Duplexer Construction – Part 2 (The Prototype)


Prototype cavity, just before resonator stub installation

The prototype cavity, just before installation of the resonator stub.

During the late part of winter 2012, I spent a great deal of time reading up on duplexer theory and construction.  I really wanted to understand what I was doing, not just blindly follow a recipe to build something.  The more I read, the more I believed it would be possible to build a properly functioning unit using parts from the former TV diplexer and sideband filter.  If the telescoping resonator stub sections could tune low enough, or be modified to tune low enough, then we should be in good shape.

By this time, I had brought John (WB2DVE) on board to help with machining tasks needed for duplexer construction.  His first assignment had three parts: 1) Coupling loops – create discs with a BNC and an N connector mounted on them; 2) Top plate – holes with shoulders to receive the coupling loop discs; 3) Inside partition – cut two pieces to length to make one complete partition.  A few holes would need to be drilled in the outside panels to match the nuts on the partition; I decided to do this on-site with a handheld drill. 

The prototype coupling loop.

The prototype coupling loop, made with thick strap scavenged from the TV diplexer.

I also used some strap pieces removed from the diplexer to create coupling loops.  The shape and dimensions of the coupling loops were copied directly from this web page, by Gary (NZ5V).  John provided the BNC connectors, I recovered the N connectors from obsolete equipment in the “junk pile.”  Two identical coupling loops were built, and two holes were cut in the top plate.  John cut an extra, blank disc to fill one of the holes so that I could test both bandpass and bandpass-bandreject (BpBr) configurations.

KD2SL Duplexer

Closeup of the holes in the top plate, which will receive the coupling loops.

Due to the size of the 6-cavity cabinet, all of the assembly and testing of the prototype had to take place at the transmitter site, which is a 20 minute drive from the office.  As soon as I had all of the machined and cut pieces from John, I eagerly arranged a long lunch break to assemble them into the prototype cavity.  John’s work was flawless, and everything fit together perfectly.  You will recall from an earlier post that one of the resonator stubs was longer than the others; I made sure to use that one for the prototype, so as to provide the best chance for success at the lower frequency.

KD2SL Duplexer

With the cavity tuned to 53.67 MHz, there was only 1/2″ of adjustment rod left!

Testing the resonant frequency of the cavity is pretty straightforward: connect a signal generator to one coupling loop, a spectrum analyzer to the other, and see what comes out!  The peak in cavity response is easy to spot.  I kept pushing the tuning rod lower and lower, and reached resonance at 53.67 MHz (our transmit frequency) with less than an inch of the adjusting rod to spare!  It was not possible to reach 52.67 MHz (the repeater’s receive frequency), but at least we now know how long a resonator stub needs to be for resonance at our repeater’s frequencies, and that it will indeed fit (barely) within the 51″ height of the cabinet.

KD2SL Duplexer

It doesn’t look so “fat” in this picture, but duplexer cavities normally use skinnier resonator stubs.

Why does it fit, when the calculation of a 1/4 wavelength at our frequency comes out a little longer than 51″?  Because the resonator stub is fat.  This same principle applies to antenna design; for example, with a yagi (beam) or dipole, making the elements fatter decreases their resonant frequency, meaning that you end up making them shorter to compensate.  It also makes them a little more broadbanded, which is usually great for antennas, but it reduces the “Q” of a resonant cavity.  We could have achieved higher “Q” and better notch depth with a skinnier resonator stub, but at our frequency, it would have been too long to fit in the cabinet.  There are always tradeoffs, it seems.  Fortunately, this is one we can live with.

Resonance alone isn’t the whole story.  How is the “Q”?  What the heck is “Q” anyway?  “Q” basically stands for Quality, and with regard to filters or other RF components, it refers to how sharp the response is.  Think of how an antenna works.  If it is broadband and covers a wide frequency range, it has a low Q.  If it has a narrow frequency range, it has a higher Q.  For duplexer cavities, we want a very high Q, because the distance between a cavity’s notch and passband is very small (only 1 MHz in our case).  The high Q means that a single cavity can attenuate 52.67 MHz by 40 dB, yet 53.67 MHz escapes virtually unscathed.

I’m happy to say that the Q of the prototype cavity was pretty good for duplexer use.  I didn’t think to make notes or take pictures of the spectrum analyzer while testing the prototype, so you’ll have to take my word for it.  In the bandpass configuration, insertion loss (attenuation at the pass frequency) was around .4 or .5 dB with the coupling loops adjusted for maximum coupling, and the response rolled off quite sharply – something like 15 dB or more at 1 MHz out from the center frequency.

Reasonably high Q, and the resonator stubs will tune low enough while still fitting in the cabinet.  Life is good!  Time to go full steam ahead with construction, and that’s where we’ll pick up the story next time.

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Duplexer Construction – Part 1 (The Components)

Fully stripped filter cabinets

The bare cabinets. Just out of view is a BIG pile of hardware!

Alright, it has been exciting to launch the repeater, and I’ve been focused on that the past couple of weeks – but let’s get back to how it got here!  When we last talked about the duplexer, I was deciding whether to build in the 6-cavity cabinet, or the 4-cavity cabinet.  You know by now that I decided on the 4-cavity cabinet, but why?

Advantages for the 6-cavity design are that I could include a bandpass filter for receive, which may or may not be necessary for protection of the receiver, due to nearby broadcast transmitters.  Advantages for the 4-cavity design would be smaller size and easier transportation (the whole thing fits in my wife’s Honda CR-V), and the inside partitions were nearly complete.  I was leaning toward the 4-cavity cabinet, but decided that the very first thing I needed to do was to build a prototype cavity in the 6-cavity box first, to see how it performed.  That way, if the duplexer is built in the 4-cavity box, I’ll still have the prototype to experiment with.

Inside the filter

Inside the bandpass filter. Click on the image for a super Hi-Res version.

Take another look at the 6-cavity cabinet (before it was fully desecrated!)  Notice that the left-most resonator has a longer bottom section.  This is because it needs to tune to a lower frequency, since it masks (or shunts) the low side of the passband.  For TV channel 3, this means about 60 MHz.  Assuming that the same design cabinet would be used for TV channel 2, which covers 54 to 60 MHz, I expected that the longest resonator could probably be tuned below 54 MHz.  The other resonators were all a few inches shorter and probably couldn’t be tuned that low.  LOTS of assumptions.  I needed answers!  “How low would it go” was the most important question that the prototype would answer.

Silver-plated telescoping resonator stubs.

Silver-plated, telescoping resonator stubs. NOT your average hardware store item!

Taking apart the diplexer and filter cabinets yielded a very nice pile of high quality metals; most of the cabinet was plated aluminum (not sure with what), and the resonators were silver-plated copper pipe with silver-plated finger stock in the slip joints.  Fancy stuff!  Building high-quality, stable, telescoping resonator stubs is the hardest part of a homebrew duplexer, so it is nice to have that work done for us!  Overall, it appeared that there was enough material that could be cut and reassembled to make duplexer cavities, but there wasn’t much room for mistakes or do-overs.  I did NOT want to buy any additional metal!  It was important to get it right the first time, important to do the research.

Generally speaking, there are three major components to a resonant cavity:

  1. The enclosure
  2. The resonator stub
  3. The coupling loop(s)

The Enclosure

The enclosures most often used for resonant cavities are usually round pipes, but they can also be square like the ones we’re working with.  Ours happen to be about 8″ square, and about 4 ft. 3 in. tall.  Eight inches square is perfectly fine, but only 4 ft. 3 in. tall?  That’s Aluminum partscutting it awfully close for a quarter wave at 6 meters.  In space (rather than in wire or metal), a quarter wave at 52.67 MHz is approximately 246 / 52.67 = 4.67 ft., a few inches LONGER than our enclosure.  We’ll have to hope that our prototype cavity has a low enough velocity factor to make it fit!

Notice that if we build the prototype at the left end of this enclosure, we already have a top (the 10mm thick plate leaning against the side), bottom, front (removed for the picture), back, and one side; we just need to build a bigger partition to close it off on the inside.  No problem, we’ll just borrow some partition material from elsewhere in the cabinet, and cut it to fill out the partition to reach all the way from the top to the bottom.  That will give us a fully enclosed box that measures about 8″ x 8″ x 51″.  If it works OK, we can build more cavities using a similar method.

The Resonator Stub

A resonant cavity is just a 1/4 wave section of coax – very LARGE diameter coax – open at one end and shorted at the other.  The length of the center conductor determines the resonant frequency.  You can’t accurately measure the center conductor and cut it to length ahead of time; you need some way to fine tune it by adjusting the length after it is in Telescoping resonator stubthe enclosure.  Usually some sort of telescoping section is used to accomplish this, and the resonator stubs from the old combiner and filter cabinets do just that.  We have TEN of them to work with, but we only need 4 (or maybe 5 or 6 if we add bandpass cavities), so maybe we can somehow combine them to make them longer, if necessary.

Something worth noting about these resonator stubs is the diameter, about 2-3/4″ for the top section, and about 4″ for the bottom.  That is somewhat larger than ideal for an 8″ square cavity.  The more air space you have between the cavity walls and the center stub, the better performance you get.  Commercial designs for duplexer cavities usually have thinner stubs, which results in a longer cavity, larger cavity volume, and higher “Q” (sharper tuning).  However, you can’t change any ONE thing in a cavity filter without affecting some OTHER thing.  In this case, that’s OK, because having a fatter stub means that it will be shorter, and our cabinet isn’t very tall.  A skinnier stub would probably be too long to fit, so having a little lower “Q” is a reasonable tradeoff for being able to make it fit!

The Coupling Loop

The diplexer and filter cabinets did not have coupling loops that were useful for our application.  We will have to fabricate our own.  Coupling loops are really just half of a transformer; they magnetically couple the RF into the center resonator stub (the other

The prototype coupling loop.

The prototype coupling loop, made with thick strap scavenged from the TV diplexer.

half of the transformer).  The basic design is shown in the photo; one end of the loop connects to the coax center conductor, the other end connects to a capacitor (for a reject cavity), or to ground (for a bandpass cavity).  Using a connector for both ends of the loop gives you the flexibility to go either way.  Different types of capacitors may be used, and can be mounted either inside or outside the cavity (we’re using a connector, so the capacitor will be outside).

As far as mounting the coupling loop to the top plate, the most common design uses a disc that fits into a hole, and is screwed in place.  To get the desired filter performance, you adjust the amount of coupling by loosening the screws and rotating the disc.  This alters the alignment of the magnetic field inside the cavity, and therefore alters the filter performance.  The top plate of our cabinet is 10mm thick aluminum, so we’ll make some discs out of material that was recovered from disassembling the cabinets, and make some holes in the top plate to receive the discs.

OK, we have a basic plan to cover the basic elements of cavity design.  I’m really itching to get a prototype assembled, and answer the BIG questions: How long does the resonator stub need to be for 53.67 (TX) and 52.67 MHz (RX), and will it fit in our cabinet?  And, how well will it work (how high will the “Q” be?)  In our next installment, we’ll see more particulars of how the prototype cavity was put together, and see how it performed.

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6m HT


The “new” official KD2SL office monitor rig. (A convenient excuse to try out the WordPress app for my phone.)

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Early Adjustments

WSTM (and KD2SL!) TowerWe’re already up to day 5 of operation for the 6m repeater.  I’ve enjoyed meeting some new hams, and reconnecting with some old friends!  Possibly this weekend some additional new users will arrive, as people have time to set up antennas, program their radios, and join in.  If you haven’t tried it yet, I encourage you to drop in with your call sign and see who is around.  If you’ve tried to get in and can’t, I’d like to know that too; e-mail me at kd2sl@yahoo.com.

For a “new” repeater, constructed out of “old” parts, it is doing very well.  A few minor adjustments have been made.  First, the CW ID was too loud, so I dropped it down a little, and raised the pitch.  I also lowered the level of the courtesy beep.  Some radio and speaker combinations give a lot of boost to those frequencies, so this should help. 

Second, each time I visit the repeater site, I turn the squelch knob just a smidge.  You might think that you would want to adjust the squelch to be “right on the edge,” so that ANY signal can open it, and let the tone decoder weed out the noise.  However, the tone decoder does an amazing job of sensing tone even when the signal is VERY noisy.  It doesn’t do anyone any good to have the squelch opening on super-noisy signals.  It annoys repeater listeners, and the person who transmitted the noisy signal hears a courtesy beep and thinks (falsely) that they can be heard on the repeater.  I will keep adjusting the squelch VERRRRRRRY slowly until I reach the point where it generally blocks unreadable signals, and lets noisy but readable signals through.

Finally, I have been investigating the source of some intermittent noise, which is sometimes heard on received signals (that is, not during IDs, announcements, or hang time).  On the very first day I noticed that wiggling the coax to the receiver input would make noise.  It was actually two cables: a longer 1/4″ Heliax from the duplexer, which mated to a short pigtail that plugs into the receiver.  Replacing the pigtail didn’t fix it, so made a single new cable that replaced both of the old ones.  My impression is that it helped, but there may yet be more noise generators at work.  We’ll have to see what happens on the first windy day, because ANY loose joint, or loose hardware ANYWHERE on the tower (even if not part of our antenna and feedline) could cause micro-arcing and RF noise generation.  Grounding of the repeater is not complete either, so THAT might be a factor.

Someone suggested that a preamp might help with the weak signals.  It might.  I am sure I will try it at some point.  My biggest concern with that is the VERY strong signals our antenna picks up from TV channels 24 and 25, and FM stations at 94.5 and 107.9.  We do not have a bandpass filter, so a LOT of energy at those frequencies would reach the preamp and possibly overload it.  I suppose I could ADD a bandpass filter at the cost of another 1 dB or so of signal prior to the preamp.  We’ll put this on the list for future experimentation.

In the next post, I’ll get back to a discussion of the duplexer construction.

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Going Viral?

It is still small potatoes in the Internet world, but within the ham universe, the 6 meter repeater is getting some attention.  This blog had been averaging around 50 hits per day, but yesterday (first full day on the air) it jumped to 862!  Thank you to everyone who helped spread the word.  It is also interesting to note the interest from international hams.

Today (Tuesday, 2nd day) the repeater has seen some use, and a bit of kerchunking (that’s OK, I do it myself).  If you’ve been standing by, listening, I encourage you to join in.

See you on the air!  73 – KD2SL

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Points to Remember

KD2SL Repeater, 53.670 MHz, Syracuse NY

The KD2SL 6m repeater! It takes up very little space, but packs a big punch on the monster antenna!

There has been a lot of activity on the first day of repeater operation.  Thanks to everyone who has given it a try!  Remember to carefully set up your transmit shift, and the tone:

  1. Dial in the receive frequency first: 53.670 MHz.
  2. Turn on the “-” repeater shift.
  3. Make sure the repeater offset is 1.000 MHz.
  4. Enable the tone on transmit.  Usually your radio will show a “T” or something similar when transmit tone is enabled.
  5. Select the tone frequency: 103.5 Hz (consult your radio’s manual).
  6. When you transmit, make sure your display shows 52.670 MHz.

This repeater is on the air for ALL hams to enjoy, and ragchew across all of upstate NY and bordering states/countries.  I am open to the idea of organized nets on the repeater, and will consider them once we’ve had a chance to get used to the coverage area and usage patterns.

I thought I found the source of the intermittent static, but it still pops in occasionally.  I’m confident we’ll find and eliminate it. 

Edit (May 15, 2012): The crackle is caused by the pigtail between the SO-239 receiver input on the back panel, and the RCA jack on the receiver.  It will be an easy fix.

Edit (May 18, 2012):  Ha!  Nothing is that simple!  See the “Early Adjustments” post above.

Thanks for all your signal reports.  Keep ’em coming!

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On The Air!

Sunday night, May 13 at around 8:00pm – the final connections were made, the power switch flipped on, and the KD2SL 6m repeater went on the air!  It is an exciting milestone in the project, and this is a good time to mention the hams who made it happen:

WB2DVE – John brought his machining skills and equipment to bear on the duplexer, turning my incoherent instructions and drawings into a skillfully assembled, one-of-a-kind unit that performs beautifully.  And, I think he enjoyed doing it!  There will be much more posted about the duplexer in the next few weeks.

W2QYT – Jack is responsible for taking scattered piles of donated low-band VHF GE Mastr II radios, picking out the best parts, turning them into a repeater, and tuning it up on 53.67.  His experience with creating amateur repeaters goes back nearly 40 years, and I don’t know how I could have gotten this repeater on the air without him.

N2FMM – Clint is a fellow TV broadcast engineer.  He provided a great deal of essential information and advice from his repeater experience in Buffalo and elsewhere.

N2VZD (Tim), WA2KFV (Al), KC2VER (Tony) – Each provided significant donations of equipment and/or cash.  Also, Tim was this project’s biggest cheerleader, encouraging me to move forward when I doubted whether it was worth the effort, and he worked hard to promote interest in the repeater.

NZ5V – Gary’s research and development on 6 meter duplexer design saved me countless hours, bringing our duplexer to the finish line MUCH faster.

All of these gentlemen embody the very best spirit of amateur radio, willing and eager to help my 6m repeater project with no thought of compensation or recognition.  Thanks also to Barrington Broadcasting, the parent company of WSTM, who owns the antenna and tower, donated the filter which became the duplexer, and is donating the space and electricity for the repeater.

Early results are pretty good, with full quieting signals being heard with low power from significant distances.  However, a couple of times I have heard some sort of staticy interference, even on strong signals – that bears watching.  Please click the comment link below and let us know how it is working for you; be sure to include your QTH, information about your rig and antenna, and a signal report.

It has been a long wait, but now it’s here!  Let’s have some fun with it!

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