A ham friend recently asked, “Why is it taking so long to get the repeater on the air?” There are several reasons, not the least of which is a shortage of my own personal time and money. If I had more of either of those, things would move a little faster. Also, working with the frequency coordinator takes time, and ordering crystals and tuning up a radio takes time; plus, you can’t order the crystals until you’re sure about your frequency!
However, the part of this project MOST responsible for the delay between “the dream” and being on-air is the duplexer (also often referred to as cans, filters, or resonant cavities). The radio was a donation, and relatively easy to modify for 6m repeater use. A great deal has already been written about the antenna, and how easy it was to prepare. But the duplexer, THAT takes some work.
So, what is a duplexer, and why do we need one? A repeater is different from a base station because it has to receive and transmit simultaneously. The moment that something comes in on the receiver, the transmitter turns on and sends it back out, on a different frequency. A duplexer (in the repeater context) is a filtering device that allows your transmitter and receiver to be connected to the same antenna, and operate simultaneously. It prevents transmitter output power from slamming into your receiver and rendering it deaf or burning it up. Think of it this way: You’re trying to read someone’s license plate at night, but their headlights are blinding you. A duplexer filters out the car’s headlights, allowing you to read the license plate. Not quite the same thing, but you get the idea.
This is a great time in history to build a duplexer, because the Internet brings a great wealth of information right to your computer. Thirty years ago, researching this topic would likely take many hours at a specialized technical library, or maybe you would be lucky enough to have friends who possessed the knowledge. I’ve understood the basic concepts of how a duplexer works for many years, but that is a far cry from knowing enough to actually build one. Fortunately for all of us, a great library of information about duplexers has been created online at the Repeater Builder web site. Over the past few months I have spent hours reading through that information, re-reading, then reading it again. I still go back there and read it AGAIN whenever I need a refresher. It is beginning to sink in!
Why am I building a duplexer? Why not just buy one, maybe at a hamfest, from a surplus auction, or on eBay? That’s a good question, and buying one or finding a donation would certainly have been my preference. You will recall from previous posts that some pieces of this repeater fell into place very quickly, for essentially no money. I had high hopes of continuing that run of good luck by obtaining a duplexer (or individual cavities that could
become a duplexer) at little or no cost. Asking around, I kept hearing the same, hopeful answer: There are at least a dozen VHF low band cavities sitting unused at such-and-such government radio site (just a few miles away from our site). I saw them with my own eyes, and have pictures to prove it. However, the short version of this story is that the governmental entity is constrained by strict policies relating to the disposal of assets, no matter how obsolete, no matter how much sense it would make to put them to use on an amateur repeater. Several of us with “connections” to various people tried to make it happen, but it just wasn’t meant to be. Maybe someday they will show up at an auction, but that’s no help now.
The next thought was finding something at a hamfest. A friend who was there when it happened told me that the cans used on the K2INH 53.05 repeater in Auburn were found at Dayton some years ago. That would be fine with me, but again, timing is a big problem. If we wait for hamfests, and then don’t find something, we’ve wasted a lot of time. I’ve been to quite a few hamfests in 26 years, but I don’t recall ever seeing cans for 6 meters or low band VHF (admittedly, I wasn’t really LOOKING for them). As much as I would love to travel to Dayton in May to look for a 6 meter duplexer, time and money constraints just don’t allow it this year.
OK then, how about eBay? Duplexers for higher bands are on eBay all the time. I just checked, and there are numerous duplexers for 144 MHz, 220 MHz, 440 MHz and higher available right now. Their size and weight are manageable for shipping at reasonable cost. However, a duplexer for 6 meters is much larger (lower frequency equals larger size). It would have to be strapped to a pallet and shipped by truck, so they just don’t show up very often on eBay. There just aren’t very many of them out there.
You could buy NEW cans, but that money thing gets in the way again – probably a few thousand dollars. So, we reluctantly arrive at the last resort: building a duplexer. Yes, it will be a lot of work, but none of the other options got us anywhere. Many people have done this successfully, and have written about it online. Here is one interesting example, using 8 inch aluminum irrigation pipe! There are other examples, one using 10 inch copper pipe (where on earth do you find THAT?), and another using scraps of Heliax transmission line.
The irrigation pipe story got me thinking about my mechanic brother, who has lots of exhaust pipe stacked up against the wall at his shop. It is coated with an aluminum finish to prevent rust, and comes in various sizes up to 5 or 6 inches. Maybe, just maybe it would be possible to build something using that? Surface conductivity wouldn’t be as good as copper, but you have to accept some amount of compromise to keep it affordable, right? Rust did concern me, though. Even with the aluminized coating, I didn’t think it would hold up well over the years. Sratch that idea.
While researching the homebrew idea, I ran across other, even less conventional ways of building duplexers, or even individual resonant cavities. Here is one that uses British beer kegs! And how about a galvanized garbage can! In reality, just about any properly sized metal can (string large coffee cans or number 10 food cans together somehow?) could be used to construct a resonant cavity, with varying degrees of success. These stories intrigued and tempted me, but I was determined to “respect” the incredible antenna and feedline we have for the repeater, and build something worthy of them.
Stay tuned – in our next duplexer installment, I’ll talk about how I found suitable raw materials sitting right under my nose!
Do you have experience building a duplexer? Have any interesting war stories about duplexers? Click the comment link below, and tell us about it!