N2PK VNA - Some initial plots

Quick sweep (3 to 29 MHz) of my G5RV clearly shows the low SWR points. It also reminds me that I need to do some minor tuneup of the antenna. The red vertical cursor is at the CW portion of the 20m band - the other SWR dips are in agreement with what should one expect from the classic G5RV. A VNA makes the ultimate antenna analyzer! One extremely valuable feature is the ability to perform calibration at the end of the coax / antenna feed point thus removing the impedance transformation effect of the coax from the measurement! It is like having the VNA connected right at the antenna feed point. (A good overview of various antenna analyzers and designs (incl. N2PK) can be found here )

The noise floor of Detector 2 (the one I am planning to use for transmission measurements) is below -125dB (at the slowest ADC speed) and flat. The plot for Detector 1 looks the same! This is a pretty good range for home-brewed equipment!

N2PK - Final Assmebly

Finally, about two months after I started working on it (with the "partial support" of my patient wife :) - the finished VNA is waiting for its aluminum cover. Really glad, that the build is over - it was a fun project but I am not sure if I am going to build a second one.
I am very satisfied with the results - a very precise, lab grade instrument in a professionally looking package - one can hardly tell that it is not a "commercial product".
The initial testing went just fine as expected. I spent a lot of time, making sure that everything is done right, inspecting every single solder joint and checking every component so I was not expecting any major problems. Everything worked the first time I plugged it in!

A note to myself: Next time I decide to build something with 7 bulkhead BNC connectors on the front panel - get a Greenlee D-hole punch. Shaping that many D-shaped holes with a regular drill and a file was tedious work.

My method of using Clear ink-jet mailing labels and 3M laminating sheets resulted in a superb quality panel finish. The colors on the panels are just a hint how much fun is to use the instrument :-)

The last touch before closing the unit was to attach a spare fuse to the PS board :) (hopefully I won't need it)
It is a pretty tight install - once everything is in place and connected, there is not much space wasted in the Hammond enclosure.

2010 Update: This picture shows the final version, using semi-rigid coax (RG-405) for Detector inputs and RF DDS out. The coax is in black heat-shrink tubing - just did not like the idea of exposed conductors crossing above the VNA PCB. The control/power cables were re-organized too and an accessory connector was installed.

The Hammond enclosure comes with two elegant bezels for the front and rear panels. When the bezels are installed, they stand between the enclosure and the panels. Under "normal" circumstances (DC/low frequency applications) this is OK. Since these bezels are plastic, the face-plates are electrically connected to the enclosure only with the 4 corner screws. In the RF world things are a little different. The electrical gap which occurs because of the plastic trim can cause signal leakage to and from the VNA. It degrades the overall RF shielding of the aluminum enclosure and affects the impedance-to-ground of the front panel connectors. I solved this issue with 6mm strips of self-adhesive copper foil - I wraped the 3 surfaces of the inside plastic edge with copper foil, covering all 4 sides of the bezel. When the plastic bezel is sandwiched between the panels and the enclosure, the copper foil serves as electrical bridge between the aluminum face-plates and the enclosure, while creating continuous RF shield on the inside.

This is the complete set - VNA, Reflection bridge and OSL calibration standards.
Since the calibration standards determine the accuracy of the measurements, I paid extra attention while building them. For the 50 ohm load I used small (0603) thin-film resistor - optimized for high frequency use (up to 20GHz) (Digikey P/N FC0603-50BWCT-ND or FC0603-50BFCT-ND). The construction of the OPEN and SHORT standards was also done very carefully. I am planing to make 2 more sets of calibration loads - N-type, SMA and UHF loads. This way I can use adaptors BNC-to-"X" and calibrate after the adaptor or at the end of the jumper cables regardless of their type and without using more adaptors.
The last thing left to do is to organize everything in a Pelican 1450 protective case.
Hmm...now thinking about this RF-IV sensor ... it doesn't look like the build is really over :-))

Feb 2011 Update - My complete VNA kit fits nicely in a Pelican 1500. This case is water-tight, very durable and makes it convenient to store and transport the VNA with all of it's accessories.

N2PK VNA Main board RF shielding

I fabricated the custom DDS ICs heatsink out of a used old Pentium CPU heatsink (the green heatsink on the picture). Using a Dremel tool, I shaped the heatsink to fit in between the nearby taller components (ICs, inductors and a transformer). It was a tricky job! The two holes on the PCB provided for heatsink mounting are hardly in their optimal location. Because the holes are located only to one side of the DDS chips, it was very tricky to mount it in a way where the heatsink is pressing on both chips with equal force without tilting to the side of the screws. I used some washers as spacers to achieve this. A second heatsink (the black square heatsink) was attached to the side of the main DDS heatsink for cooling the Valpey-Fisher Master Oscillator. Two smaller heatsinks were attached to the ADC chips using self-adhesive heatsink interface.

I made the RF shield cans out of tin-plated brass sheet. This material is very easy to work with and solder. Due to the high density of components and the really narrow solder pads for the shield, a great level of precision is needed while fabricating the RF shields. There is no need to solder completely the shield along its whole length - just a few solder points per side is sufficient. The extra heat used during a complete soldering is unnecessary and dangerous to the components. In addition, if the shield ever needs to come off it will be much easier that way.

This is the finished board ready to be installed in the enclosure. The lids of the two RF screening cans are attached with self-adhesive copper tape (with conductive adhesive). Again, this method provides sufficient electrical connection and allows for an easy access to the the detector circuits if ever needed.

N2PK VNA Power supply module

I mounted the power supply module onto a piece of double-sided FR4 board. The board is exactly the same width as the main VNA PCB and just slides in the lowest channel of the Hammond enclosure - the same way the main VNA PCB is mounted.

This makes up for a very nice mounting solution with no drilling and no screws on the bottom of the enclosure - I really wanted to keep the enclosure clean and free of unsightly screws on the outside. The actual PS PCB is screened with a tin-plated brass RF shield (soldered to the top copper layer of the FR4 board). Copper tape strips on each side of the module are used to improve the electrical connection between the aluminum enclosure and the two copper planes of the FR4 board, once it is inserted in its channel. The lid of the RF shield is attached with self-adhesive copper tape (with conductive adhesive). This allows for an easy removal of the lid should a fuse change is necessary. If needed, the whole power module can be completely removed for servicing just by detaching the connectors and sliding it out of the channel.

The PCB is bolted to the FR4 board inside the shield using small brass stand-offs/spacers (there are components on the bottom side of the PS PCB and some clearance is needed between the solder joints/components and the top copper plane).

The bottom side of the FR4 plate. I decided to move the +12V linear voltage regulator from the bottom side of the PS PCB to the bottom side of the FR4 mounting plate. When the plate is inserted in the very bottom channel of the enclosure, there is just enough space (aprox. 5mm) for the linear regulator to fit in. This, actually turned out to be a pretty good cooling solution. On the right side of the board are visible some current limiting resistors for the two LEDs and the VNA Detect circuit as well as by-pass and filter caps. There is also a second LDO voltage regulator for +9V line (along with some filter caps). The +9V line is wired to the Accessory connector on the back, powering Transverters, RF-IV sensor or S-parameter test set.

The linear regulators are using the bottom side of the aluminum enclosure as a giant heatsink. Some thermal grease and a small copper insert (shim) ensures the good mechanical contact between the aluminium wall and the IC. The bottom copper layer of the FR4 board serves as a secondary heatsink - the regulator is mounted with its metal tab facing down and it is "sandwiched" between the FR4 mounting plate and the bottom wall of the enclosure. I used thermal double-sided self-adhesive tape to attach the regulator to the FR4 board (to the copper shim actually, the shim is soldered to the FR4 along one of its edges so it can flex). Soldered to the board is a little brass stub that goes into the regulator's mounting hole. This provides extra mechanical stability when the board is installed/removed.

N2PK VNA Reflection brdge

The reflection bridge was fairly easy to build. Most of the work was mechanical. The two male bulkhead BNC were very difficult thing to find. I could not find a local source so I had to buy them from Hong Kong (tnx eBay!) . All RF connectors are attached to the aluminum enclosure using stainless steel hardware.

Great deal of precision is needed when drilling the holes for the male bulkhead BNC connectors. There is not much mechanical "play" in these connectors and they need to match perfectly the female BNCs on the front panel of the VNA. Both connectors are exactly 2 inches apart. This picture also shows the two L-shape brackets connecting the bolts of the female BNC to the ground plane of the PCB. The brackets are soldered to the ground plane on both sides of the center pin of the female connector.

Same L-shaped brackets for the male BNCs. I had to remove some of the solder mask in order to make better electrical connection. Good ground connection is essential! The male BNCs have their own ground pins on the back side (a nice feature) - also soldered to the ground plane (visible at the very left and right of the board).

The finished reflection bridge. This side is "up" when the bridge is fitted onto the Detector 1 port - the normal position for the bridge during reflection measurements.

The reverse side of the bridge. This side is "up" when the bridge is fitted onto the Detector 2 port (optional).

N2PK VNA Rear panel

Cutting out the rectangular openings for the switch and the DB25 connector was the real challenge here. The left side of the panel is dedicated to graphics - there is not much room behind it because of the power supply board. I left some empty space on the panel for the future installation of a small Accessory connector so I can connect different VNA accessories (like the RF-IV test head or Transverters). The power switch has a built-in LED connected to the regulated +12V line. The front panel's LED is connected to the +5V line (DDS power).

The rear panel is completely detachable. This makes life easier if I need to open and disassemble the unit. Ferrite chokes are installed on the control / data lines and power supply input. The little 4 pin connector supplies power to the +12V LED, VNA Detect line and +9V to the Accessory connector.

N2PK VNA - Power supply PCB and front panel

I received the PCBs for the PS board from the board fab house (Sunstone circuits) and I must say that the fabrication quality is pretty good. The schematics are based on the N2PK/OM3LZ power supply board with very minimal changes on my part. Basicly, there are two voltage regulators: +12V Low-Dropout linear type and +5V switching regulator, LC noise filters for each regulator and two "crowbar" type over-voltage protection circuits combined with over-current and reverse-polarity protection. In other words - the power conditioning is very solid and provides a good protection for the VNA board from a number of potentially dangerous events.
There is an additional +9V regulator circuitry (installed on the bottom of the PS support plate) for the Accessory connector - it is used to power transverters, S-parameter test fixture or the RF-IV sensor.
The PCB layout design was done with Cadsoft Eagle. I started with the OM3LZ board as a reference but at the end I changed the layout a bit. These pictures are of my ver. 1.0 board. The final version of the PS PCB is ver. 2.0 and that one is even more compact and with a smaller footprint. Not sure if the 2.0 board will ever see daylight since I am all set with my PS needs for now.

The PS is done with mixed thru-hole/SMD technology - first the SMDs are installed and then the rest of the components. I added pads for an SMD LED on the +5V line and a current limiting resistor for it. This LED serves as a reminder that the unit is powered when operating with the covers off and it is totally optional to install.

On the bottom side I have a space for an optional LDO 12V linear regulator (Digikey p/n 576-2206-ND (Mircrel MIC29150-12WT)) . This allows for an external power supply with a wide range of voltages - typically filtered DC 13V to 24V. Because of the extremly low drop-out voltage ( 0.35V @ 1.5A) of this chip, the VNA can be powered by a standard 13.8V PS and still be in regulation. For portable use I'll probably power the unit from a 2 x 7.2V Li-Ion battery packs. This regulator can be omitted and bypassed with a jumper but then extra attention needs to be paid to the input voltage in order to prevent the over-voltage protection from triggering (at the price of a blown fuse). There is a couple of extra bypass capacitors associated with this regulator.

Unfortunately, it wasn't cost-effective to put a silk-screen for the SMD components on the bottom but I don't think it is a big issue.

The front panel of the VNA. First step is to drill all holes in the panel. The graphics for the panel are actually a "sandwich" of two layers - printed layer and protective layer. I used Corel Draw to design the graphics layout (any other vector graphics software like Adobe Illustrator will work too). The layout is then printed with a Color Ink Jet printer (printer driver: best quality, transparency) on a sheet of "Avery Clear Full-Sheet Labels" (Avery 8665 or 18665 (better) from OfficeMax/Staples). The aluminum panels must be cleaned, de-greased with alcohol and dry. During application of the printed layer, one should be careful not to smudge the printing or leave fingerprints and must try to prevent any air bubbles from forming at the same time. (I used a piece of the base paper from the label stock (the glossy, waxy side as an "applicator", rubbing the sheet while applying it to the surface). The graphics must be carefully aligned with the holes on the panel during application. Then, the print layer is protected with a second layer of a durable clear self-adhesive plastic sheet - 3M Scotch Laminating Sheets (LS854- 10M or -10G (the last number shows how many in a package, M for Matte, G - for Gloss). After applying the first (printed) layer, compressed air and soft brush were used to remove any dust particles, then applying the protective layer is done the same way - applied slowly while watching out for air pockets . (Remember - it is a "one shot" deal - if something goes wrong during the application of the laminating layer there is no going back - you have to start over with new print layer).

The resulting surface is smooth, dirt and scratch-resistant and because the printed layer is transparent, the front panel has almost the same brushed aluminum/metallic look as the enclosure. Instead of using transparent print layer, a solid-color stock can be used too, but IMHO it looks "flat" and not as attractive as the natural metallic look. Once the two layers were "sandwiched" and pressed well together, I use scalpel blade to carefully cut out the openings and the excess around the edges.

The finished panels came out very nice and professionally looking - practically "commercial product" grade. I am really happy with the results - I think this will be my method for printing front panels from now on.

While looking for a front panel layout solution, I came across an interesting product - Ink Jet printable laptop skin (sold in Office Max). It is a white, self-adhesive vinyl sheet and I think it could be used for front panel labeling as well but I really wanted to preserve the aluminum finish look so I opted for the see-through label sheets. Another possibility is to use one of the products by http://www.texascraft.com/

The front panel is installed on the enclosure along with the BNC connectors and LO jumpers. I decided to color code the connectors because of their number on the front panel. It will be easier to work with the VNA and keep track of all connections.

All of the RF interconnects are done! Initially, I was going to use microwave semi-rigid coax as it provides the best shielding and phase stability but this stuff is too exotic (read: difficult to find/install/work with/using specialized connectors) so I opted for a special mil/aerospace version of the RG-316 by Semflex called SI316. The regular RG-316 is double-shielded with two silver-plated round braids. The SI316 is the same silver/teflon coax but it is triple shielded - it has metalized kapton foil layer between the outer round braid and the inner flat braid. This results in lower attenuation and much better shielding (aprox. 35 db better or >90 dB) than the regular "plain-vanilla" RG316 - the only thing better then this cable would be to use semi-rigid coax (shielding >110 dB).

Next item on the list is the wiring harness and the power supply board mount.

Update: I made a set of bulkhead (f) BNC to (m) SMA internal RF interconnects, using semi-rigid hand-formable RG-405 coax. I left the old SI316 cables for the LO DDS and replaced only the ones connecting both detector inputs and the DDS RF OUT to the front panel. I did not observe any better detector noise figures.

N2PK VNA

Here is the first look of the newly born N2PK Vector Network Analyzer. This weekend I completed the main PCB. It is a ver. 4.3 (dual detector) board. The Power Supply PCB gerber files (layout generated with Cadsoft Eagle) were sent to the board house and I am expecting the new boards any moment. Hopefully there are no mistakes and soon I'll have a power supply ready for the VNA. All of the PS components were ordered together with the VNA'a bill of materials.

The N2PK is a serious SMD soldering exercise - over 300 components on the main PCB and over 700 solder joints. The blank 4.3 PCB was ordered from VE3IVM. All parts are from Digikey. The only additional components were the Valpey Fisher Master Oscillator and the Minicircuits transformers. The VP XO is a very difficult component to find! There is an alternative with Connor-Winfield MO, available from Digikey but I was lucky to find the Valpey Fisher part. I installed all components on one side of the PCB and then moved to the other.

The N2PK VNA is an advanced project - high part count / desnsity on both sides, some fine pitch ICs (not very easy to solder pin-by-pin) and some components in very small packages. Good soldering skills are a must as well as a good set of tools, soldering materials and soldering iron with fine SMD tips (I used Weller WESD51 with ETP / ETH tips). I worked on the board for about 2 weeks spending a couple of hours almost every evening. I could have done it much faster but I was taking my time, double-checking and making sure that there are no mistakes. Troubleshooting is difficult with SMDs and the capacitors are not marked which can cause a lot of headaches should one makes a mistake with the components. My approach was to install all components (small-to-large size order, but starting with the ICs) of the same type/value together, while working on the top side first. After the top side was completed, I did the same thing on the bottom side leaving all connectors for last. I made sure that there is only one type/value components out on the bench at a time to prevent confusion. How well I've done we will see once it is powered up.

I have also prepared custom heatsinks for the MO, DDS and ADC chips. The only thing left to do on the main PCB is the RF shielding cans for both detectors. I need to make screening boxes out of tinplate. After I am done with the power supply / power conditioning board and the wiring harness I'll be able to finally test the VNA.

"Third hand" aid for SMD soldering

We are continuing with the "Hints and Kinks" category. I recently started working on my N2PK VNA project. Normally, when I do manual soldering of SMT devices I rely on "sticky flux" and tweezers to prepare the components for soldering. First, I'll apply flux solution on the pads using flux pen (like the Kester 186 pen) and then I'll wait a bit, until the solvent evaporates leaving sticky flux residue behind. After applying the flux solution, the right timing is very important for placing the SMD - too soon, while the pads are still very wet and the board will be slippery making it dificult to position the SMD - waiting for too long to dry and it wont be "sticky" enough for the flux residue to hold the component in place. In the next step, using tweezers I'll place and position the component and then press on it gently with the tweezers (small screwdriver, toothpick or even fingernail might work too). The sticky flux residue helps a lot as the component adheres to the board and doesn't move easily or slides across the PCB when I try to hold it in place by applying pressure. Then I'll just tack one (or a few) of the leads, using very little or no additional solder (only what is already on the pads/leads). When the component is secured in place, I'll release the pressure and continue soldering the other lead(s) first. Then, Ill come back with solder to the ones I have just "tacked".
The N2PK VNA project is done entirely with SMDs and because of the sheer amount of components (over 300), I needed some sort of aid for this soldering procedure.
Here is a very simple gadget for SMD soldering which I constructed out of parts laying around on my garage workbench. The Bill Of Materials includes 3.5-4" copper pipe (1/2" diameter, plumbing type), 7" small diameter brass tubing stock (~0.13" OD, ~0.1" ID, usually sold in Hobby stores), 6" steel rod which fits nice and tight in the brass tubing (~0.094" diameter, again from the local Hobby store), some sort of weight (slingshot ammo, lead fishing sinkers, etc) and an empty plastic canister from 35mm film cartridge.

First, I drilled a small hole right in the middle of the 1/2" diameter copper plumbing pipe. I inserted and soldered the brass tubing to the pipe, while making sure that the assembly is a nice straight angle T-shape (because of the large thermal mass of the assembly, I had to use a small propane torch to solder it). The piece of steel rod should be formed into the shape shown on the picture above. Next, I drilled a small hole in the bottom of the film canister and another one in the plastic cap. The holes need to be a tad smaller than the brass tubing's OD so the tubing fits in very tight (with significant friction). I inserted the brass tubing through the plastic canister and the cap, filled the canister with weight (it could be anything small and heavy - nuts, bolts, lead fishing weight, marbles or slingshot ammo (as in my case)). Than, I inserted the end of the steel rod into the brass tubing.

Done!

This simple 15 minutes project will give you a great "third hand" for SMD soldering. The cool thing about it is that it is very easy to control the amount of downward pressure on the components - just slide the film canister up or down on the brass rod for fine control or open it and add / remove weight for coarse. Also, the film canister could serve as a solder dispenser - it can hold a small spool of solder wire which can be pulled through another small hole in the cap. Yet another possibility is to house inside the film canister a couple of 3V lithium batteries, on-off switch can be installed on the plastic cap and connnected to a White LED at the end of stiff insulated wires.This can server as "goose-neck" type of miniature work light, illuminating the immediate area of soldering. Small magnet could be glued to the canister to keep handy small parts and components. Even a small magnifying glass can be attached somewhere - what other features can be added is limited just by your imagination.

The soldering weight is pressing onto chip, ready to be soldered. If the steel rod rotates too easy inside the brass tubing, glue can be used to fix it in a straight vertical position.

This gadget has a small footprint on the workbench and it can be used with almost any size boards. It can be used inside larger equipment enclosures without the need for removing the circuit boards.

In addition, special tips could be fabricated for the end of the rod and used with different shape and size components. The tip in the picture was made out of an old food thermometer probe - something I just had in one of my junk boxes.

The very tip was shaped to have a flat face. This picture shows a 0805 SMD resistor ready for soldering. One added benefit is that the metal (nickel-plated copper) tip works as a miniature heatsink for the component being soldered, reducing the chance of overheating and thermal damage.

Palstar AT2K - 160m Mode Indicator Mod

The AT2K is probably one of the best tuners I've ever used but as we all know - "nothing is perfect" and this tuner is not excluded from the above-mentioned statement. It has one somewhat annoying flaw - it lacks a good indication when the extra inductor is switched in circuit for 160m band operation. As a result from it, every now and then I'll forget that I have had the 160m switched on and I'll try in vain to find a match for my dipole antenna on the higher bands. The only indication on the front panel that 160m mode is selected is the button itself but this is not very obvious (especially in a dark shack) so I decided to add an LED indicator.
This is an extremely simple and quick modification - nothing special but it improves the tuner's ergonomics IMHO.

The LED is connected to the coils of the relay responsible for switching the 160m inductor in series with the main roller inductor. Current limiting for the LED is done with a 1.8 kOhm - 2.2 kOhm resistor. The LED can also be connected straight to the 160m switch board but it is very hard to get to the board with a soldering iron without removing the top peak-hold/switch board.

The key component for the mod is a 3 mm LED in a small yet elegant black bezel - Digikey P/N 67-1192-ND. It looks excellent on the black front panel just above the 160m button. This LED bezel assembly requires a 4.1 - 4.3 mm hole. One needs to make sure that all of the metal particles from the drilling are completely removed from the enclosure!

Maintenance tip: I noticed that the roller shaft was very dry and needed lubrication. I applied very small amount of conductive grease using a fine brush. The grease needs to be brushed as a very thin film just on the shaft. (!) One must be very careful not to get any grease on the roller itself or the inductuor's coil as this might cause huge arcing problems. The tuner normally comes with factory applied conductive grease but on my unit it wasn't enough. I moved the roller up and down the shaft a few times to evenly spread the grease while making sure there is no excess that can fall onto the inductor.

For DIY conductive grease : I used some white lithium grease (sold as Genie Screw Drive Garage Door Opener GLU-3 lubricant - this grease doesn't gum up easily) and added a good amount of fine graphite powder (I used a fine file (! not sandpaper) and soft carpenter's pencil to produce the powder). I mixed it well, while adding more graphite powder until the mixture thickened a bit.

2010 Update: Palstar changed the design a bit for their current version of AT2K. They no longer are using the 18 uH inductor which had to be padded with extra 10uH inductor for the 160m band. The new inductor is 28 uH therefore there is no longer need for a relay and switch in order to add extra inductivity and tune 160m so this mod is irrelevant.

"Fancy" Bird 43 Dummy Element

This year at Dayton, as the flea market was concluding on Sunday and I was hunting for last-minute bargains, I came across a few non-functional Bird 43 slugs in one of those "Everything $1" bins.
My first thought - I'll repair them. As it turned out, they were for frequency/power range I had no interest whatsoever. So what one can do with a damaged Bird 43 slugs besides repair? A few things actually - it makes nice dummy slug for the Thruline section, it can be reworked into a RF sampler or just keep them for parts.
The original aluminum dummy slug could actually be improved on - it has a small design flaw IMHO - when inserted, it keeps the contact finger of the Thruline section pressed and under tension. This might lead to metal fatigue and eventually the contact finger could loose its spring action, causing poor electrical connection with the slug. Once, I saw a third party "dummy slug" where this issue was addressed with a shallow groove allowing the contact finger to rest. The normal Bird element already has two small lateral indentations on each side where the contact points are protruding.
First I "gutted" the slug, completely disassembling it and removing everything from inside including the internal contact plate. This was simple enough and left room for the contact finger of the Thruline section to rest without pressing onto anything. But why stop there? How about a hygroscopic dummy slug to absorb trapped moisture inside and to keep the Thruline cavity nice and dry?

I sealed the two contact point holes of the slug with copper/kapton tape from the inside. Then I made 6 small holes on the bottom of the white Teflon cover (which normally protects the coupling circuit).

I placed a piece of the material used to make the little bags with Silica gel on the bottom of the cover, over the holes and then filled the element housing and the Teflon cover with Silica-gel beads. The piece of "silica-gel bag" material serves to stop any Silica-gel beads from going through the holes, while allowing air to flow in and out of the slug. (tea-bag material could also be used)

I reinstalled the Teflon cover on the slug, pressing it until it snapped into place.
Silica-gel is very easy to find - people normally discard the Silica-gel bags after they buy a pair of new leather shoes or a hard drive - I collect the little Silica-gel bags in a jar and re-use them.
Re-cycling Silica-gel is also easy - the Silica-gel needs to be "baked" for about 2 hrs in a toaster oven on "high" (250 - 300 F). This process will evaporate the moisture absorbed by the beads and the batch will be as good as new, ready to absorb more moisture again after it cools down.

The new dummy slug is lighter, preserves the contact finger of the Thruline section, absorbs moisture and keeps the cavity dry and most importantly - makes a cool rattle sound when you shake it :-) Not bad for $1.
Update: Another idea for enhancing the functionality of the "dummy slug" even further - small piece of felt/cloth/foam can be attached inside each contact bed of the slug. Then, (optional) a drop of Deoxit D5 solution can be used to moist this "cleaning pad". Each time when the slug is inserted or removed, it will wipe clean the contact finger.
In addition, instead of using only silica-gel beads, the dummy slug could be filled with a mixture of silica-gel and activated carbon. (another option, if moisture is not an issue, is to replace the silica-gel entirely with activated carbon). The activated carbon will absorb any sulfuric gasses in the air (like hydrogen sulfide) - sulfur is the main cause for the black tarnish on silver/silver-plated surfaces (the Truline cavity is silver-plated).

SB-200 - correcting the filament voltage

Most of the SB-200 amplifiers (including mine) suffer from "high filament voltage". One theory I've heard is that when the amplifier was designed in the 60s, the power transformer was calculated for AC mains of 110V/220V and today we are using 120V/240V AC. While a bit higher plate voltage is welcomed , higher filament voltage is a bad thing! According to an EIMAC study: " A 3% increase in filament voltage above the maximum rating will result in a 50% decrease in tube life."
The factory specification for 572B heater voltage is 6.3V +/- 0.3V @ 4A. My calibrated (NIST traceable) DVM measured 6.65V AC (True RMS) for the filament voltage @ 240V AC mains. This reading is beyond the maximum allowed and it should be corrected!
I decided to bring the voltage down, just slightly less than the recommended 6.3V.
The increase in the filament voltage is of no benefit to the output of the valve. 572B is performing at full output with even less than 6.0V filament voltage! My target voltage was 6.25V - about 0.4V down. A thing to note is that the voltage of my 240V AC line does not fluctuate a lot over time - just a couple of volts. Using resistors to correct the filament voltage is the obvious solution except sometimes it is hard to find the right ones for the job! Each amplifier will require custom values for the resistors (determined by the individual transformer and maximum AC line voltage).
To drop the voltage across a resistor by 0.4V (in my case) while drawing 8A of current (2 x 4A for each valve) results in calculated total resistance of 0.05 Ohms. To complicate the matter further, this resistor should be capable of dissipating at least 4W. To preserve the circuit symmetry (the filament secondary is center tapped) I decided to use 2x 0.025 Ohms /2 W resistors - one for each side of the secondary.
Precision current-sensing power resistors could be one possibility. Another solution would be to use a coil of small gauge Teflon insulated wire - AWG #22 for instance. The small gauge will cause the wire to heat up increasing the resistance and corresponding voltage drop. (Even better - coiled on a ferrite rod, thus creating a second filament choke - more inductance equals better choking in this case). I might actually try this someday but there is not much space in the tube's socket compartment and I want this to look neat.
Instead, I made DIY resistors out of Nichrome-60 AWG #22 wire (0.65 mm), left over from my Low-Q VHF suppressors kit. This wire has very high electrical and thermal resistance. To reduce the heating and thermal stress on the short piece of wire required for each 0.025 Ohm resistor (I am using two of them in series for 0.05 ohm total resistance), I actually made each resistor out of two paralleled 0.05 Ohm "resistors".
Recap - i need a 0.05 ohm resistor but for circuit symmetry, I'll use 2 x 0.025 ohm in series (one on each end of the filament winding, just before the filament choke). To increase the power rating of each 0.025 ohm resistor on the other hand, I'll be using 2 x 0.05 resistors in parallel to form the actual resistor.

I used a little over 3cm length of Nichrome wire, bended into a "U" shape. Both ends of the wire are soldered together, creating small elongated loop. The solder points for the resistor are at each side of the elongated loop. This way, each 1.5 cm length of the wire would have to dissipate about 1W for a combined power rating of 2W per resistor.

I inserted each leg of the U-shape wire into Teflon tubing before soldering them together, forming the "resistor". This will prevent an "internal short" in the "resistor". My LCR meter shows exactly 0.025 Ohm.

Each 0.025 Ohm/2W resistor was placed in-line with the filament secondary, just before the filament choke. For soldering the Nichrome wire I used the instructions, flux (corrosive!!!) and silver solder supplied with the suppressor kit. The solder is a tin-silver alloy with high melting point and it should be used also for soldering the "DIY resistors" to the filament choke terminal strip and to the transformer leads. The heat produced by the resistors is substantial - I barley can keep my finger on the resistor (right after I shut the amplifier off (!)), so high temperature solder is needed in order to prevent the joints from failing over time. The result is as expected - filament voltage now measures exactly 6.25V @ 240V AC Mains!

I was ordering parts from Mouser for another project so I ordered a few high-power current sense resistors. Here is a more "commercial" modifications for those who don't feel like making their own resistors. The end result is the same as it is with the DIY resistors. I have the feeling that the commercial resistors heat up a bit less (obviously, larger surface area - better cooling) than the nichrome wire but it is hard to tell just by touching them with my finger.

Common-Mode Current Choke (Ver 2)

There are two major changes in Version 2! The first change is purely a mechanical one- no enclosure anymore - the choke is not as compact as it is in the original design (Ver 1). It is not a "one-box" device but rather an in-line jumper-like device. Before, the enclosure was keeping the coaxial from damage by garden critters and the brittle ferrite rings from mechanical damage. Now, with the open design, one should be more careful while handling/installing the choke. On the bright side - there is one less M/F Type-N connector pair (less insertion loss and one less connection to waterproof) - now the balun has one Male and one Female connector on each side, oppose to the two female bulk-head connectors mounted on the box. Because the antenna side pigtail (/w Male Type-N connector) is longer, there is no need for the short Male-to-Male jumper to the antenna. There are only two connections to waterproof. The open construction also allows for better cooling during Legal Limit power levels.

The second change is electrical - since the space is not limited anymore, now the choke has more ferrite rings (24 pcs vs 18 in the old one) and less coaxial cable turns (2.5 turns in the new version vs 3.5 turns in the old) through the binocular core. This improves the upper frequency impedance (speaking of which, I have a couple more mix 43 single turn ferrite beads on the coax pigtails, for a total of 3)!  More ferrite rings in the core - more inductance (to make up for the lower number of turns). Less turns on the other hand means a decreased capacitive coupling between the turns on higher frequencies. My MFJ-259B is limited in measuring the complex impedance (Z) up to 650 Ohms. The choke was measuring >650 Ohms up to about 20 Mhz. The impedance is around 400-500 Ohms at 30 MHz, mainly due to the capacitive coupling between the turns. I have installed an additional plastic "jacket" over the coaxial in attempt to increase the space/dielectric between the turns and decrease the capacitive coupling but can't really tell if this has any real effect! I am using the same LMR-240 coaxial (RG-8X type with a solid center conductor). The choke should handle well power levels up to 1.5 kW but I am not planing to use it with more than 1 - 1.1 kW anyway (ACOM 1000 amplifier).
For steady 1.5 kW use or with antennas, exhibiting very high SWR, I'll recommend the use of high-temp cable such as RG-142 (silver/teflon) for the balun. This cable has a solid teflon center insulator and there is no danger for the center conductor to migrate to the shield and short it, if substantial heating occurs - something that is possible with the polyethylene-foam based center insulator of the RG-8X / LMR-240.
The ferrite toroids  I used are made by FairRite (Part# 2643801002) - mix 43, OD 28.5mm, ID 18.5 mm, H 7.5mm. Price is about $6.10 for 10 pcs - Mouser Part #: 623-2643801002.

The new Common-Mode Current Choke takes a little more linear space but this is not a big deal. I am planing to put extra mechanical support (another plastic stake) for the choke itself.
Update: I tested the current balun with my VNA and here are some plots:

Insertion loss on HF <0.1 dB (due to connectors and coax cable insertion loss).

Complex impedance (Z) for Common-Mode current is 2200 - 2500 Ohms on the 80m -40m bands and drops down to ~300 Ohms on 6 meters (just as expected). On the 20m band chocking is still > 1kOhm.
Overall, for a current balun of this size and cost, the result is excellent and it completely serves the purpose. I wanted to have it primarily for the lower bands - 80m and 40m but get some good values on 20m as well. If mix #31 is used for the ferrite,  the optimization will be even better in the lower HF spectrum.
2-3 kOhm Chocking impedance is completely adequate for a tunable antenna like BigIR where the SWR never exceed 2:1 (given that the number of radials is sufficient and they are properly sized). I actually get SWR of 1.1:1 to 1.2:1 on 80 meters.
The VNA test just gives me a nice confirmation that I have achieved my design goal when building the choke.
If more chocking impedance is needed, more of these in series can be built on the same piece of coax and folded to save linear space.

A transmission scan shows attenuation >30 dB on the lower bands but this is a less relevant test to a real-world application as the measurement of attenuation is done in a true 50 ohm system. In reality, the antenna feedpoint (Z) is never exactly 50 ohms.

Common-Mode Current Choke Disaster!

It is Spirng time and Spring means rain - lots of rain! Yesterday I wanted to use my SteppIR vertical antenna after a couple of months rest (I've been using my dipoles meanwhile). When I did, the SWR was very high on all bands regardless of the SteppIR tuning. It was 4.3:1 even when the element was completely retracted and the same thing with completely extended element - 40/80 meters tuning - a sure sign of trouble. No matter what I was doing the SWR was not changing and very strong signals were extremely attenuated - about 1/3 of what the dipole was receiving! MFJ-259B was showing the same thing - the impedance was changing a bit on different bands but the SWR stayed very high. Next thing to try was the DVM - aha!!! - DC resistance between ground and antenna was 6-7 kOhm and constantly changing up and down (ideally it should be just a few ohms of DC feedline resistance as the 80m coil has a built-in balun showing as 0 ohm for DC)! After going to the antenna site, everything looked normal until I started the troubleshooting by inspecting first the common-mode current choke/balun in the base of the antenna!
Here is what I have found upon removing the cover of the balun enclosure! Totally flooded - just missing a few toads inside! Not only that! Note the left antenna connector, between the two top screws of the flange connector, where the coaxial is soldered to the center pin receptacle of the female N connector - the pin receptacle, together with the white dielectric insulator (visible between the bolts) were (!) pulled out of the N connector housing and were almost entirely inside the enclosure! As a matter of fact - they were so far out of the connector's barrel - the center pin of the male antenna connector was not making contact - not even close! The solder connection of the coaxial shield was broken off too! I have no idea what happened there but looks like some serious force was in play - I can't do such a thing with my fingers and even with a tool will be a chore! One speculation is ice. In a solid block of ice this might be possible during freezing / melting cycles during the winter but none of  the ferrite toroids were broken! Another possibility is a miniature steam explosion. When I used the antenna last time, was with 1kW according to my logbook, water inside the tightly coupled male-female N connectors could have vaporized and the resulting steam could have pushed out the dielectric inside the box and ripped off the shield's solder joint- the middle insulator is the only part that can easily relief such pressure. Who knows... Bottom line - the whole balun is badly damaged and I'll be making Version 2 once the parts arrive!
The reason for this disaster is very simple - having too much faith in labels such as "Weather resistant electrical box" (aka the balun enclosure) from Home Depot. By "Weather resistant" they actually mean - "water resistant", forgetting that Sun exposure is part of the "Weather". The seal around the cover failed. It is a ring type gasket and either the gasket material changed over time or it was no good in first place. After removing the gasket, the seemingly dry gasket produced a lot of water just by squeezing it with my fingers. Seems to me that the gasket is made of some sort rubberized open-cell foam material (?!? huh?) and it was SOAKED with water. It is possible that the UV exposure broke down the rubberized coating and changed the properties of the material - the gasket no longer functioned as gasket but as a wick letting water to fill the box.
Looks like Carlon - the manufacturer of the so-called "Weather Resistant Electrical Box" needs to learn more about gaskets and materials or revise the description. (Needles to say - the cover was screwed very tightly so the only thing really to blame is the gasket!). There was also a very small discoloration and deformation of the cover (due to the sun exposure) but that was well within the tolerance of the gasket. I should have removed the original gasket when I built this choke and use some silicon sealant instead but back then I didn't even think they will sell electrical boxes with such poor choice of gasket material - now I know - they do! Lesson learned!
Construction notes for the original choke (Ver. 1.0) are on my antenna site. Before the damage, the choke worked just great! The new one, I am planing to build will be of open type design, with no enclosure.
If anyone decides to built this version - my recommendation is to have a small drain hole on the bottom or use a bead of black RTV sealant around the edge of the gasket, once the box is closed to protect the material from UV damage.

Hamshack RF Signal Path Diagram

I've updated the diagram to reflect the latest changes in the RF paths / equipment. The main change is the way the output power is measured. In the new configuration I am using one watt-meter for output-to-antenna power and another watt-meter for the output-to-dummy load power. In the old configuration, each amplifier had its own dedicated watt-meter for the output. In addition, the main high-power dummy load is now an oil-filled Bird 8201.