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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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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Inside the Magic Box

TV broadcast diplexer and filterThe debate in my mind between building a duplexer with 6″ copper transmission line vs. a TV broadcast filter ultimately came down to exactly WHAT was inside the filter cabinets.  For years I had been walking past the filter boxes, wondering what was inside.  Documents I found at the transmitter site indicate that the smaller box (on the right) is the diplexer, and the larger box is the bandpass filter.  There were even drawings of the diplexer and filter that showed the version for higher TV channels, with correspondingly shorter cabinets.

You might be thinking, “If that is a diplexer, you’re all set, right?”  Not exactly.  Duplexers and diplexers are very similar.  In fact, similar enough that when I try to describe the difference between them, I really struggle to find words that clearly define the difference.  Try it yourself!  They both combine and/or separate signals of different frequencies.  What makes them different?  Hmmm…  Normally we think of a duplexer being used to allow a repeater to transmit and receive simultaneously on a single antenna, but a diplexer can do that too with a dual-band rig.

Anyway, while some readers take a moment to ponder that question, let’s move on.  An analog TV transmitter generates separate carriers for sound and picture; it’s really like two transmitters in one.  The diplexer (4 cavity cabinet) combines those two feeds, then passes it on to the bandpass filter (6 cavity cabinet).  I decided to start with the latter, so I found a 4mm hex head driver, and started removing screws.  Literally hundreds of them!  Finally, the cover came off.  Refer to the picture below, and notice that the filter cabinet contains six resonant cavities. 

Inside the filter

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

The first thing that caught my eye was the bright, silver-plated resonators.  These are very similar in construction to the resonators in duplexer cavities, just a little larger diameter than average.  The resonator is tuned by adjusting the length; the larger diameter section at the bottom slides up and down on the smaller upper section.  The longer the stub, the lower the resonant frequency.  Rather than a threaded rod, the adjustment is made by loosening a compression nut on the top, and sliding the rod up and down.  To provide thermal stability, the rod is made of Invar, an alloy of 36% nickel, 64% iron.  At this ratio, the metal exhibits virtually no expansion and contraction due to temperature changes, and helps keep the filter in tune.

Coupling loopsNotice that the partitions between the cavities don’t go all the way to the top and bottom.  That’s because these cavities are directly coupled to each other right through those holes.  The amount of coupling is adjusted by rotating the coupling loops.  I don’t know for sure, but I believe very few (if any) duplexer designs use directly coupled cavities.  It is far more common to couple signals in and out of individual cavities using coupling loops attached to coax through the top of the cavity.


Coax feed coming through the back wall of the cabinet, with the center lead tied directly to the shaft of the resonator.

It appears that the TV signal entered via direct coupling to the 2nd resonator, from 1-7/8″ rigid copper feedline that is visible on the back wall of the cabinet.  Energy below the filter’s passband was shunted into the 1st cavity (the longest resonator, therefore the lowest frequency), coupled into it by the adjustable loop at the top of the wall between the 1st and 2nd cavities.  The 2nd through 5th cavities are tuned to form a bandpass response with a broad “nose,” several megahertz wide, and energy above the passband is shunted into the 6th cavity (the shortest resonator, therefore the highest frequency).  The output signal is coupled from the 5th cavity resonator to a coax connector, in a manner similar to the input.

This is all very interesting hardware, with very heavy-duty construction to handle high power levels (about 8kW in our case).  But I had to wonder, could this filter with a fairly broad passband be re-tuned to have a sharper response, and function as a two-way radio duplexer?  Or would I have to make fundamental changes to the construction?  Could I use the direct coupling between cavities, or would I have to fill in the gaps and use traditional coupling loops that are fed from the top?  I even contemplated removing the resonators and possibly installing them inside pieces of 6″ copper pipe, but their large diameter would be too tight a fit.

Before deciding what to do, I would need to see inside the smaller diplexer cabinet – coming in the next installment.

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Wire Matters

There are many things to learn when taking on a project like this.  One of the more surprising for me was that very few types of coax cable are suitable for use on a repeater.  I thought I understood coax pretty well, but I really didn’t.  There is a good chance that some of what YOU think you know isn’t quite right either (check the YouTube video below, I think the gentleman isn’t quite clear on certain coax concepts either!)

Cable explained

Click the image for an unintentionally funny YouTube video about different kinds of coax.

When it comes to 50 ohm cable for two-way radio use, just about all hams are familiar with RG-58 and RG-8.  I got involved in CB radio during the craze of the 1970s, and that’s all there was for coax.  We used RG-58 for mobile installations, and if possible, RG-8 for base stations.  We could afford to generally ignore the quality and construction specifics of our cable because at HF frequencies, coax cable is very forigiving.  Even at 6m, 2m, and on up to 70cm, in many situations you can get generally reasonable results with modest RG-58 and RG-8 coax.

However, certain situations require you to raise your game, and use coax that is designed with special properties.  Repeater service is one such situation.  First of all, a repeater’s antenna is often very far up a tower.  It is very important to preserve as much received signal as possible, and at the same time transfer as much transmitter power as possible to the antenna.  The lower the feedline loss, the better.  Bigger coax generally has lower loss, so any decent repeater installation typically uses 1/2″ or larger Heliax for the main run, and short, more flexible jumpers to complete the run at the ends.

But there is more than loss to consider.  In repeater service, shield coverage is also important.  Lower quality RG-58 and RG-8 has shield braid coverage with lots of holes.  Signal can find its way in and/or out through those little gaps in the braid, and totally ruin the receiver/transmitter isolation that you worked so hard to create with the duplexer.  Heliax has 100% shield coverage (because it is essentially a solid copper pipe), but it is very stiff and isn’t suitable for locations where you need to move the cable now and then, or bend it into tight turns.  So then, what we want in our ideal cable is low loss, 100% shield coverage, and flexibility.  No matter how good the RG-58, it will not have low loss or 100% shield coverage.  RG-8 can have reasonably low loss, but 100% shielding is not possible and flexibility is so-so.  (Believe it or not, plenty of budget-minded hams have installed, and are using repeaters cabled entirely with RG-58!)


Illustration of the basic design of LMR-xxx cable.

One of the more recent arrivals on the scene is Times Microwave LMR-400 cable.  It’s basically the same size as RG-8 and can use most of the same connectors.  The construction uses a foil wrap, covered by a braided shield for 100% coverage.  Loss figures are very low, and there is a “superflex” version with a stranded center that is fairly flexible.  It isn’t super-cheap, but is still within reach for most hams at around $1/ft.  What’s not to like about that?  It might surprise you to learn that experts in the two-way radio field warn STRONGLY against using ANY LMR-xxx cable for repeater service!

Wait, what?  If it has very low loss, 100% shield coverage, is flexible and generally affordable, why can’t we use it on repeaters?  The answer lies in that aluminum foil shield, covered by a tinned copper braid.  Dissimilar metals in contact with each other are bad news.  In the presence of high RF power levels (such as you get when the repeater is transmitting while simultaneously receiving), all those millions of contact points between the braid and foil become millions of little tiny diodes, signal is rectified, and noise is heard on receive.  It might not happen when the cable is new, but eventually it WILL happen – especially if you have it outside, blowing in the breeze.  (An excellent article on repeater duplex noise is available here.  I highly recommend you take time to read it.)

With all the cable types out there, how do you choose the best ones for a repeater?  At the risk of oversimplifying, I think you can break it down this way:

  1. Andrews Heliax, 1/4"

    Andrews Heliax, 1/4″, shown next to a BNC connector for perspective.

    Use Heliax or other hardline wherever possible.  When that isn’t possible due to flexibility concerns, or when you need a short jumper at the end, then:

  2. Use silver-plated, double-shielded coax such as RG-400 (small, flexible, but higher loss) or RG-214 (large, stiff, lower loss).  There may be other types that will work, but stick with these recommendations and you will have the greatest chances of repeater success.

By the way, a few important corollaries:

  1. Avoid adapters; use the correct connectors on your cable instead.
  2. If adapters can’t be avoided, shop around for silver-plated, Teflon versions from reputable dealers (NOT eBay, probably not at a hamfest).
  3. Use genuine, high-quality MIL-spec coax, not cheaper imported versions that are often advertised as RG-214 “type”, for instance.
  4. It bears repeating: For duplex (repeater) service, do NOT use LMR-xxx (where xxx is any number).

(For more detailed information on the points above, visit this page at the Repeater Builder web site).

RG-400 Jumpers on the KD2SL Repeater Duplexer

In this sneak peek at the KD2SL repeater duplexer, note the light brown RG-400 jumpers

Sure, these types of cable cost more.  However, this is one area of repeater installation where you can’t afford to compromise performance.  Fortunately for this repeater, I was able to find 1/4″ Heliax jumpers and RG-400 jumpers in the “junk pile” that could easily be shortened to the lengths needed.  With high-quality coax costing nearly $3.00/ft, and proper N connectors costing $10-$20 each, I saved a lot of money.  Also, a local ham donated 50 ft. of 1/2″ Heliax that finished most of the inside run between the duplexer and the power divider.  The connectors on that Heliax were in poor shape, so I found and used two nearly-new N connectors from a short junk pile jumper.

LMR-400 Coax

Before I knew better, I bought LMR-400 and prepped the ends with nice N connectors. It serves as a reminder to do your research before you buy!

I’ll conclude with a confession: I bought LMR-400 for use on the repeater, before my research uncovered the reasons NOT to use it.  It still sits on the shelf of my shack, looking pretty with its shiny silver N connectors!  I’m annoyed at the money and time wasted, but I learned a lesson, and I’m thankful that it won’t be used on this repeater!

In our next post, I’ll get back to details about the duplexer, and how it was made from a diplexer and sideband filter from an analog TV transmitter.

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Finding Duplexer Parts

View from WSTM-TV, Syracuse NY

I can't resist yet one more view from the WSTM tower, about 75 feet BELOW our repeater antenna. This view faces north, right in to Syracuse, and FAR beyond.

Knowing the basics of duplexer theory are one thing, actually building one is quite another.  There are several viable methods of construction, with varying degrees of performance and pros/cons.  There is no way I can describe them all, so if you’re interested in details of duplexer theory and construction, the best place to go is the Repeater Builder site.  If you spend some time reading and re-reading the great information available there, you can develop a very good understanding of how duplexers work, how to build them, and how to tune them.

Building a duplexer requires a good bit of research, because you can’t just download a “recipe” and parts list, and start building.  For one thing, there are several design variables: band, frequency split, TX power, location.  Perhaps the biggest variable is the availability of raw material; some items are very hard to find, and very expensive when you do.  I think that’s why most homebrew duplexer projects are adapted to fit the materials a person happens to have on hand.  That’s why having a good understanding of the theory is essential.

The basic building block of a duplexer is a resonant cavity, which is essentially a 1/4 wavelength section of coax (very BIG coax), shorted at one end.  It typically looks like a big can.  Inside is a smaller tube, attached to the lid of the can.  The lid has one or two connectors for coax.  Usually, either 4 or 6 of these cavities are cabled together to provide the needed isolation between transmitter and receiver.  One interesting thing about the 1/4 wavelength resonant cavity is that you can use it to either absorb the resonant frequency (a notch filter), or pass the resonant frequency (a band pass filter), or you can even combine both actions and create a filter that both notches and passes (band-pass band-reject, or BpBr).  A complete duplexer can be built with one, two or all three of these filter types combined together.  It all depends on the design goals of your system.  In our case, a basic four cavity system using band-pass, band-reject filters (BpBr) should work well. 

How large does a resonant cavity filter need to be?  Resonant cavities need to be 1/4 wavelength long, so for the 6 meter band that is 1.5 meters, or about 5 feet long.  As far as the diameter, the bigger the better – up to a point; 6 to 12 inches is a good range for band pass and BpBr cavities, while smaller coax (1-7/8″ Heliax) can be used effectively for band reject (notch) filters.  The pipe also needs to be made of (or at least coated with) very low resistance metal, such as silver, copper, gold, or aluminum.  So, right away you can see the challenge: the materials are heavy, expensive and not commonly available.

Metals prices are very high these days, so even if you CAN find suitable pipe, it will likely cost you BIG money.  This cost of raw materials is certainly a big part of the reason why it is so expensive to buy commercially made cavities.  Therefore, most homebrew duplexers are adapted to whatever material someone has available.  A good example is the irrigation pipe duplexer that NZ5V describes here.  Another good example is the Heliax notch duplexer described here.

Miscellaneous feedline parts

Miscellaneous copper feedline pieces, several could be made into notch stubs.

In my case, I’m very fortunate that being a broadcast engineer gives me access to raw materials that would otherwise be very hard to come by.  The first avenue I explored was building a notch duplexer using scraps of 1-7/8″ rigid copper coax, pieces that I saved when our analog TV transmitter was removed.  This approach appealed to me at first because I thought it would be possible to build it using only a hacksaw and drill press, and very few additional parts (read: cheap!).  However, a notch-only duplexer affords no protection from other nearby transmitters, and we have plenty of them at our site.  Perhaps the tight front end of the GE Mastr II radio would be able to deal with that, but I decided to keep looking for another option.

W2IVB Repeater, Colden NY

Numerous resonant cavities at the W2IVB site, Colden NY. Note the 6m copper cans in the middle.

The next possibility was band pass and/or BpBr cavities built from 6″ rigid copper feedline.  I had some discussions with another broadcast engineer friend of mine, Clint N2FMM, who built a four-can BpBr duplexer for the W2IVB 6m repeater in Colden, NY (near Buffalo) from 6″ transmission line, and the results were excellent!  I knew my employer had some leftover 6″ line, and I figured it couldn’t hurt to ask.  To my surprise, I was given permission to purchase enough to build four cans! 

6" rigid copper feedline

6" copper coax, with brass flange.

A big plus here is that one end of the pipe has a brass flange that would make it easy to attach an end plate.  The performance would likely be very good, based on the success of the W2IVB duplexer.  Construction would be a little trickier, because of the size and weight of the pipe.  I would need help cutting and machining various parts.  It would also be necessary to purchase some expensive metal stock for the end plates and center pipe flanges.  Doable, but I was still in search of a more elegant (read: cheap!) solution.

Finally, I turned my attention to the old sideband filter from the analog transmitter.  WSTM operated analog on VHF channel 3, which is 60 to 66 MHz, just a little bit above the 6 meter band of 50 to 54 MHz (which, by the way, is where the original TV channel 1 used to be!)  In one of my earlier posts, I described how WSTM rebuilt the analog transmission WSTM-TV Analog Sideband Filterfacility in 2002 while installing digital.  A new sideband filter was part of that installation.  After the analog shutoff in 2009, the sideband filter was rolled off into a storage room and largely forgotten.

I don’t know much about filter design, so I had no idea just what was inside.  From the outside, it appeared that there were adjustable plungers similar to what are used in duplexers.  The cabinet size was about right for resonant cavities, because TV channel 3 runs from 60 to 66 MHz, and the cabinet was 1.3 meters tall.  Screw patterns suggested that the cabinets were divided into chambers, although not all the way from top to bottom.  Obviously, there is only ONE way to find out what’s really inside: Tear it apart!

Stay tuned – in our next duplexer installment, we’ll find out what’s inside the boxes!

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Finding a Frequency

View from WSTM-TV Tower

Not much population in this direction. View from the tower looking S-SE, toward Fabius, Pompey, LaFayette, Cincinnatus.

Have you ever wondered how a repeater gets its frequency?  Some people probably assume the FCC handles that.  I don’t know if that might have been true years ago, but these days repeater frequencies are coordinated by volunteer groups.  Syracuse falls under the jurisdiction of a group called UNYREPCO, the Upper New York REPeater COuncil.  If you’re interested in how coordination is done, and what the application looks like, you can poke around their site and read the information.  That’s what I did back in early February when I decided to build a repeater.

You have the option of letting your coordinator select an open frequency, or you can do your own research and request a particular frequency.  I decided to do the research myself, because I had a feeling that my own criteria for identifying an open frequency might be more stringent than that of the coordinators.  It turned out to be true. 

One of my frustrations with repeaters in general is that it is impossible to get an accurate list of repeaters that are currently on the air.  Any list you find ANYWHERE is going to be full of errors.  There really is no way to avoid this, because people move, people die, repeaters die, etc., and unless they tell someone, their repeater will still be listed.  Some hams don’t even bother to work with the coordinators; they can, and do put repeaters on the air without telling anyone.  So, the best you can do is do your research, work with the frequency coordinators, and hope everything works out.

The first thing I did was to make a list of all the known repeater frequencies for this part of the country.  Next, I crossed off everything listed by UNYREPCO, then ARRL, and EVERY other online list I could find, whether or not it was known to be a current listing.  I felt it was best to avoid any frequency that had been, could be now, or might be re-used in the future.  That brought it down to only a handful of possibilities.  Then, I crossed off anything that had an adjacent frequency in use within 60 miles or so.  Finally, I started Googling the remaining frequencies, which turned up various web references that were not mentioned in anyone’s lists.  Just to be safe, I crossed those off the list also.

Our monster antenna means that this repeater will likely have longer range than average, so I expanded the search all the way to northeastern Ohio, all of Pennsylvania, northern New Jersey, and just about all of New England (except Maine).  That may seem like overkill, but it made sense to give this repeater every advantage for long range without interference.  In the end, this particular search pointed me toward 53.25 MHz.  I dutifully submitted my application, and about a month later the offical coordination approval was e-mailed by UNYREPCO.

What happened next was a genuine, hard smack of the forehead moment!  Literally minutes after I read the e-mail that contained the good news, I thought, “Wait a minute, I didn’t check Canada!”  I don’t know WHY this didn’t occur to me before, and wouldn’t you know it, I found a 53.25 repeater in the Toronto area, and it even had the same PL I had requested!  There was no doubt in my mind that someone in the Rochester area could easily bring up BOTH repeaters.  Why did UNYREPCO approve it?  Basically because it met their criteria for distance between co-channel repeaters, so there was no reason not to.

With heavy heart, and my head hung in shame because I hadn’t included Canada in my search, I started all over again.  This time I ended up with 53.67 MHz.  The closest repeaters on that frequency are in Oakham, MA (210 miles), Budd Lake, NJ (160 miles), Honeybrook, PA (195 miles), and NOTHING in Canada!  There is a 53.69 in Elmira, and a 53.65 in Burdett (near Watkins Glen), but those are far enough away that it shouldn’t be a problem for an adjacent channel.  I contacted UNYREPCO, modified my application, and a month later had my official notice that I was coordinated for 53.67 MHz.  That’s a good thing, because the crystals for the GE Mastr II repeater had already been ordered by then!

So, it took longer than it needed to because of my ineptness, but we’re now “official” for 53.67 MHz (-1.000 MHz offset, and 103.5 PL if we choose to use it).  I missed the ARRL Repeater Directory deadline by a couple of months, so it won’t be listed there until the 2013-2014 edition at the earliest (updates from our area are notoriously slow to reach the ARRL Directory).  By the way, I don’t expect to be using PL on the receiver unless it is absolutely necessary to prevent spurious squelch opening; if it is needed, it will be 103.5 Hz.

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Status Update

I am pleased to annouce a few updates:

HP Spectrum Analyzer

It is good to have friends - especially ones who are willing to loan a good spectrum analyzer with tracking generator!

  1. The crystals have arrived for the GE Mastr II repeater!  They will be installed, and the tune-up performed within a couple of weeks.  (Thanks to the “anonymous” ham who is handling this part of the project for me!)
  2. I have obtained a loan of a very nice HP spectrum analyzer with tracking generator (picture at left).  This will be essential to the tuning of the duplexer.
  3. My machinist John, WB2DVE, reports that work is progressing nicely on the duplexer modifications.  Much more will be posted about the duplexer work in the coming days.

All of this means that we are still on track to put the repeater on the air by the middle of May!

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