Demo of Custom Built Arduino-Based RF Power & SWR Monitoring/Alarm System

This video demonstration shows the custom-built, Arduino-based RF Power & VSWR Monitoring/Alarm system which I recently built as part of my engineering work for a client who owns a multi-site paging company and uses Glenayre and Quintron paging systems. They are great systems — built like tanks and extremely reliable. We had a few of their Power Monitor panels sitting around, but despite much searching, nobody seems to have the documentation, schematics, wiring diagrams, etc. on these power monitors, which haven’t been manufactured in many years. To wit, I decided to just “gut” the thing and use the chassis and the old meter bezel/lens and build one that does everything I wanted it to. While this project was originally intended with the client’s Quintron and Glenayre paging transmitters in mind, the monitor/alarm system is just as usable in other transmitter systems or repeaters — it’s really just a matter of having an RF power sensor of the right type (proper operating frequency range and output voltage range.)

Starting with an Arduino Uno R3 development board, a dbProducts power sensor, a 16×2 LCD display, I went to work building this RF Power & SWR

Monitor/Alarm system, which constantly measures the transmitter’s forward and reflected power in Watts, and uses an algorithm to precisely calculate the VSWR, all of which is in turn displayed on the front panel.

In the event of a high SWR event, the LCD display shows the actual VSWR and an audible alarm is activated. In addition, an SWR alarm LED turns on, which remains lit until a momentary contact button is pressed, resetting the alarm condition. I wrote the Arduino IDE sketch code to “latch” the LED so that in the event of a temporary high SWR event (ice on the antenna, for example) the system can continue to update and display real-time power and SWR readings but there will be a visible indication on the front panel that a temporary high SWR issue occurred.

The above video shows the system running my v2.4 Arduino sketch code. There will be future updates to the code and the hardware, as I intend to add features such as temperature monitoring, remote access/monitoring, automatic alerting of alarm conditions via RF signaling and/or internet linking, etc. I most likely will end up designing and building a custom Arduino “shield” for a much neater, quicker, easier installation and deployment. The main thing is that the first working version of this monitoring/alarm system is now built, installed, operational, and doing everything I originally intended to do.

This initial build has a few cosmetic imperfections, but hey… in the future I’ll use a better grade of paint when “masking off” all but the desired portion of the original lens/bezel assembly to accommodate the actual LCD display size and repainting the front panel to do away with the original labeling, etc. Until then, it does the job, so who cares if it doesn’t look all spit-polished like it was made in a factory (with a price tag to prove it) ?!?!?!

Much Ado About Isolators – Demonstration and Testing of db Products DB4613-1A

I often get asked “Do I really need to spend the money to purchase an RF isolator and have it tuned for installation on my repeater or transmitter? And, if so, why?” In this video, I demonstrate (and test) a db Products (Decibel Products) DB4613-1A isolator on a 100 Watt paging system, showing how the isolator protects the P.A. from high SWR (even full reflected power) in the event of broken, damaged, shorted, or disconnected feedline, damaged or iced-over antenna, etc. Just take five or six minutes to watch this video and you’ll fully see for yourself why an isolator is an excellent investment for your system. provides professional, precise tuning and testing of isolators, with quick turnaround and full testing — including a custom report showing the tuning and performance of your isolator when it’s finished.

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 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, 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 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.







Update On the “Uncle Wobbly” Tower

We have been monitoring the status of a tower in a community south of Opelika which was supposed to have been taken down a while back. This

The curving of the tower in this photo is not camera distortion. It really is bowing and bending as bad as it looks.

tower has been (not so affectionately) nicknamed “Uncle Wobbly” because you can watch it precariously sway with the wind. It is also bowing and leaning very badly. All of this is because it only has three of the original guy wires still intact, and only two of those are actually supporting the tower. The guy wires are old and extremely rusted and six of the original nine guys have broken. Two guys remain on the back side of the tower at approximately the 60′ level, and one on the front at approximately the  80′ level.

The guy wire at the front (from the vantage point of the road — and utility lines — is the only top guy remaining, and it’s slack. That “slackness” is actually a good thing in this instance, because with the other two top guys missing, one can venture a pretty accurate guess as to what would happen if someone put much tension on that top/front guy.

Two of the three front guys have been broken for quite some time, and you can see the extremely rusted condition of the guy wires in this photo.

This tower is only about 50 feet from the utility lines running parallel to the road. That has created a seriously hazardous situation. The tower is 110′ tall (including appurtanances.) Obviously, if it falls in the direction it is leaning, there is a high likelihood it will fall across the utility lines and the roadway. This decommissioned tower was slated for dismantling in the Fall of 2017 according to the tower owners, but that work has not yet been done.

We will not be the least bit surprised to get a call at any moment letting us know the tower has completely failed. We also would not be surprised to hear that it damaged the utility lines (and quite possibly the utility pole nearest the tower) and caused interruption of utilities for a lot of folks in the area.

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.

DuplexerRepair Is A Proud Supporter of AFCRAS

AFCRAS is proud to support the Alabama Frequency Coordination and Repeater Advancement Society (AFCRAS), Alabama Amateur Radio’s new ham radio frequency / repeater coordinating entity. AFCRAS was formed by a group of Alabama hams determined to provide the Alabama Amateur Radio enthusiast community the quality and level of service they have been wanting for many years.

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.

New: Service Photos & Documentation Page!

DuplexerRepair will now be creating pages here on the website dedicated to communicating and documenting services and repairs performed. While not all service and repairs will have a special page, you can check the list on the Service Photos & Docs page to see if yours has its own page. If you receive an email with a link to online documentation of work performed for you, that link will take you directly to the page dedicated to your specific equipment or system.

New “Flat Rate” Duplexer & Cavity Tuning Pricing Just Added To Our Website

We naturally get lots of telephone calls and emails asking how much we charge to tune duplexers and filters. In the interest of saving you time and getting  your equipment in, tuned, and back to you even faster we have a new page on our website with “flat rate” pricing so you can quickly determine what the fees will be. Note that these prices apply only to equipment which arrives in our lab in good working order with all necessary cables, connectors, adapters, hardware, etc. attached. Just click on this link to see view our Flat Rate Tuning Service Price List. Quick turnaround. Precision work. Excellent customer service. All at prices quite friendly to your equipment and maintenance budget.