Conversion of DB4060 and DB4062 Duplexers for 2 Meter Amateur Radio Use

The db Products / dbSpectra DB4060 and DB4062 duplexers are awesome in terms of performance for 2 Meter repeaters. That is… if they are built as or converted to true “-B” models (and they are in good overall condition.)

There have been numerous different “cheats” and workarounds published on the web on how to use a set of DB Products (now DBSpectra) DB4060 and DB4062 duplexers which were manufactured for frequencies other than the Amateur Radio /  Ham band on 2 Meter ham repeaters. We’ve probably seen all the same “tricks” you have seen, and by and large they are all compromise solutions which don’t result in very good performance and — in some cases — can result in costly damage to the duplexers and radio equipment, not to mention frustrating down-time.

Let’s get real…

The bottom line is this: if the loop assemblies (more specifically the copper loops inside the enclosures) are not correct, they won’t perform to factory specifications of a DB4060 (4-cavity) or DB4062 (6-cavity model) with the “-B” model suffix. The “-B” model is a 143-156 MHz duplexer; however, most of what hams find for sale or salvage  is the “-C” model, which was/is manufactured for 154-174 MHz operation. Trying to use a non-converted “-C” set for 2 Meter, 144-148 MHz purposes is a very good way to end up pulling your hair out. They won’t provide the factory-specification Return Loss (SWR), Insertion Loss, or the notching/isolation performance which Amateur Radio repeater owners are truly looking for. But have no fear — they can be converted to a -B set and meet or exceed factory performance specifications.

DuplexerRepair.com offers multiple solutions for converting DB4060 and DB4062 duplexers to true “-B” models.

Depending on your budget, location, skills, and available resources, the following options are available for converting a set to a -B model:

  • Bring (or ship) the whole set to DuplexerRepair.com and we’ll do the conversion, inspect the cavities, completely overhaul the internal workings of the cavities if needed, do any other needed repairs, and convert the loop assemblies. When we’re done with them, you will have a set of great working “-B” duplexers, tuned and ready for use on your repeater.
  • If you have someone local who can do the tuning for your, you can just send us your loop assemblies and we’ll convert them (including testing the trimmer capacitors and replacing any which have failed or show signs of impending failure.) When you get the loop enclosures back, just install them on the proper cavities with the proper cables and have someone tune them for you (or do it yourself, if you have the proper equipment and skills.)
  • If you are VERY comfortable with your soldering skills (or have someone close by who is), we can sell you a set of custom-made copper loops, which — when installed in place of the original loops in your duplexer — will
    Loop assemblies converted to “-005” loops by DuplexerRepair.com

    convert the set to a “-B” model. Note that we highly emphasize the soldering skills issue, here, because the precision trimmer capacitors used in these duplexers are extremely sensitive to heat — including too much heat during soldering. Those capacitors are expensive, and it’s very easy to destroy one or more during loop replacement if you’re not well-versed in soldering or using the wrong equipment and tools. Our DB4060 and DB4062

    Another view of loop enclosures which have been converted to -005 enclosures in the DuplexerRepair.com labs.

    replacement/conversion loops are made by hand right here in the lab. Depending on whether or not we have some already made up and ready in our inventory, it might take us a few days to custom fabricate the loops and ship them to you.

  • If you are interested in purchasing a set of replacement/conversion loops, sending us your loop enclosures for conversion, or bringing/sending your DB4060 or DB4062 duplexer set to us for conversion, just contact DuplexerRepair.com to make arrangements or to get more information.

“How do I know if my set actually needs conversion to a ‘-B’ model?”

First, if your duplexer set has anything other than a “-B” in the model designator, it definitely is not a set which was originally built for 2-Meter ham use. If you are still not sure, there are two dead giveaways that your set needs conversion:

  1. The cables BETWEEN the low pass cavities should be 10-1/2″ long (measured tip-to-tip of the N-connector center pins.) All other cables in the set (between the high pass cavities, and on both sides of the antenna TEE-connector to both branches) should measure exactly 10″ in length.
  2. The loop assemblies on the LOW PASS side should be “-004” assemblies, and the HIGH PASS cavities should all have “-005” loop assemblies. The
    Factory “004” loop enclosure, identifiable by the penned “004” on the bottom of the housing’s mounting flange.

    three digit number denotes which length copper loops were installed at the factory, thereby determining the pass and reject frequency range for the cavity/loop assembly combination.

  3. Herein lies the catch: dbSpectra (formerly db Products) starting stamping the assembly numbers on the outside of the mounting flanges of the enclosures at some point, but almost every set we’ve seen here in the lab pre-date that practice. On non-stamped enclosures, you have to remove the cabling from the loop enclosure, remove the eight screws used to attach the enclosure to the top of the cavity, then look at the underside of the enclosure. They marked them at the factory (sometimes with a permanent “Sharpie” type marker, sometimes with a pencil, and sometimes by just scratching the number into the metal. You will find either the number “-003,” “-004,” or “-005” marked on it. In order for it to be a “-B” set of duplexers, it has to have -004 assemblies on the low pass cavities, and -005 assemblies on the high pass side. Anything other than that means the set definitely needs to be converted.

A little more information regarding our custom-made replacement/conversion loops

The custom-made replacement/conversion loops we make here at DuplexerRepair.com are painstakingly hand-made from high quality, 30-mil copper. They are measured and cut by hand, drilled, de-burred, shaped,

Custom-made replacement/conversion loops being ultrasonically cleaned prior to polishing and installation in the loop enclosures.

ultrasonically cleaned, and polished before installation in the loop enclosure. When we do the full loop assembly conversion job, we clearly mark the new designator on the underside of the flange, and typically label the outside of the enclosure as well with the conversion date and the new designation number. If we replaced the capacitors in the loop, we like to mark that on the outside as well, including the date the capacitor was replaced. Repeated failure of capacitors — especially in the same cavity position in a set — is a sign that something else in the repeater/transmitter system may be occurring which needs to be corrected.

 

Overhaul and Conversion of DB Products 4-Cavity Duplexers to 6-Cavity “B” Models

We’ve recently converted a couple of sets of Decibel Products (a/k/a/ dbProducts and dbSpectra) 4-Cavity 4060-WOC-C duplexers to 4062-WOC-B models. Translation:

4062-WOC-B Duplexer built by converting a new set of 4060-WOC-C cavities to “B” models, and overhauling and converting two additional “C” cavities from a failed set, resulting in a great working 6-cavity “B” model with over 100 dB of isolation in each branch.

we’ve converted 4-cavity sets which were built for operation at higher, public safety and commercial frequencies to 6-cavity sets which are optimized for operation in the 2-Meter Amateur Radio / ham frequency bands.

Doing so involves a fair amount of time and work, including total overhaul of each cavity, custom manufacturing of the correct length loops, capacitor replacements, and the building of a new frame for the expanded 6-cavity set, not to mention final tuning and performance testing.

Tuning Plunger Removal, Inspection, and Polishing

After disassembly of the old cavities, one of the many steps in the overhaul and conversion process involves getting the tuning plungers back in good

Aged, oxidized tuning plungers BEFORE cleaning and polishing.
The same two tuning plungers after careful cleaning and polishing.

shape. As can be seen in these photos showing the plungers before and after the application of some TLC, the difference is more than visible. So is the resulting performance and ability to be accurately tuned (and for that tuning to remain stable.)

Custom Copper Loop Fabrication and Loop Enclosure/Cavity Conversion

In order to obtain the best cavity SWR (lowest insertion loss) along with the maximum branch notching/isolation performance, the copper loops in one

Original capacitor with signs of excessive heating, old flux residue which had not been cleaned off after soldering, etc. This capacitor had failed, causing the set to be taken out of service.
New capacitor and custom made loop installed. The enclosure has been relabeled from the original “003” part number to its new, proper “005” designation.

branch of the duplexer assembly have to be replaced with loops of the correct length. At DuplexerRepair.com, we handcraft the replacement copper loops. We start with high quality, 30-mil copper and carefully cut, shape, drill, and polish the replacement loops.

During the installation of the new, replacement loops of proper dimensions for 2-Meter operation, we also replace the trimmer capacitors. This is actually a delicate process, as these capacitors do not tolerate excessive heat. It is quite common for us to discover signs of overheating from the combination of the original capacitor installation, soldering, and RF heating over time (mistuning, high SWR, and lightning will destroy these capacitors pretty easily.)

Frame and Mounting Rail Fabrication / Conversion

The original 4-cavity frame and mounting rails for assembling the cavities into a set have to be replaced in order to accommodate six cavities. We

Custom machined mounting and frame bars for conversion to 6-cavity set.
New custom made rails with cavity braces and clamps installed.

 

custom machine these from square aluminum tubing.

Reassembly, Cable Harness Inspection and Service, and Final Testing

With all the components overhauled and reassembled using the new frame and mounting rails, each cable, Tee-connector, etc. is inspected and cleaned

Another 6-cavity 4062-WOC-B set we recently converted, overhauled, and built, reassembled and ready for final tuning and testing.
Duplexer assembly being put through final tuning and performance testing and verification. The client gets a complete report of the testing, including the network analyzer graphs showing proof of performance.

or replaced as necessary.

Each cavity is then individually performance tested, followed by connecting each branch as a set and testing it, and finally the entire harness is secured and the entire set undergoes its final tuning and performance testing.

The End Results

A complete conversion, expansion, and overhaul job such as the ones described here, commonly involves between 15 to 20 hours of labor, plus

With 113.37 dB of isolation (factory spec is 100 dB or higher) with only 1.45 dB of IL (Insertion Loss (factory spec is 2.2 dB or less), this set is working considerably better than factory specifications. Well worth the investment of time, money, and energy.

materials. It’s not exactly cheap, but the results are worth it. What usually starts out as a 4-cavity 4060 “C” model (not built for factory spec operation in the 2-meter Amateur radio band, and often a set which has failed and been pulled from service) becomes a great working 4062 “B” model — as though it left the factory as a set intended to work to specs in the 2-Meter band. The 4-cavity 4060 models are rated at 80 dB or more of branch isolation; whereas the 4062 6-cavity sets are rated for 100 dB or higher isolation. At the typical 600 KHz “split” used in 2-Meter band, this extra isolation makes a world of difference, especially at transmitter power levels above 40 watts or so, and can be a game-changer when trying to get better performance out of certain repeaters, such as the Yaesu Fusion DR-1/DR-1X series, which tend to have lower receiver selectivity compared to most of the commercial grade repeaters with highly selective physical filtering on the front-end. Very often we deal with duplexers sent to the DuplexerRepair.com labs with complaints of “They worked great for years with our old Mastr II repeater running 40 Watts, but when we bought and installed a new Fusion repeater (or D-Star repeater, DMR  machine, etc.) everything went to crap.”

We deal with such all the time. And we’re here to help. Call or contact us if you’re experiencing similar problems. We’ll be delighted to help you get things working the way they should. As they say, “A chain is only as strong as its weakest link.” Duplexers which aren’t up to the task will result in a poor or totally useless repeater setup. It doesn’t have to be that way. We’re here to remedy that.

 

 

 

 

 

 

Forensic Inspection of Damaged Tower

Earlier this week, we had to go and do a little forensic inspection of a tower which was severely damaged by an electric company bushhog operator who was cutting and clearing the right-of-way. Here are a few photos of the damage and how the tower failed as a result of the incident.

This is how the tower looked when the owner went to investigate the loss of signal from his equipment.
This guy wire anchor and spreader had been struck by a utility company “bushhog.” It went unreported by the operator of the bush hog. The tower owner could tell that the site and utility right-of-way had just been cut, and he and the deputy finally made contact with the equipment operator who admitted he had struck the guy hardware, causing the tower collapse.
Top guy, which had been pulled with enough force to pull the upper part of the tower over.
One of the two tower legs which literally bent and folder over, in a hinge-like fashion.
The second of the two tower legs which bent and folder over.
This tower leg was opposite the side of the force of the top, front guy wire being pulled by the bushhog. As can be seen, the bolts remained intact in the mating leg, and the force was sufficient to cause the bolts to rip through the galvanized metal tower leg.

New 10 MHz GPS Disciplined Oscillator Lab Calibration Reference

Having accurately calibrated equipment here in the DuplexerRepair labs is a must, but sending instruments out to have them calibrated so that we know we’re calibrating and tuning your equipment accurately is very expensive. It also takes the equipment out of service if it has to be sent to an outside lab. Knowing that the ideal solution to keeping the equipment in service, accurately calibrated, and controlling costs would be to have an accurate in-house frequency standard, we decided to explore options such as calibrated rubidium oscillators, cesium driven units, etc.

Rubidium and cesium units have a few disadvantages. First is the cost. They can be very expensive, on the order of several $K for a new one. Second, they have to be re-calibrated periodically due to the natural aging of the oscillator devices. The ongoing cost and the uncertainty between calibrations makes them even less appealing. The solution we were looking for needed to be a combination of:

  • Accuracy
  • Reliability
  • Affordability
  • Availability

After much research, the decision was made to go with a GPS Disciplined Oscillator (GPSDO) system. Such a system, as the name implies, utilizes signals from the GPS satellites to precisely control a local oscillator. Each of the GPS satellites in operation are equipped with cesium and/or rubidium clocks and timing circuits. These are in turn all synchronized to the National Institute of Standards and Technology (NIST) cesium clock, which is considered the standard for time and oscillator frequency accuracy in the U.S. By knowing the maximum levels of uncertainty (error) of each device in this chain, it is possible to create a reference system for use in the lab which is traceable all the way back to the NIST. The maximum error in this chain is small — very, very small. We’re talking down to a fraction of a billionth of a second variability. When you convert that time error to a frequency error, it is equally small. It’s much smaller than the level of accuracy required for communications equipment and the manufacturer calibration standards for the equipment we use here in the lab to work on that equipment. The next question was whether to purchase a ready-built system or build it in-house. After much consideration, the decision was made to just put one together right here in the lab using readily available, affordable modules and devices, and design and build any additional components needed for the system.

Left to right in this photo, are the GPS module and buffer board (sitting atop the breadboard which was used to initially design the buffer board circuitry), the Arduino Uno module, and the LCD display module, all interconnected and functioning. All of these will be assembled into a rack-mount unit.

Our GPSDO system, as seen in the photo as it was being assembled and tested here on one of the lab benches, consists of essentially four subassemblies, three of which are off-the-shelf items, and one module/board which was created right here in the lab:

  • Ublox Neo-M7M GPS Receiver module
  • Arduino Uno module
  • 16×2 LCD Display module
  • Custom designed signal buffering/conditioning board

Once all of the off-the-shelf items were in hand, the task of designing and refining the special buffer board which takes the

Buffer Board schematic.

oscillator output from the GPS module (a square wave) and conditions and converts it to a usable sine wave at a very specific frequency took about a week (squeezing it in between other work.) This buffer board also provides impedance matching so that it can be connected to the instruments in the lab as a frequency standard, phase synchronization device, and calibration reference for radio equipment, frequency counters, and other instruments. Our final working prototype is not exactly the most aesthetically attractive looking thing in the world, but it sure does work well. How well? A good example can be seen in the following photograph showing that the IFR (running on the GPSDO system as a 10 MHz reference oscillator) indicates that WWV’s 15 MHz signal to be 15.000000 MHz with an error of 0.0 Hz.

The carrier frequency of WWV’s 15 MHz transmission out of Boulder, CO, as read by the IFR 1600 with our GPSDO connected as its frequency/oscillator reference.

Since we can only see one decimal place beyond the Hz digit in this example, the fact that the 1/10 of a Hz digit is rounding to the digit “0” means the value the IFR is coming up with must be in the range of -0.05 and +0.04 Hz, which equals a calculated maximum error of +/- 0.05 Hz. In the case of our 15 MHz frequency (15,000,000 Hz) that’s equivalent to an error of  0.00333 PPM (or 3.33 PPB, depending on which reference you prefer.) That is way better than the level of accuracy required for communications work. As an example, the FCC says that in land mobile radio equipment, the permissible carrier frequency error for a fixed transmitter operation in the range of 50-450 MHz is 5.0 PPM. Do a little math and you’ll see that our GPSDO has the IFR operating at roughly 1,500 times the level of accuracy required by the FCC rules. Sweet! Even sweeter is the fact that — unlike a rubidium frequency standard which has to be sent off for calibration once a year or so — our GPSDO is essentially updating its own calibration several times a second by constantly receiving the GPS signals and synchronizing itself. With an active gain GPS antenna attached, it rarely indicates being locked on less than 10 satellites simultaneously. It only takes four satellites to achieve a super-accurate “lock” but ours typically registers being locked on 11 and even 12 satellites. Did we mention that is with the antenna still inside the lab? Although it appears quite unnecessary, in the interest of maintaining the best lock and avoiding any potential interference to the GPS performance from equipment running here in the lab, we’re going to be installing an active, weatherproof, marine grade GPS antenna in an elevated position outside the lab. Another benefit of this GPSDO system is the fact it can achieve an accurate frequency lock in less than three seconds of power-up; whereas, rubidium standards employ temperature compensated ovens and typically have to be powered up for several hours for their oscillators to warm up before they stabilize and can be used as a reliable lab reference.

The icing on the cake in this recipe is the affordability of the project. Here’s a rough breakdown of the cost of creating this:

$15.30 — uBlox Neo-M7M GPS Module
$19.48 — Active GPS antenna and necessary adapters and cables
$5.45 — Arduino Uno (“knock-off”/compatible 3rd party clone)
$8.79 — 16×2 LCD Display with I2C interface module
>$10.00 — estimated cost of perf board, electronic components, etc. to build prototype “buffer board”
——————————————————
$59.02 — estimated final cost

That’s less than the one-way shipping cost of sending just the IFR analyzer off for calibration (and don’t even ask the actual cost of the calibration service — it’s astronomical.) With a hyper-accurate 0.00333 PPM frequency reference in-house, that’s exactly where the calibration work on our instruments will be done if/when needed.

PCB layout design for the version 1.2 GPSDO Buffer Board.
3D rendering of the v1.2 Buffer Board.
3D rendering of the reverse side of the v1.2 Buffer Board.

A rework of the schematic and accompanying PCB layout have now been completed and we’ll soon be fabricating a nicely laid-out version 1.2 of the  Buffer Board. Once we’ve fully assembled and tested that prototype board, we’re considering having several of the boards manufactured so that others who are interested in putting together a similar system can simply purchase one of our buffer boards and the parts to populate it (which we might offer in a kit form.) Estimated time to solder all the components is roughly 30 minutes to an hour, depending on how well versed you are with a soldering iron.) There might be a version 2.0 board in the works as well — using SMD components to further reduce the cost, board size, power consumption, and to reduce the amount of RF emission.

Stay tuned! (Pun only slightly intended.)

Thank you, New Florida Clients!

We’re seeing a spike in equipment coming in from Florida for tuning and repairs. The majority of it is “word-of-mouth” referrals, with some manufacturer referrals in there as well. Thank you all! It is indeed a pleasure to be of service to all of you and to provide you with top-notch repairs, tuning, and customer service.

Do Your Duplexer Cavities Look Like This Internally?

This short video shows what years of oxidation can do to the internal workings of a set of duplexers. This is the inside of one of the tuning cylinders in a set of EMR 65544 UHF cavities. These cavities are known for

great performance; however, time takes its toll on anything metal and RF duplexers are no exception. These duplexers would not tune for proper pass or notch frequencies. This is what an endoscopic inspection revealed — lots of corrosion and contamination. Along with damaged trimmer capacitors, these sort of problems often lead to duplexers being replaced with new ones at tremendous expense. A replacement set of these duplexers lists for $1,710. At DuplexerRepair.com this refurbishing job would run somewhere in the range of $250-$400… less than 1/4 the cost of buying replacements, and the duplexers will be ready for many more years of service when the job is done.

If your duplexers no longer perform correctly, contact us about having them repaired and refurbished. Call (334) 787-9005 or send an email to service@duplexerrepair.com to arrange repairs, overhaul, or retuning services.

Trimmer Capacitor Failures in Decibel Products Duplexers

This post could appropriately be subtitled “The Costs of Doing Good RF Business.” I’m a fan of Decibel Products duplexers because they do an excellent job in repeater applications. But like any piece of hardware, they are not immune to damage or failures. Many of Decibel Products’ duplexers (most notably the DB4060 and DB4062 VHF models) utilize precision trimmer capacitors which are the sort of component you love to hate.

Capacitor removed from service due to failure. Discoloration can be seen on the white ceramic portion which resulted from excessive heat.
Capacitor shown with retaining nut and protective cap. This capacitor required replacement due to failure. Discoloration can be seen on the white ceramic portion which resulted from excessive heat.

On the plus side, the capacitors (commonly referred to as “Johanson capacitors, after the company which manufactured the bulk of the caps used) are known for the ability to be very precisely tuned, largely due to the fact that it takes around 20 turns of the capacitor’s adjusting screw to go from one end of its capacitance range to the other. Compare this to many open plate type capacitors which go through their full capacitance range in only 1/2 turn and it’s obvious that very accurate and precise tuning is much easier using the Johanson type capacitor; however, utilizing them in a duplexer cavity also makes the cavity a bit more prone to fail under certain conditions. That’s the trade-off involved.

The very design elements which make the Johanson capacitors so great also make them much more fragile and more easily damaged. First of all, they are very small. The diameter of the outer ceramic cylinder of these capacitors is only 0.244” (6.20 mm to be more precise.) The component is hermetically sealed from the outside environment, which is good, because moisture and dust are enemies to such devices. At the same time, the very small enclosure size means that any heat in the device pretty much STAYS in the device until it dissipates through by conductive and convective methods — not always a fast process.

Capacitor disassembled to show the concentric, cylindrical “plates,” which mesh with free space between them, thus forming a capacitor.

The actual capacitance creating part of these capacitors is a combination of concentric cylinders. One set of concentric cylinders is permanent attached to a stationary end inside the ceramic cylinder. A second set of cylinders is attached to the adjusting/tuning screw which has very, very fine pitched threads, giving the capacitor highly precise tuning ability because it takes so many turns to adjust it from one end of its range to the opposite end. As the screw is adjusted, this second set of metal cylinders moves in and out of the stationary set. If you’re familiar with electronics theory, you’ll recall that the two ways of varying the value of a capacitor (without altering the material used) is to either increase/decrease the size of the plates, or alter the spacing between the plates. A sort of combination of both variables has been accomplished in open-plate capacitors for ages by physically rotating one set of plates in and out of a second set in a horizontal fashion. This is the type of variable capacitor we see commonly used in everything from tiny transistor radios to the large open-plate variable capacitors used in high wattage antenna tuners/matchers. The difference in the Johanson capacitor’s operation is that they accomplish the same thing by changing how deeply the sets of concentric metal cylinders are meshed. They simply move in or out in a linear fashion.

The largest of the metal capacitor plate cylinders measures approximately 0.193″ (4.90 mm) outside diameter. Keep in mind that’s the LARGEST of the cylinders, which get increasing smaller in size, and there are seven total cylinders between the two sets. The smallest of the capacitive plate cylinders is approximately 0.063″ (1.60 mm) outside diameter. Metal objects this small can’t handle much heat, for starters. As those of us in the electronics and communications world all know, heat is not our friend. At the least, heat causes metals to expand. Given the miniscule size of these cylindrical capacitor plates, it wouldn’t take much heat to totally alter their size — thus the effective spacing between them — thereby changing the capacitance. These capacitors are used in the duplexer cavities to tune the rejection notch; therefore, if the capacitance changes, the notch tuning changes. If the notch tuning changes, the pass/reject performance of the cavity changes. In short, excessive heat buildup in the capacitor will cause the cavity and the duplexer set to fall below designed performance specifications. This obviously is not what we want happening.

Pitting and burn marks were visible on microscopic inspection of the cylindrical plates inside this failed capacitor.
Pitting and burn marks were visible on microscopic inspection of the cylindrical plates inside this failed capacitor.

What would cause excessive heat buildup? A number of factors contribute to heat in the capacitors. First, RF energy by its very nature produces some amount of heat. That’s unavoidable. Many of the repeaters built or serviced by DuplexerRepair.com operate upwards of 100-125 Watts maximum. The higher the power from the repeater’s power amplifier, the more heat is generated in the cabling, duplexer cavities, and most importantly in the capacitors and other “loop” components. While RF energy that’s flowing through the devices is just part of the process, there are other causes which aren’t part of normal repeater operation which can be downright catastrophic, especially to these fragile capacitors we’re discussing.

High SWR in the antenna and feedline system can cause abnormal amounts of RF energy that’s supposed to be leaving the antenna in the form of electromagnetic waves to actually reflect back down the feedline to the duplexers. Since these capacitors are intended to help block the unwanted frequency from getting through by essentially creating a short-circuit to ground (the cavity cylinder itself is grounded) any increase in RF through the capacitor means increased heat. If the heat becomes great enough, the tiny capacitor cylinders themselves can expand to the point that they make physical contact between cylinders, essentially creating an RF short-circuit to ALL RF energy (meaning the frequency that’s supposed to be getting blocked along with everything else.) This short-circuit only leads to more and more heat, and it is commonly a cause of power amplifier/transmitter damage and failures as well. The end result is a catastrophic failure. If your antenna is mistuned or has problems leading to increased SWR, your duplexers are more likely to fail because of damage to these capacitors. Likewise, feedline problems such as physical damage (or even poor grade cabling/feedline) can cause high SWR and eventual failure. Lastly, mistuning of a cavity — even relatively slight — means needlessly increased RF energy is hitting places and components where you don’t want it to. Tick, tock… it’s just a matter of time until trouble occurs.

There’s also our other enemy in the radio world: lightning. Let’s face it, there’s not much need for technical explanation of the effects of lightning here, because we know it’s just downright destructive to communications equipment, and if it directly strikes an antenna (or even in close proximity) even the highest quality lightning protection devices can only do so much. If a hundred or so watts of misdirected energy can damage one of these capacitors, just imagine what a few megawatts from a lightning strike will do in terms of destructive effects. Enough said about that.

Micron-sized dust particles and/or moisture incursion into these caps will cause them to fail prematurely.
Micron-sized dust particles and/or moisture incursion into these caps will cause them to fail prematurely.

Dust particles and contaminants are theoretically kept out of these capacitors by sealing the plate cylinders inside the capacitor’s housing. The key word here is “theoretically.” Nothing is perfect. It only takes a few micron-sized particles of moisture or dust getting inside one of these capacitors to create a failure scenario. Moisture causes corrosion. Corrosion on the capacitive cylinders not only alters the capacitance but also creates potential conduction paths for electrical current which lead to RF short circuits. Dust particles similarly create unwanted conduction paths. Bottom line: we want to minimize any risk of contamination by moisture, dust, or other contaminant as much as

A Johanson-type precision trimmer capacitor properly dressed for work, with its cap in place.
A Johanson-type precision trimmer capacitor properly dressed for work, with its cap in place.

possible. They aren’t healthy for radio equipment, duplexer cavities, or these capacitors in particular. These capacitors come with protective screw-on sealing caps. They should be kept on the capacitors and fairly firmly tightened at all times, the only exception being while tuning is being performed on the duplexer assembly. Otherwise, moisture and micron sized particles of dust, etc. can and will eventually make their way between the tiny threads and into the capacitor, causing an eventual failure. Likewise, although it should not need to be pointed out: proper control of the ambient environment where the duplexers are installed is worth all the effort you put into it.

If this adjusting screw feels too easy or too difficult to turn, it is advisable to go ahead and get it replaced.
If this adjusting screw feels too easy or too difficult to turn, replacement of the capacitor is most likely in order.

Something to watch for (or perhaps we should say “feel for”) if you’re tuning a set of duplexers which use these caps: very loose or tight feeling threads when turning the adjustment screw/slug. The screw should offer some/moderate resistance to being rotated for tuning; however, if it seems to spin pretty freely or feels as though it takes a lot of torque to turn it, the capacitor needs to be replaced. A loose turning adjustment screw usually means the capacitor is cracked (we’ve seen them broken completely into two halves, totally separated physically.) A tuning screw that feels excessively bound/tight typically means the capacitor has significant heat/lightning damage. In either case, you’ll only prolong the inevitable and end up “chasing your tail” trying to make the duplexers work properly and stay properly tuned and functioning. More often than not, Decibel Products duplexers we see in use by Amateur Radio folks are surplus/repurposed sets which had been used in commercial or public safety communications prior to being pulled out of service, eventually ending up in the hands of ham radio folks. We often suspect that the reason they were pulled from prior commercial/public safety use was due to… wait for it… failing capacitors which led to a two-way shop making the recommendation to just replace the whole set with new ones due to time constraints, and possibly because the radio shop can usually make more money selling a new set than they can by repairing the failed set. Plus, they often have to install another set immediately in order to get the system back in operation, so instead of the system being down twice it often makes more sense in those circles to just install new ones immediately. All that said, there’s probably a reason some ham got hold of the things for considerably less than the going price for new ones, and that reason is often undisclosed, unknown, and undetectable until attempts are made to actually put them to use in a repeater system. For those who have never done in-depth duplexer work, it’s worth mentioning that duplexers can often seem to work great when you’re testing and tuning them at power levels down in milliwatt/microvolt range but prove to be total garbage when you hit them with the sort of power levels coming out of a repeater’s transmitter. Experienced duplexer repair techs are better versed in things to look for which could spell trouble for duplexers in the real radio world.

There is another issue which can contribute to failure of these capacitors which should be pointed out: improper installation of the capacitors in the duplexer cavities. The capacitors most commonly used in the design of Decibel Products duplexers are attached to the cavity loop assembly by means of a stud/pin on the end of the capacitor which is soldered to the copper resonating loop. The opposing end is attached to the loop enclosure by means of a retaining nut. Because they are very easily damaged by heat, it’s extremely critical that the temperature and soldering time be precisely controlled. Not enough heat when soldering the capacitor in place will result in a poor solder connection (cold solder joint) which will have added resistance (heat buildup from RF current) or can open entirely, causing the cavity to fail. Too much heat during soldering will either destroy the capacitor immediately (and these things are not cheap) or do damage which will shorten the life of the capacitor tremendously. Proper installation of the retaining nut on the end of the capacitor which protrudes through the loop enclosure is also important. Too tight and the capacitor can be deformed and damaged. Too loose and the contact (conduction path) can be poor, plus the entire capacitor assembly may try to spin during adjusting, which can in turn break the capacitor assembly (if the solder joint at the other end is nice and strong) or torque the loop, resulting in deformation, pass frequency shifting, and a loop which might try to slowly move back into  its proper shape (usually well after you’ve reinstalled the duplexers and left the repeater site) causing everything to go haywire — which Mr. Murphy delights in seeing happen at the most inopportune times. Decibel Products typically painted the loop enclosures AFTER installing the components including these capacitors. Some people complain about that making it more difficult to remove that tiny, ultra-fine threaded retaining nut. Don’t be a hater: by doing so they actually reduced the chances of the nut working loose and allowing things to move around which should be doing so. Kudos to Decibel for that decision. It’s not uncommon for the retaining nuts to bend, break, and generally become unrecognizable (not to mention unusable) when they have to be removed. That’s okay, because we only remove them to repair a loop enclosure (or modify it for operation at a different frequency range than it was originally manufactured for by fabricating replacement loops of the proper length for the desired frequency range.) If we break the nut, we’ll replace it, and it’s not uncommon for the entire capacitor to already be in need of replacement during the conversion. If we see signs of heat damage (such as discoloration on the ceramic enclosure) the capacitor will be replaced. At DuplexerRepair.com we are extremely careful and diligent when inspecting and installing these capacitors because we want you to get the longest possible life and best performance out of any equipment we repair or recondition. This in turn helps to minimize the risk of damage/failure of the other equipment in your communications system, such as your receivers, exciters, and power amplifiers.

By now you probably know more than you ever imagined you would (or wanted to) about the precision capacitors used in Decibel Products duplexer cavities. We just thought you deserved to be well informed of the pros, cons, and factors you should take into consideration regarding certain Decibel Products duplexers because of the capacitors used. We do highly recommend these duplexers because their design is excellent, their performance is fantastic, and the lifespan of Decibel Products duplexers between repairs/overhauls is wonderful, provided they — and the rest of your communications system — are installed and maintained properly. While we can’t control when and where lightning strikes will occur and cause damage to equipment, we can reduce the chances of system failures through proper attention to the details we do have some control over.

If you are the owner of a set of these Decibel Products duplexers and you need tuning, repair, conversion, or complete overhaul/refurbishing of a set, get in touch with us by calling (334) 787-9005 or by emailing service@duplexerrepair.com. We’ll have them in proper working order and back to you as quickly as possible (usually within 2-3 days, although sometimes it takes a bit longer, especially if parts/cables have to be fabricated or special-ordered.)