Inside the TV Diplexer

I’ve been thinking about the question I posed in the last post: What’s the difference between a DUplexer and a DIplexer?  On a basic level, they both have to do with using multiple transmitters and/or receivers on a single feedline and/or antenna.  Diamond MX72HGenerally speaking, when we say DIplexer, we’re talking about a small filter that combines or separates signals in different bands, such as the unit pictured to the right that allows either: 1) Two antennas (144 MHz and 440 MHz) to be connected to a common antenna port on a dual-band rig, or: 2) Two radios (144 MHz and 440 MHz) to be connected to a common, dual-band antenna.  There are quite a few variations, but that’s the basic idea.  They don’t have to be very large because the frequency bands involved are broad, and far apart.

WSTM diplexer and filterOn the other hand, when we say DUplexer, we usually mean a series of tuned cavities that allows a repeater to transmit and receive simultaneously on the same antenna, using frequencies that are very close to each other.  But, a DIplexer could also be made with tuned cavities, and could combine two or more transmitters into a single feed.  I guess you could also just call that a combiner.  For whatever reason, the smaller cabinet in the picture to the left is called a DIplexer by the manufacturer, because it combines the visual and aural transmitter signals into one feed.  Click the image to enlarge it, and you can read the labels I added which identify what went where.

Alright, enough about what it USED to do.  We’re here to build a duplexer, so let’s rip it apart and figure out what we can MAKE it do!  Once again, it took quite a while to remove all the screws and get the cover off.  Screws around the edge of the cover were pretty straightforward, but things got weird in the middle, because the screw heads were INSIDE the box, with a nut tightened onto it OUTSIDE!  Four cavity diplexerI’m still scratching my head over that one, because I can’t understand how they held the screws in place while tightening the nuts, nor why they would WANT to do it that way!  I had to spin the nuts off, then poke the screw in, where it fell inside to the bottom of the cabinet.

Inside, the diplexer cabinet was fairly similar to the bandpass filter cabinet shown in the previous post.  There were four resonant cavities, with direct coupling ports between some of them.  Some differences were that these resonators had plastic spacers attached to the top of the moving section, and the center partition almost completely separated the adjacent cavities – just a small gap at the bottom.  The signal was coupled into and out of the cavities using little round plates that were positioned in holes on the side covers.  Inside the diplexerAttached to each side cover was an interesting box that contained some sort of inductive or capacitive coupling device; at RF frequencies, it isn’t always clear which it is – probably some of both.

I spent several days pondering the possibilities: Should I attempt to use these filter cabinets basically as they are, with minor modification for the different frequencies?  Or should I completely tear them down, and rebuild them using repeater duplexer construction techniques that are tried and tested?  I love a good technical mystery as much as the next guy, and the thought of experimenting with the filter as I found it was appealing…sort of.  In particular, I thought the idea of using direct coupling between the cavities would be rather unique for an amateur duplexer.  I knew it could work, but how well?

This is where I had to make a big decision.  My choices were:

  1. Create a traditional duplexer “can” construction using the 6 inch copper feedline I mentioned in a previous post.  PROS: I already had the copper, and the techniques were well documented online.  CONS: Cutting and transporting the copper would be difficult, and I’d have to purchase and machine additional metals at significant cost.
  2. Experiment with the analog TV filter cabinets, and attempt to retune them with minor modifications.  PROS: Lots of the machining and assembly was already done.  CONS: It would take too long to experiment, and the end result might not work anyway.
  3. Completely strip the cabinets, and rebuild the cavities using traditional duplexer construction techniques.  PROS: Adequate raw materials could be scavenged from the filters, and the resonators were already constructed.  CONS: Significant machining would be required, and transportation could be a challenge.

I settled on option 3, with option 1 as “Plan B.”  So, the next several extended lunch-time trips to the transmitter site were spent removing hundreds of screws, and tearing the filter cabinets down to the bare walls.  All the while, I kept considering the NEXT big question: Should I rebuild in the 6-cavity cabinet, or the 4-cavity cabinet?  That question, and more, will be addressed in the next duplexer post.

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About kd2sl

I'm a lifelong geek, interested in anything electronic, but especially ham radio, radio and TV broadcasting, and computers. Employed as a television engineer for several Syracuse, NY TV stations.
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