixture. On-off switch and the power connector are wired on the left side. Very little space is wasted inside the enclosure and the PCBs are densely populated as well.

This is the finished meter. This meter is using the 4-wire probe method for measuring DUT (Device Under Test). 2 wires are supplying and measuring the AC (0.5Vrms) current thru the DUT and 2 other wires are measuring the voltage across the DUT. This allows for a very accurate measurement canceling out the error introduced by the probe. Furthermore, I increased the accuracy by calibrating the meter with my own 0.01% resistors instead of using the supplied 0.1% calibration kit. If an accurate Ohm-meter is available, the firmware allows for correction of the calibration values and then the supplied resistors can be used just as a "transfer" standard. On this picture high-current 270 uH inductor reads 263.8 uH.

During measurement, the display shows additional information such as the test frequency used for measurement (adjustable up to 15.6 kHz), the Auto-range mode, currently used range, test model (parallel or series), secondary value (ESR in this case) etc. Capacitor with marked value of 5.6 pF and tolerance 5% reads 5.56 pF. There is a menu option for the averaging mode - higher averaging results in more stable measured value. The test frequency is very low - only up to 15.6 kHz (there is an EU version with max freq. of 25 kHz) - not really RF range measurments but gives an idea.

The power supply for the meter should be between 9V-13V. Current draw is about 200 mA . Using the small 9V alkaline battery is out of question - it will last just a few minutes. The backlight can be switch off but the current is still too high for such battery. I am using 2.9Ah 12V SLA battery to power the meter.

This picture shows the inductance and Q for 330uH inductor in "series" model (Ls).

I built a few different sets of probes - Kelvin clips, 4-wire Pomona-style clips and SMD tweezers.
The Kelvin clips probe is the best all-around type. One half of each clip carries the Drive and the other half the Sense signal. It has the best accuracy for testing leaded components. The 4-wire clips probe is useful for testing transformers (there is a special mode for this), already installed components or using it with a small PCB fixture for quickly checking / sorting multiple components (there is a SORT mode alowing a tolerance value to be set and and audio beep indicating if the tested component is conforming or not). The SMD tweezers are a bit less accurate when measuring capacitors due to stray capacitance in the actual tweezers.

M3 Electronix Semiconductor Analyzer

A little known company called M-Cubed Electronix is selling an excellent little kit - PIC based Semiconductor Analyzer. It is one of those "I don't know how I managed without it?" things. On their web page - http://www.m3electronix.com/ there is a plenty of information about the features of this Analyzer - it fills a whole page so I am not going to relist them. From hardware point of view there is nothing special - just a PIC, LCD, 3 analog multiplexers and an array of precision resistors. used to set different measurment ranges. What is amazing is the firmware inside the PIC - the Analyzer is using mathematical models of semiconductor devices to detect the DUT (Device Under Test) and measure its parametters accordingly. I must say that this little Analyzer has an impressive accuracy and set of features and it is "a must" for every electronics workbench.

All components are high quality - no surplus stuff. Components are sorted by type. The plastic enclosure (sold separetly) is somewhat bulky anc could be better but it works. Assembly instructions and user manual are in digital form (CD).

Here is the complete PCB and Display Board. On this picture, the PIC is not istalled yet in it's socket. Upgrades to the software are free when available (requires shipping of the old PIC back to the company or buying a new PIC with the upgraded firmware)

The finished analyzer. Device Under Test is an ancient Bulgarian-made Germanium Transistor (GT2 306). There is no specific order to connect the test clips - the analyzer will automatically detect and display the DUT pinout. Every 5 seconds the display changes, scrolling thru a few data screens and showing various test results. After 30 seconds the LCD backlight is switched off to conserve power.

Here is another data screen. Diodes are connected only between the left and right test clips. The feature, detecting internal short (fault condition) in components can be used as ohm-meter for up to 50 ohms.

One small modification I have done is to drill a hole to the right of the LCD and glue a micro-switch connected to the calibration jumper. This allows me to perform calibration without opening the eclosure. I might drill another small hole for access to the trim-pot controlling the display contrast.

Palstar AT2K

Palstar makes some of the best antenna tuners out there! Last year after much research I got their AT2K model. The build quality is simply amazing. This tuner almost tunes my standard G5RV on 160 meters. It also work on 6 meters due to the smaller roller inductor. The built-in Peak-reading watt/SWR cross-needle meter works really well and with very good accuracy.

Internal picture of AT2k. Most components are manufactured in-house by Palstar, including both variable capacitors and the roller inductor. The quality of the components is impressive - everything is very solid and heavy duty - indeed "built like a tank". All solder joints are perfect.

Roller inductor, 160m inductor (toroid) and switch-in relay. The high-power balun transformer (toroid) is visible at top-right.

Load and tune high-voltage variable capacitors - top-left is the power/SWR detector, top-right is the PCB of the peak-and-hold circuit. At bottom-left is visible the ceramic rotary RF switch.

Palstar AT2K and Heathkit SB-200 in my shack. AT2K looks stylish and it is very easy to operate! The two vernier dials allow for very fine tuning.

2010 Update: The current version of AT2K was changed from the original design. As a cost-saving measure, the internal 4:1 balun (used with balanced antennas) was removed from the tuner and it is now sold separately as an external option. Another change is in the inductor - the original inductor was 18 uH and was padded with extra 10 uH inductor for 160m only. The new inductor is 28 uH, eliminating the extra inductor, relay and switch at the expense of the tuning resolution.

"Gettering" GU74b / 4CX800A

There is no such thing as a perfect seal! Vacuum tubes (especially high-power transmitting tubes) not used for a few years might exhibit serious problems if put into service without a prior conditioning of the vacuum. With time, gas molecules leak inside and/or are released by the tube's internal components. With years and years of storage, the vacuum could deteriorate to a dangerous level and once the tube is used for the first time it could "flash-over" - the gas molecules inside will become ionized by the electron flow and this will create a flash of high-temperature plasma between the cathode and anode, damaging the grid(s) and other internal components. A chemical composition, called "getter" is factory deposited inside the tube to complete and maintain the quality of the vacuum (the getter is visible as the shiny, black-metallic area on the inside wall of the glass envelope (in smaller tubes)). In power tubes, the activation of the getter is done by heat. Therefore, it is recommended, before putting into service a power tube with very long on-the-shelf life (more than a couple of years) to condition the vacuum first. This is done by applying power to the filament (cathode heater) only and leaving it on for a period of time. The hot filament will heat up the getter and also will improve the vacuum by itself (gas molecules will react with the hot tungsten filament and the cathode surface, forming chemical bonds and effectively extracting them).

Here is a simple fixture I used to "getter" my newly acquired GU-74b tubes (NOS, manufactured in 1990-92).

The "chimney" is made by cutting the top portion of a plastic soda bottle ("Classic Seltzer Water" sold at Costco to be more specific :). The cooling fan is from computer power supply. The tube MUST (!) be cooled with forced air while being "gettered" or the high temperature will damage the metal-ceramic seals and destroy the tube. The fan is raised about an inch from the surface to allow for air intake. I slowly raised the filament voltage from 3v to 12.5V (12.6 is the nominal voltage) over a period of 5 hours in 5 steps (3v, 5v, 7.5v, 10v, 12.5v) using variable power supply. I, then left the tube running with the nominal filament voltage for about another 8 hours. The fan should run continuously, powered by a separate 12V supply. The current drawn by the heater is around 3.6A (maximum allowable is 3.9A). Absolute maximum voltage for the filament is 13.3V but it should never be reached! Recommended operational voltage for the heater is 12.6v and exceeding this voltage is not healthy for the tube. Measures must be taken to avoid short in the power leads (at the tube's pins) - best is to use proper tube socket but wire-wrapping with solid copper wire (AWG 18 or 16) and heat-shrink tubing insulation could work too.
At the end of the "gettering" procedure (after power is disconnected) is also a good time to conduct a few electrical checks for possible internal short between various tube components - using just a simple ohm-meter, while the tube is still hot (! be careful handling it to avoid burns - use gloves) and one more time when the tube completely cools down. Check for short between the heater and the Cathode, between Cathode and 1st Grid (G1), between G1 and G2 and finally G2 and Anode. While the Cathode is still hot, it is normal to see somewhat lower resistance between the Cathode and the grids or the anode - it is a vacuum tube after all and it will conduct current if electrons are emitted by the hot cathode. This resistance will gradually increase as the tube is cooling down.
After installation in the amplifier it is recommended to start using it at low power and low duty cycle (maybe just starting in SSB mode) and gradually increase the power output.

AC Line Voltmeter

In most tube amplifiers there are at least a couple of supply voltages which are not regulated! Usually, these are the anode (plate) voltage (B+) and tube's heater (filament) voltage.
It is difficult and expensive to regulate a few kilovolts power supply in the case of the anode supply and it is big-n-heavy to regulate 8-10A of filament voltage. Instead, a form of regulation (or rather adjusting the voltages in the "ball park") is done via power transformer taps in the primary winding. Selecting the proper transformer tap is very important for proper operation and tube's health but this means that a stable AC line voltage is as important. Often under heavy load, the AC line "sags". Sometimes the utility company delivers power which is "out of specs" or your neighbour is welding in the garage - all are things that can affect your line voltage and might yield for adjustments in the way the amplifiers is used.
In other words - monitoring the AC line gives useful information during high-power amplifier operation.
I decided to build a digital AC line voltmeter for my shack. (as mentioned above - a useful thing but It also looks cool - I like red glowing numbers in the darkness of my shack, it contributes to the ambiance :-)))
AC power is delivered to residential buildings with 3 conductors from a "single phase" center tap 240V utility transformer (located near by). The center tap is the "neutral" conductor (usually grounded too) and there are two "hot" lines - 240V between both "hot" conductors and 2x120V (each half of the secondary) between each "hot" (end of the secondary) and "neutral" (center tap) conductor. In the electric panel the loads (circuits) should be distributed evenly on each half of the winding but for 240v, power is taken from both ends of the secondary.
If a 240V line is available for the amplifier this will allow also for monitoring each half of the secondary - both 120V lines coming to the house. I decided to implement this ability in my voltmeter. This way I can keep an eye of what exact is coming to the house and determine if my utility transformer or house electric panel is not loaded evenly.
The shopping list included the front panel mountable digital AC voltmeter (500V) UP5135 - $12 from eBay, electrical box from Home Depot, 12V/300 mA transformer, 7805 IC, bridge rectifier and a few switches (all from Radio Shack) as well as some other parts from my junk box (filter caps, etc).
The digital voltmeter unit (UP5135, AC 500V version) I am using doesn't have an isolated input so it is important that the meter's +5V power supply is electrically isolated from the measured voltage. This is done very easy by using a small transformer for the built-in 5v supply and not grounding the "negative" side but leaving it to "float" with whichever voltage is measured. (I let the smoke out from one of these meters by testing it using my *grounded* Alinco power supply. When connected to measure the 240V line, it shorted trough the negative side (ground) of the power supply - it destroyed the main IC and evaporated a few PCB traces in the meter.)


Internal look of the AC voltmeter. The board of the meter unit is on the left, transformer and rectifier on the bottom-right and the voltage regulator and filter is on the top-right.


3 switches are recessed in the enclosure - two on each side are switching the input of the meter to measure the 240V line and both 120V lines (by connecting one of the 2 inputs to the "neutral" line and the other to a "hot" line). The switch in the middle is an ON/OFF switch for the meter.


This is the finished meter in my "amplifier corner" showing the 240v line voltage!

Aspire One - Bluetooth modification

Acer Aspire One - NetBook

One thing which always bothered me when operating “portable” is the logging process - logging the contacts on paper and then entering them in the computer! Using laptop solves this issue but laptop computers are still relatively big and heavy to carry around and the battery life is limited….until now… Enter the NetBook – new generation of portable computers – UMPC or Ultra Mobile PC.
The Net Book is excellent candidate for logging machine. NetBooks are a relatively new market but with many entries by some “ big boys” – Dell, Asus, Acer, HP etc. I decided on Acer Aspire One!

The version I got is with 160 GB HDD, 1 GB RAM (which I later upgraded to 1.5 GB) and 6 cell Li-ion battery powering the machine for up to 6 hours! The Aspire One is very small and light machine – 2.2 lbs. It is using the new 1.6 GHz Intel Atom CPU. The only thing missing to a full-blown laptop is the CD-ROM/DVD drive – an external USB CD-ROM can be connected if needed. Aspire One is well equipped for connectivity – LAN, WiFi, 3 USB ports, built-in camera and audio card /w microphone (for digital modes).

I got an external “Prolific chipset” based USB-to-RS232 converter for connecting it to the radio via CAT interface. Ham Radio Deluxe runs just fine on this machine. The built-in wi-fi card does a great job connecting the PC to Internet hot-spots (eQSL, propagation, even DX cluster to mention just a few ham applications) . The screen is extremely bright and sharp! One very useful feature of this machine is also the ability to run from solid-state memory module instead of a hard-drive – they sell a different version equipped with solid state memory for those who like better shock-resistance. I decided to go with the hard drive as I needed the storage space – it is nice to have space for pdf manuals, books, music and even a few movies.

The Li-Ion battery is 11.1V /5200 mAh (operating) and DC charging/operating voltage is 19V / 1.58 A. I’ll look into a modification to power it from external battery or solar charger. Another thing I would like to see is a built-in Bluetooth interface which will complete the “interface arsenal”.

Meow!

Spotty - the HAM CAT!

"A mouse! On my operating desk?!!
"

"It is nice to have a callsign! I can dream about DX now!"

"I'll be watching the DX cluster for you!"

"CATuner - solution for high SWR"

"Tuning antennas with manual tuner is exhausting and I have the perfect spot for nap"

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.