Available for Purchase Soon – Reconditioned DB Products 2M 4-Cavity Duplexer

  • 2-Meter, 4-Cavity Decibel Products Duplexer Set To Be Available for Purchase Soon!

We are about to do the conversion of a 4-cavity set of Decibel Products DB4060-WC-C (154-174 MHz) duplexers into a DB4060-WC-B model (143-156 MHz) ideal for use on 2-Meter Amateur Radio repeaters. They are already disassembled down to every single nut, screw, washer, etc. and will also be undergoing a full refurbishing and performance testing. This set of duplexers will be for sale for only $400, which will include precise tuning to the buyer’s frequency pair, along with a full performance verification report showing their performance as tuned and ready for the buyer to install. They won’t sit around or collect any dust when they’re done — reconditioned and performance-verified duplexers from www.DuplexerRepair.com stay in high demand. If you or your club are interested in this set, we recommend you contact us as soon as possible. We will not ship these, so the buyer must arrange to pick them up in the Auburn/Opelika area. You can contact us at service(at_sign)duplexerrepair.com, or by telephone at (334) four-four-four-6796.

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 logoDuplexerRepair.com 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.

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.

Yaesu Fusion vs. G.E. Mastr II — Selectivity Causes Huge Differences In Desensing

Okay, so your old Mastr II was pumping out 50, 75, maybe even 100 Watts with no problem using your trusty old duplexers, but then you installed a shiny new Yaesu Fusion and the proverbial “stuff” hit the fan. Desense galore. Poor sensitivity. User complaints about not being able to carry on a QSO like before — all other factors being equal. You’ve even tried backing the Fusion’s transmit power down to half or less of what you’d been running the old Mastr II’s PA at (or maybe it’s a Motorola) but you still feel as though you’re chasing your tail. I’ve received countless calls, messages, emails, etc. asking for help and advice with this problem. I have known since my very first encounter with a Fusion repeater essentially what the problem was — and various solutions — but today I actually went to the effort to do in-depth technical testing and put some real numbers with it. In all honesty, it’s a bit worse than I had estimated.

The problem is one of “selectivity” — not to be confused with “sensitivity.” Two totally different beasts. My testing does show the sensitivity of the Yausu Fusion to be pretty close to that of the Mastr II and other commercial grade repeater equipment. Where it can’t compare is in selectivity — the ability of the receiver to effectively hear what it’s supposed to while suppressing and not getting swamped by other signals, including (especially) its own transmit frequency. Because the Fusion is much less selective (just how much so you’ll see shortly) you can’t get the same performance out of the Fusion that you were accustomed to with a Mastr II, Motorola, or other truly commercial grade repeater if you try to use the same duplexers. High RF environments such as broadcast transmitter sites or sites with multiple repeaters in operation also become a problem at times with the Fusion; whereas, the old Mastr II and other systems seemed to work okay.

There are a combination of factors which make the receiver in one repeater more or less selective than others. I’ll spare you all the discussions of “Q” and how amplifiers, transistors, etc. actually lose gain at the desired receiver frequency when there’s a lot of RF on frequencies close to it (such as just 600 KHz or so away — typical of ham repeater in the 2-Meter band), but we can’t avoid mentioning it altogether. True, your duplexers help reduce much of the unwanted signals, but they never block 100% of it. The less selective the repeater’s receiver is, the more isolation between the receive and transmit frequencies is needed in the duplexers. By contrast, higher selectivity in the receiver means you can get away with lower “dB” isolation in the duplexers. For example, a Mastr II might work fine at 50 Watts on a high quality set of 4-cavity duplexers, or at 100 Watts on a 6-Cavity set, but the Fusion often takes repeater owners on a not-so-fun journey into desense land at 50 Watts (sometimes 25 Watts) on the same 4-cavity set that the Mastr II had played nicely with in the past. My testing this morning shows how much so.

In order to conduct side-by-side, “apples to apples” testing for comparison purposes, here’s how I set everything up and conducted the tests.

Lab testing setup used for measuring comparative selectivity/desense levels of the Yaesu Fusion DR-1 repeater and a G.E. Mastr II repeater. Tests conducted in VHF/2-Meter band only.

With the setup above, I sent a signal from the IFR-1600S to a combiner, the output of which was in turn routed to the receiver port of the repeater being tested. With the repeater activated, the transmitter output was sent to a 100 Watt dummy load with a reduced level sampling port. The output from that sampling port was then fed to a precision attenuator capable of 1-101 dB attenuation in 1 dB steps, the output of which was combined with the IFR’s output at the second input port of the combiner. This setup allowed me to combine a precisely adjustable amount of the transmitter’s output signal with the desired receive signal. By slowly decreasing the amount of attenuation of the transmit signal allowed into the combiner, I was able to determine how much RF power getting back into the repeater’s receiver would cause desensing. I kept adjusting until the level of the transmit signal going back into the combiner was high enough to cause total desensing of the receiver, meaning that the receiver was unable to discern enough receive signal to even do “cyclic” type key/unkey/key/unkey repeated desensing. The levels I created and measured resulted in total loss of useable reception in the receiver, effectively deafening and shutting down the repeater’s functionality altogether.

It is important to note that this testing does not address the levels at which “white noise” from the transmitter starts causing voice/modulation quality degradation in the repeater’s output. “White noise” and transmitted voice/modulation quality actually start occurring at much lower levels of unwanted transmit RF getting back into the receiver. Such noise gets progressively worse as the level of unwanted signal creeping back into the receiver increases, until it reaches the total desense level. Thus, the amounts of transmit/receive isolation needed (discussed a bit later in this article) in order to have clear, enjoyable QSO’s is actually considerably higher than the levels of rejection/isolation necessary just to avoid total desensing. Don’t use the isolation numbers shown in a bit to be the duplexer performance level you need for your repeater. Rather, the levels discussed in this article allow side-by-side comparison of the selectivity — or ability of each repeater’s receiver to block troublesome, unwanted signals and actually “pick out” and properly demodulate the desired signal.

For testing purposes, I used the following signals:

  • Rx signal for the repeater: 147.660 MHz, modulated with a 1 KHz audible sine wave tone modulated at 3.0 KHz deviation, and a 123.0 Hz CTCSS tone modulated at a 0.6 KHz level for repeater PL tone operation. Signal level from IFR-1600S set to 0.224 uV for testing of both repeaters.
  • Tx signal from repeater: 147.060 MHz with no tone generation being added by the repeater.

I chose these particular frequencies and tones for a simple reason: I had recently done a Mastr II repeater conversion for a local amateur radio club using that frequency pair and tones but it was still here in the lab, so a well functioning, properly aligned Mastr II set up as such was readily available. The Yaesu Fusion — being fully programmable and agile — was thus easy to set for identical frequencies and CTCSS operation.

Once I fired up each of the repeaters and had adjusted the level of the transmit signal being intentionally looped back into the receiver so that total desense occurred, I simply disconnected the feedback line from the combiner, which allowed the repeater to start repeating again. The level of the transmit signal coming from the test port of the dummy load and going into the precision attenuator had been measured just before conducting each desense level test. By calculating the resulting level after attenuation and factoring back in the amount of loss presented by the combiner,  I was able to calculate the power level (calculated in uW (microwatts) in this instance) and record it. After putting both repeaters through this same test, I had the desired numbers at hand. I already knew that the differences between the Mastr II and Fusion would be noticeable, but they turned out to be a bit more so than I had suspected.

The Fusion repeater would totally desense with unwanted transmitter signal power of 4.77 uW. Contrast that to 80.8 uW that it took for the G.E Mastr II to reach total desense and a lot of things start becoming clearer. This table lists what I’ll refer to as the “Critical Rejection Level” — the amount of transmitter/receiver isolation necessary to prevent total desensing of the receiver for each

Calculated Critical Rejection Levels for comparing the selectivity and desense avoidance capabilities of the Yaesu Fusion DR-1 and the G.E. Mastr II operating in the VHF/2-Meter band.
Calculated Critical Rejection Levels for comparing the selectivity and desense avoidance capabilities of the Yaesu Fusion DR-1 and the G.E. Mastr II operating in the VHF/2-Meter band.

repeater. The table shows this estimated value calculated for operating each repeater at four power levels: 5, 25, 50, and 100 Watts. Please keep in mind, as mentioned before, that these are not intended to be absolute values to use for specifying duplexer parameters, etc. They are, however, numbers useful for comparison of the selectivity and desense avoidance capabilities of the Fusion and Mastr II. In my overall experience, the amount of transmitter/receiver isolation necessary to actually have relatively white noise-free, clear QSO’s in a repeater are generally 30-40 dB or so greater than what I refer to as the Critical Rejection Level. Shooting the middle and adding 35 dB to the numbers in this table yields numbers very comparable to the actual values of duplexer Tx/Rx isolation I’ve found necessary in real-world practice. I have found that the G.E. Mastr II VHF repeater will typically operate well in a relatively RF-clean environment with 80 dB or greater Tx/Rx isolation up to around 40 Watts, and do well up to 100-125 Watts with at least 100 dB of isolation. By contrast, the Yaesu Fusion is best operated with at least 80 dB of duplexer Tx/Rx isolation at 25 Watts or less, and definitely needs 100 dB or better in order to operate at its 50 Watt transmitter power level or with an outboard power amplifier running between 50 and 100 Watts (again, in a relatively CLEAN RF environment.) Also, these numbers relate only to transmitter/receiver isolation. Operating a repeater in a high-RF environment such as a commercial/broadcast transmitter site will create the need for specialized filtering (such as pre-filters), increased duplexer isolation rating needs, and frequently create intermodulation problems that call for lots of creativity and sometimes end up being impractical to try to overcome. I also have not done extensive testing to see whether or not the Fusion’s transmitter final/PA output is as good as the Mastr II in terms of any harmonics, spurious artifact, etc. in its “Fixed Analog FM” output mode. I suspect that it isn’t, but that’s research for yet another day. For now, suffice it to say that if it’s not, then things get even worse, but for the moment let’s at least imagine that the Fusion and the Mastr II have exactly the same output signal quality.

I am considering repeating these tests using the Fusion in UHF/70CM band and a Mastr II running at the same frequency/band. Almost all complaints I’ve heard regarding the Fusion having desense problems have been in the 2-Meter band, so that was my focus for today’s testing and documentation purposes.

I hope that this information sheds some light on why Fusion repeaters installed as replacements for Mastr II repeaters (and other commercial grade repeaters, such as Motorola equipment) so often create frustration and sleepless nights for so many repeater operators. My hat is off to the folks at Yaesu for creating some exciting, feature-rich repeaters in a price range that put them within the reach of many repeater owners who thought they’d never own a brand-spanking new repeater. Anyone considering purchasing a Fusion just needs to keep in mind that Fusion repeaters are ultimately ham/consumer level equipment and understand that they will not perform as well for analog communications as will a Mastr II or other commercial/professional grade repeater. Installing a Fusion using the same antenna, duplexers, connectors, and other hardware that was previously used with a properly working and aligned commercial grade repeater can and will quickly lead to great frustration unless you’re prepared to make a combination of changes and/or concessions.

73 Everyone!